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  1. #1
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    Thumbs up Evaporating / condensing temperatures



    As a rule of thumb:

    Refrigeration condensing temperature is ambient + 10
    Refrigeration evaporating temperature is space temp - 10

    Air conditioning + or - 15 deg

    Would anyone care to agree or disagree with this?

    Regards and thanks



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    Re: Evaporating / condensing temperatures

    Hi there,

    Tc = condensing temperature
    Ta = ambient temperature
    te= evaporating temperature
    tei = air inlet to the evaporator

    TDc = Tc - Ta
    TDe = tei - te

    For air cooled condensers :
    Hot climates ------- TDc = 6 to 8°C
    Normal climates ------ TDc = 8 to 10 °C
    cold climates ---------- TDc = 12 to 15 °C

    For water cooled condenser :
    TDc = 15°C

    For cold room evaporator :
    TDe = 5 to 6 °C for fruits and vegetables (high relative humidity)
    TDe = 8 to 10 °C for frozen products (low relative humidity)

    For air conditioning evaporator :
    TDe = 15 to 20 °C

    For water chiller evaporator :
    TDe = 10°C

    Hope this helps
    Cheers
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    Re: Evaporating / condensing temperatures

    Quote Originally Posted by lana View Post
    Hi there,

    Tc = condensing temperature
    Ta = ambient temperature
    te= evaporating temperature
    tei = air inlet to the evaporator

    TDc = Tc - Ta
    TDe = tei - te

    For air cooled condensers :
    Hot climates ------- TDc = 6 to 8°C
    Normal climates ------ TDc = 8 to 10 °C
    cold climates ---------- TDc = 12 to 15 °C

    For water cooled condenser :
    TDc = 15°C

    For cold room evaporator :
    TDe = 5 to 6 °C for fruits and vegetables (high relative humidity)
    TDe = 8 to 10 °C for frozen products (low relative humidity)

    For air conditioning evaporator :
    TDe = 15 to 20 °C

    For water chiller evaporator :
    TDe = 10°C

    Hope this helps
    Cheers
    For Air-Conditioning purposes

    I think for air-cooled condenser:

    Hot climates ------- TDc = 15 to 20°C
    Normal climates ------ TDc = 10 to 15 °C
    cold climates ---------- TDc = 8 to 10 °C

    For water-cooled condenser

    Tc - Twi (inlet water temperature) = 5 to 8°C

    and Twi depends on the wet bulb temperature of the surrounding air , it is larger than the wet bulb temperature by approach value of 6 to 8°C.

    Cheers

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    Re: Evaporating / condensing temperatures

    I think for air-cooled condenser:

    Hot climates ------- TDc = 15 to 20°C
    Normal climates ------ TDc = 10 to 15 °C
    cold climates ---------- TDc = 8 to 10 °C
    Complitely the oposite. When low TDc is selected for the condenser the size (surface area) will be bigger which is required for hot climates.
    Higher TDc gives smaller surface area, good for colder climates.


    For water-cooled condenser

    Tc - Twi (inlet water temperature) = 5 to 8°C
    and Twi depends on the wet bulb temperature of the surrounding air , it is larger than the wet bulb temperature by approach value of 6 to 8°C.
    What you mentioned is for evaporative condensers not water cooled.
    For water cooled condenser which is cooled by cooling tower water then :
    water inlet to the condenser = 27°C
    water outlet from the condenser = 32°C
    and condensing temperature is = 42°C
    These figures may change if the cooling tower performance changes, i.e. wet bulb of the area. These figures are for WB=23°C.
    Of course these figures are not exact but normal conditions.
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    Re: Evaporating / condensing temperatures

    Quote Originally Posted by lana View Post
    Complitely the oposite. When low TDc is selected for the condenser the size (surface area) will be bigger which is required for hot climates.
    Higher TDc gives smaller surface area, good for colder climates.




    What you mentioned is for evaporative condensers not water cooled.
    For water cooled condenser which is cooled by cooling tower water then :
    water inlet to the condenser = 27°C
    water outlet from the condenser = 32°C
    and condensing temperature is = 42°C
    These figures may change if the cooling tower performance changes, i.e. wet bulb of the area. These figures are for WB=23°C.
    Of course these figures are not exact but normal conditions.
    Hi Lana

    thank u for ur input. Ok u have mentioned that u should select the TDc is low for hot climate to get a large area, and TDc is high for cold climate to get a small surface area. ok firstly it is not neither large area nor small area as u mentioned it is called the design area which is responsible to reject a certain amount of heat at this condensing temperature. Secondly ok i would like to ask u for a cold weather u say the TDC should be high that means definitely u will burden on the system compressor power and size to offset the impairing occured in its volumetric efficiency. In addition when the ambinet temperature drops significantly (normally in cold climate) this leads to a high drop in condensing pressure and an inefficient performance of TXV due to the lack of pressure difference and the problem will be aggravated if it is a rather cooling load on the evaporator. Furthermore, the designated system will consume a large amount of energy due to it is designed at higher condensing temperature in a scarce times of design ambient temperature occurrence. In hot climate u mentioned that in it the TDC should be small ok suppose that there a high shooting in ambient temperature (this is normal in hot climate as my country) that means the compressor has to squeeze itself to raise up its condensing pressure by multiple factor to overcome this dilemma uless it will trip off by HPG. Secondly i did not mention any word related to evaporative condenser (i said water-cooled condenser) and water-cooled condenser sometimes uses cooling tower and sometimes uses dry cooler. Even the evaporative condenser depends on the wet bulb temperture. Hope that it is clear

    Cheers

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    Re: Evaporating / condensing temperatures

    Hai,

    For water cooled units should we do the same ?


    Quote Originally Posted by Contactor View Post
    As a rule of thumb:

    Refrigeration condensing temperature is ambient + 10
    Refrigeration evaporating temperature is space temp - 10

    Air conditioning + or - 15 deg

    Would anyone care to agree or disagree with this?

    Regards and thanks

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    Re: Evaporating / condensing temperatures

    Ok u have mentioned that u should select the TDc is low for hot climate to get a large area, and TDc is high for cold climate to get a small surface area. ok firstly it is not neither large area nor small area as u mentioned it is called the design area which is responsible to reject a certain amount of heat at this condensing temperature.
    We are saying the same thing. That is the design condenser heat transfer area.

    Secondly ok i would like to ask u for a cold weather u say the TDC should be high that means definitely u will burden on the system compressor power and size to offset the impairing occured in its volumetric efficiency.
    Why? When a condenser is selected for an operating condition then it is OK and it will reject the heat at the design conditions. Why would the compressor consume more power? If smaller condenser was selected then this would increase the condensing pressure and therefore, the power consumption of the compressor.

    In addition when the ambinet temperature drops significantly (normally in cold climate) this leads to a high drop in condensing pressure and an inefficient performance of TXV due to the lack of pressure difference and the problem will be aggravated if it is a rather cooling load on the evaporator.
    If condenser is selected for a cold climate then the size of the condenser is such that it will give the design condensing temperature. For example for ambient temp of say max 30°C you take TDc = 15°C and therefore, condensing temperature would be 45°C. Now selected condenser is suitable for this condition.
    If ambient temperature drops so will the condensing temperature. And this is why we use "head pressure control".


    Furthermore, the designated system will consume a large amount of energy due to it is designed at higher condensing temperature in a scarce times of design ambient temperature occurrence.
    NO. The system is not designed for high condensing temperature- as explained above.

    In hot climate u mentioned that in it the TDC should be small ok suppose that there a high shooting in ambient temperature (this is normal in hot climate as my country) that means the compressor has to squeeze itself to raise up its condensing pressure by multiple factor to overcome this dilemma uless it will trip off by HPG.
    Again NO. When low TDc is selected which gives bigger condenser then again the condensing temperature is what it should be. For example the ambient temp is max 42°C and TDc = 8°C now condensing temperature is 50°C. From the beginning the compressor should be selected for these conditions and at these conditions compressor consumes design power not more. If ambient temperature increases above the design value then yes power consumption will increase.
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    Re: Evaporating / condensing temperatures

    Quote Originally Posted by mohamed khamis View Post
    ...when the ambinet temperature drops significantly (normally in cold climate) this leads to a high drop in condensing pressure and an inefficient performance of TXV due to the lack of pressure difference and the problem will be aggravated if it is a rather cooling load on the evaporator.
    Why should the performance drop so low that resulting in an inefficient performance? Danfoss can work with a DP as low as 4 bar.
    What will give you the biggest savings: a high DP over the valve or an overall low DP over the valve?
    It's better to keep your mouth shut and give the impression that you're stupid than to open it and remove all doubt.

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    Re: Evaporating / condensing temperatures

    To remarks,

    * in general (warm, cold or moderate climat) : keep the temperature difference (dT= Tc-Tamb) to a minimum, keeping in mind:
    - you need a minimum dT to have a good heat exchange
    - you need a minimum dp over the expansion valve to keep a stable operation, aspecially on low superheat applications (electronic expanion valves are the solution here)
    - small dT means (larger exchange surface and) more expensive condensor, higher content, ....

    * the unit for temperature difference is K (Kelvin) (not °C); let's use it, to avoid any confussion

    Best regards

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    Re: Evaporating / condensing temperatures

    Unless there is a constraint on space, I feel specially in our country havine a larger surface area for an air cooled condenser is more effective. There are times when ambient in the Northern part of our country goes up to 44 Deg C to 45 Deg.C.

    In some part water is a scarcity and as such Air Cooled Condensers are te ones that has to be used.

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    Re: Evaporating / condensing temperatures

    Quote Originally Posted by Samarjit Sen View Post
    Unless there is a constraint on space, I feel specially in our country havine a larger surface area for an air cooled condenser is more effective. There are times when ambient in the Northern part of our country goes up to 44 Deg C to 45 Deg.C.

    In some part water is a scarcity and as such Air Cooled Condensers are te ones that has to be used.
    In the southern part of our country you get 55°C ambient also. You have to measure and feel to believe it.
    In these areas small condensers (high design TDc) don't work. Also in these parts ambient Wet Bulb is too high to use an efficient cooling tower or evaporating condensers.
    Cheers
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    Re: Evaporating / condensing temperatures

    Quote Originally Posted by lana View Post
    Why? When a condenser is selected for an operating condition then it is OK and it will reject the heat at the design conditions. Why would the compressor consume more power? .
    What I want to say that the cold climate does not need a high condensing temperature or pressure when u design with a high condensing u will select compressor featured by high consumed power and size. The increasing in condensing pressure design is followed by an amplification in Energy consumption…check this website http://electronics-cooling.com/artic..._august_a1.php

    Quote Originally Posted by lana View Post
    If smaller condenser was selected then this would increase the condensing pressure and therefore, the power consumption of the compressor.
    I don not understand what u mean…firstly ur design with high TDC in cold climate will lead to smaller area as u mentioned and now u say “If smaller condenser was selected then this would increase the condensing pressure and therefore, the power consumption of the compressor.” I don not know what does it mean!!!!!!!!


    Quote Originally Posted by lana View Post
    If condenser is selected for a cold climate then the size of the condenser is such that it will give the design condensing temperature. For example for ambient temp of say max 30°C you take TDc = 15°C and therefore, condensing temperature would be 45°C. Now selected condenser is suitable for this condition.
    [

    Firstly the design ambient temperature in cold climates I think it is under 20°C not 30°C because in clod climates as my knowledge the ambient sometimes reach to under 0°C therefore our colleges in these zones adjust the TXV screw in spring and fall weather to save energy in these times otherwise it is recently orientations in commercial refrigeration to use EEV “AKV” to reap this benefit
    If ambient temperature drops so will the condensing temperature. And this is why we use "head pressure control".



    Quote Originally Posted by lana View Post
    NO. The system is not designed for high condensing temperature- as explained above.
    I meant that the design with high TDC in cold climate means it is overdesigned condenser

    Quote Originally Posted by lana View Post
    Again NO. When low TDc is selected which gives bigger condenser then again the condensing temperature is what it should be. For example the ambient temp is max 42°C and TDc = 8°C now condensing temperature is 50°C. From the beginning the compressor should be selected for these conditions and at these conditions compressor consumes design power not more. If ambient temperature increases above the design value then yes power consumption will increase.
    Ok I will take ur example the ambient temp is max 42°C and TDc = 8°C now condensing temperature is 50°C. If we assume the subcooling is 5 K (tightwad design) that means the exit refrigerant temperature is 45°C that it should be above the inlet air temperature by at least 5 as mentioned in ASHRAE fundamentals and ur design will make this difference is 3 J. Suppose the ambient temperature will be heated by 10 K (Normally the air is heated by 10 to 15 K in the air-cooled condenser design) that means the exit air will be 52°C in turn the pinch temperature difference will reach 0 or as maximum 2 K unless it is negative and this violets the pinch technology to improve heat exchanger design.

    Cheers

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    Re: Evaporating / condensing temperatures

    Quote Originally Posted by lana View Post
    In the southern part of our country you get 55°C ambient also. You have to measure and feel to believe it.
    In these areas small condensers (high design TDc) don't work. Also in these parts ambient Wet Bulb is too high to use an efficient cooling tower or evaporating condensers.
    Cheers
    No. Firstly u have mentioned 55°C as the ambient temperature this is we call it sol-air temperature = ambient temperature + effect of solar radiation. It should be noted the design ambient temperature is not taken as the hottest day temperature this will inevitably waste unnecessary vast energy consumption. The design ambient temperature is taken the average temperature around all the year times day and night so there is minimum design temperature and there is a maximum design temperature and as I know the maximum design temperature which I came a cross is 40°C to 45 °C therefore the condensing temperature 60°C of R-134a.

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    Re: Evaporating / condensing temperatures

    If you take the avarage temperature as the design temperature, then what happens to the system when the maximum temperatures are prevailing. Maybe the maximum temperatures lasts for quite a while. Resulting in the failre of the system.

    I feel that it would be better to consider the peak temperature as the design temperature so as to avoid any conflict with the users.]

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    Re: Evaporating / condensing temperatures

    Quote Originally Posted by lana View Post
    In the southern part of our country you get 55°C ambient also. You have to measure and feel to believe it.
    In these areas small condensers (high design TDc) don't work. Also in these parts ambient Wet Bulb is too high to use an efficient cooling tower or evaporating condensers.
    Cheers
    Lana I would like to know as to how do you treat and design the air conditioning system under such circumstances. This is more like approaching a steel plant conditions.

    This information I am seeking just to learn more under various situation.

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    Re: Evaporating / condensing temperatures

    Hi Mohamed,


    I meant that the design with high TDC in cold climate means it is overdesigned condenser
    NO, NO, NO , NO . It's exactly the opposite.


    Q = U x A x TD for any heat exchanger, assuming U stays constant (0.05 kW/m².K)and we want to have a condenser with TDc = 10°C and capacity of 100 kW.
    100 kW = 0.05 x A x 10 ---- A = 200 m²

    Now calculate the area for TDc= 5°C
    100 kW = 0.05 x A x 5 ----- A = 400 m²

    It is so simple. If you increase temperature difference then the area decreases. If the temperature difference is decreased then area would increase.

    In hot climates you have to take TDc= 6°C so the the condensing temperature would not rise to a high value.

    I think the problem is you confuse the temperature difference here. TDc = Tc - Ta

    IN HOT CLIMATES THE AIR COOLED CONDENSERS ARE BIGGER FOR A CERTAIN CAPACITY THAN IN A NORMAL CLIMATE. IF NOT THE CONDENSING TEMPERATURE WOULD RISE, I.E. YOU WOULD GET HIGH PRESSURE CUT OUT.

    Cheers
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    Re: Evaporating / condensing temperatures

    Quote Originally Posted by Samarjit Sen View Post
    Lana I would like to know as to how do you treat and design the air conditioning system under such circumstances. This is more like approaching a steel plant conditions.

    This information I am seeking just to learn more under various situation.

    Hi there,

    These extreme temperatures accrue for a few days during summer. Also the humidity is very high in those parts.
    As for the design of A/C we have to consider the ambient temperature and then select the equipment with those conditions. Some manufacturers do not consider the peak conditions in order to build smaller units and therefore cheaper. These units do cut out by high pressure during off design ambient temperature.
    Personally I don't like that. Air conditioning is for comfort and if I don't have it in such conditions then whats the point of having it?

    Cheers
    Last edited by lana; 15-06-2007 at 05:40 PM.
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    Re: Evaporating / condensing temperatures

    This thread has become pretty interesting. But where is Contactor who started the thread.

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    Re: Evaporating / condensing temperatures

    Quote Originally Posted by Peter_1 View Post
    Why should the performance drop so low that resulting in an inefficient performance? Danfoss can work with a DP as low as 4 bar.
    What will give you the biggest savings: a high DP over the valve or an overall low DP over the valve?
    Hi Peter_1

    Of course If we are taking about the biggest savings the low DP over the valve accounts for

    a low power consumption in compressor. However, the design with low DP has a limitation of TXV performance that means if the design is performed with a very low DP

    Let us say 7 bar, during low ambient temperature with a rather cooling load on the evaporator the DP can decrease below the minimum limit

    as u mentioned (4 bar) and in this case the TXV meters unsufficent a mount of liquid refigerant less than the required at this particular load

    and the system performs unefficiently. i wish it is clear

    Best regards

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    Re: Evaporating / condensing temperatures

    Hi Lana

    Quote Originally Posted by lana View Post
    Hi Mohamed,


    NO, NO, NO , NO . It's exactly the opposite.

    Well...Sorry for using inproper terminology "overdesign condenser" but i mean overestimated operating conditionins for the Ac system in the clod cliamtes

    as u said a higher condensing temperature or pressure. I came across a book entitled "Hand book of AC and refi. 2nd Edit. Wang" discusses in detail this issue particullary Chapter 10

    I am suggesting all forum users read it carefully and he cites the proper condnesing temperature should be used as follows

    In air aiconditiong system purposes:

    TDC or CTD = condenser temperature difference = 11 to 16.7 K

    In refrigeration system puroses:

    TDC or CTD = condenser temperature difference = 8 to 11.0 K

    And i still insist that the condenser or the system should be designed with a samller CTD (like above 8-11 K) to save compressor energy consumption

    even there is a space stringent limitaions the last thoughout of decreasing condenser area is the design with higher CTD but there are many thoughout to achive that (more compact area) as follows:

    1)- Incraese the fan capcity (it takes place arround 10 - 15% of the total power consumption for the system and the remainder in the compressor).

    2)- Increase tubing number of deep-rows

    3)- Using an enhenced fins configuratons " corrugated, offest-strip, ..ect)


    Quote Originally Posted by lana View Post
    Q = U x A x TD for any heat exchanger, assuming U stays constant (0.05 kW/m².K)and we want to have a condenser with TDc = 10°C and capacity of 100 kW.
    100 kW = 0.05 x A x 10 ---- A = 200 m²

    Now calculate the area for TDc= 5°C
    100 kW = 0.05 x A x 5 ----- A = 400 m²

    It is so simple. If you increase temperature difference then the area decreases. If the temperature difference is decreased then area would increase.

    In hot climates you have to take TDc= 6°C so the the condensing temperature would not rise to a high value.

    I think the problem is you confuse the temperature difference here. TDc = Tc - Ta

    IN HOT CLIMATES THE AIR COOLED CONDENSERS ARE BIGGER FOR A CERTAIN CAPACITY THAN IN A NORMAL CLIMATE. IF NOT THE CONDENSING TEMPERATURE WOULD RISE, I.E. YOU WOULD GET HIGH PRESSURE CUT OUT.

    Cheers
    I know very well this equation, I used it more than 100 times in M.sC degree, by the way the temperature difference here is logrithmeic temperature differnce not aritmatic diffence as u wrote unless u will design the condenser without desuperhating and subccoling zones or the temperature profile is straight line.


    No in a hot climate the design condensing temperature excreeds the design air temperature by 15 - 20 K as i mentioned before.

    Oh i got it, i think u have confusion of the design ambinet tmperature that it is taken as the peak or maxium air temperature arround the year ...

    This is totally not right [1] the design air temperature is taken as the temperature whose annual accumalative frequency of occurrance is about 10% of the year hours (8760) [2]

    i.e. it is taken as the temperature which is frequntly occurred as 88 hours only that means it is not a mandentry taken the maxium air temperature and in case of the installation of a roof-top or industrial Ac an additional value of 2.8°C should be added to the design air temperature

    to account for the solar radiation effect [2] and this is the answer of "Samarejiy son" question. The most standard ambinet temperature desgin as recommended by ARI is 35°C for cooling efficent performance [3]. I wish it is clear.



    [1] http://www.hudsonproducts.com/produc...fan/FfFaq.html

    [2] Wang : Hand book of Ac and Refi. 2nd edition, Chapter 10

    [3] Stweret susan: Enhanced condenser design and optimization, PhD thesis 2003



    Kind Regards

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    Re: Evaporating / condensing temperatures

    Quote Originally Posted by mohamed khamis View Post
    Let us say 7 bar, during low ambient temperature with a rather cooling load on the evaporator the DP can decrease below the minimum limit
    as u mentioned (4 bar) and in this case the TXV meters unsufficent a mount of liquid refigerant less than the required at this particular load
    You're giving yourself the explanation for this so called problem: if the load decreases on the evaporator, then the TXV also has to meter less refrigerant, so in your case, it's compensating itselves.

    You said in some posts above this one that you change the SH setting of the TXV when outside conditions change (spring and fall) and his to save energy
    This isn't quite clear for me. The TXV tries to maintain a stable SH and especially in your case where you use head pressure control.
    You better should lower the HP to save energy, this will be much better.
    It's better to keep your mouth shut and give the impression that you're stupid than to open it and remove all doubt.

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    Re: Evaporating / condensing temperatures

    Hi Mohamed,

    And i still insist that the condenser or the system should be designed with a samller CTD (like above 8-11 K) to save compressor energy consumption
    even there is a space stringent limitaions the last thoughout of decreasing condenser area is the design with higher CTD but there are many thoughout to achive that (more compact area) as follows:
    1)- Incraese the fan capcity (it takes place arround 10 - 15% of the total power consumption for the system and the remainder in the compressor).

    2)- Increase tubing number of deep-rows

    3)- Using an enhenced fins configuratons " corrugated, offest-strip, ..ect)
    In your first post you gave the opposite explanation. Now you insist that the condenser must be selected for small TDc. Small TDC gives high surface area which in turn gives low condensing pressure. This saves energy.
    This is exactly what I said from the beginning, now you confirm that.


    I know very well this equation, I used it more than 100 times in M.SC degree, by the way the temperature difference here is logrithmeic temperature differnce not aritmatic diffence as u wrote unless u will design the condenser without desuperhating and subccoling zones or the temperature profile is straight line.
    I also know that this relationship is not linear. For your information, my main job is design and manufacturing of air coolers, air cooled condensers, cold room evaporator... You see I use this equation all day.
    In order to simplify the explanation I assumed the "U" is constant, which is not, I assume the LMTD is TDC which is not....BUT the final result is this :
    For a given capacity, area of air cooled condenser is samll if design TDc is large. The area is big if TDc is small.


    No in a hot climate the design condensing temperature excreeds the design air temperature by 15 - 20 K as i mentioned before.
    Assume air temperature is 50°C then if you design the condenser for TDc = 20°C then your condensing temperature will be 70°C which is not accepted and will cause major problems and it is a waste of energy.


    Oh i got it, i think u have confusion of the design ambinet tmperature that it is taken as the peak or maxium air temperature arround the year
    ...

    In order to evaluate the air conditioning load, I repeat Air conditioning load, you take ambient temperature as an average, that is completely correct and this is why we evaluate the load all year around.
    BUT air cooled condensers MUST be designed for peak temperatures. Imagine a cold store with air cooled condensers. Assume that this area only three days a year get to 45°C ambient temperature.
    Can you say that because this is only three days a year I don't take the peak temperature? Let it do High pressure cut out?
    What we are talking about is not comfort air conditioning. Yes one might agree if this is an air conditioning package then let it not work for three days a year. BUT I say this is not acceptable also. Because we need that air conditioning for the worst days. If I suffer there days which are the worst in the year then whats the point of having the air conditioner?


    This is totally not right [1] the design air temperature is taken as the temperature whose annual accumulative frequency of occurrance is about 10% of the year hours (8760) [2]
    i.e. it is taken as the temperature which is frequntly occurred as 88 hours only that means it is not a mandentry taken the maxium air temperature and in case of the installation of a roof-top or industrial Ac an additional value of 2.8°C should be added to the design air temperature

    to account for the solar radiation effect [2] and this is the answer of "Samarejiy son" question. The most standard ambinet temperature desgin as recommended by ARI is 35°C for cooling efficent performance [3]. I wish it is clear.

    Again your explanation is true if we were evaluating air conditioning load. NOT air cooled condenser design.

    Cheers
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    Re: Evaporating / condensing temperatures

    Quote Originally Posted by Peter_1 View Post
    You're giving yourself the explanation for this so called problem: if the load decreases on the evaporator, then the TXV also has to meter less refrigerant, so in your case, it's compensating itselves.

    You said in some posts above this one that you change the SH setting of the TXV when outside conditions change (spring and fall) and his to save energy
    This isn't quite clear for me.
    I said that our collogues in clod climates change the settlings of TXV in winter “freezing conditions” in commercial supermarkets to ensure there is a proper pressure difference between the condenser and evaporator to save energy and to offset the significant drop in condensing temperature, this is if the TXV was selected with high DP.

    Quote Originally Posted by Peter_1 View Post
    The TXV tries to maintain a stable SH and especially in your case where you use head pressure control.
    Yes it is well known that the TXV tries to maintain a stable SH unless in the low ambient temperature and there is a rather high cooling load without provision of head pressure control the TXV will not able to meter the sufficient refrigerant.

    Also, I did not mention “head pressure control” and I all mentioned “HPG” and this is mistyping error and I meant by this to symbolize to “high pressure cut out” but unfortunately all the fellows understood it as high pressure control and I mentioned this symbol when I talked about the design of condenser in hot climate with small TDC it makes the system properly to trip off when the ambient temperature is increased significantly over the design one.



    Quote Originally Posted by Peter_1 View Post
    You better should lower the HP to save energy, this will be much better.
    Yes u come to the point of conflict I said that the system should be operated with a lower HP in COLD CLIMATE to save Energy, am I right or wrong? On the other hand on hot climate the system should be operated on high HP (TDC = 15 to 20 K) to ensure an efficient performance? Could u clarify this is point?

    Best Regards

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    Re: Evaporating / condensing temperatures

    Hi there,

    On the other hand on hot climate the system should be operated on high HP (TDC = 15 to 20 K) to ensure an efficient performance?
    If you design air cooled condenser for hot climate with TDc = 15 to 20K then you will get high pressure cut out.
    HOT Climate DB = 43°C
    TDc = 15 °C
    Then Tc = 58°C means HP = 22.2 bar(g) = 326 Psi(g)for R22 ------means high pressure cut out!!!!!

    I attach a file which is a page from Dossat book "Principle Of Refrigeration" 2nd edition pages 333 and 334.
    Hope this clarify this issue.

    Cheers
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    Re: Evaporating / condensing temperatures

    Quote Originally Posted by lana View Post
    Hi Mohamed,

    In your first post you gave the opposite explanation. Now you insist that the condenser must be selected for small TDc. Small TDC gives high surface area which in turn gives low condensing pressure. This saves energy.
    This is exactly what I said from the beginning, now you confirm that.
    Hi Lana

    Okay unfortunately as I see there is misunderstanding, I meant small TDC in COLD
    CLIMATE COUNTRIES


    Quote Originally Posted by lana View Post
    I also know that this relationship is not linear. For your information, my main job is design and manufacturing of air coolers, air cooled condensers, cold room evaporator... You see I use this equation all day.
    In order to simplify the explanation I assumed the "U" is constant, which is not, I assume the LMTD is TDC which is not....BUT the final result is this :
    For a given capacity, area of air cooled condenser is samll if design TDc is large. The area is big if TDc is small.
    I totally agree that


    Quote Originally Posted by lana View Post
    Assume air temperature is 50°C then if you design the condenser for TDc = 20°C then your condensing temperature will be 70°C which is not accepted and will cause major problems and it is a waste of energy.
    Unfortunately, I have to say it is very scarce unless not practical to use 50°C as a design ambient temperature and the standard temperature at which the AC systems are tested for cooling capacity is 35°C [1]. And as I mentioned last post that the design ambient temperature is taken which is occurred with a frequency of 10% in all year, (ARI, 1989). By the way, as u have experience in air-cooler design, if u check the Hudson product website [2] and [3] u will find that they recommend the design ambient temperature not be exceeded for about 2% for reference [2] and 5% for reference [3]. Why not accepted and there are many problems, R-22 is being used and the condensing temperature sometimes reaches this value and add to it 25 K to give the discharge temperature to be 95°C. Yes it wastes energy due to the selected design temperature is unreasonable not the condensing temperature.
    ...

    Quote Originally Posted by lana View Post
    In order to evaluate the air conditioning load, I repeat Air conditioning load, you take ambient temperature as an average, that is completely correct and this is why we evaluate the load all year around.
    BUT air cooled condensers MUST be designed for peak temperatures.




    Sorry I totally disagree this idea , the cooling load of air conditioning is evaluated on the design ambient temperature as I mentioned above and the required of knowing the cooling load or ambient temperature pattern to know the suitable controller settings to match the system cooling capacity with imposed cooling load because the system is design on the design condition “Peak load”.


    Quote Originally Posted by lana View Post
    Imagine a cold store with air cooled condensers. Assume that this area only three days a year get to 45°C ambient temperature.
    Quote Originally Posted by lana View Post
    Can you say that because this is only three days a year I don't take the peak temperature? Let it do High pressure cut out?
    What we are talking about is not comfort air conditioning. Yes one might agree if this is an air conditioning package then let it not work for three days a year. BUT I say this is not acceptable also. Because we need that air conditioning for the worst days. If I suffer there days which are the worst in the year then whats the point of having the air conditioner?
    Excuse me could u cite the reference for ur claim. I GAVE u more than two reference emphasize that from the condenser should be designed at 10% of all the year. Ok who said that the system will not operate at 45°C (IN THREE DAYS) yes it will work with low efficiently due to the significant increase in condensing pressure. If someone says the high pressure cut out will be activated at this moment, ok its screw can be adjusted to some extent and sometimes by laymen’s term the technician Pour water on the condenser to reduce the pressure. This is for cold room. However, in AC conditioning if the condensing unit seizes to work due to the high pressure cut out the users will understand that the system exerts its effort to work in very harsh scarce days but no luck with it and the user have to use only fan to adapt the problem. And this actually has been happened once day in my country 99 % of the air conditioning is tripped off due to very harsh day. Even if the condenser is designed with 45°c it may be occurred also the harshness lead to the system trip off. It might be known that the decreasing one 1°C in Condensing Pressure can lower the energy bill with 2% and if I say to these users ur system will save to 20% (as the design of 35°C instead of 45°C) of electricity bill provided it will not working or stopped in 3 days from 362 day, what is his feeling?

    Cheers

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    Re: Evaporating / condensing temperatures

    Hi Mohamed,

    Excuse me could u cite the reference for ur claim. I GAVE u more than two reference emphasize that from the Condenser should be designed at 10% of all the year
    .

    Reference ASHRAE handbook - HVAC Systems and Equipment 2004 - page 35-11. It states :

    " When selecting a condenser, determine the maximum air temperature entering the coil by referring to the weather data in Chapter 27 of the 2001 ASHRAE Handbook-Fundamentals. The 1% occurrence value is suggested for summer operation.
    The specific design dry-bulb temperature must be selected carefully, especially for refrigeration serving process cooling. An entering air temperature that is higher than expected quickly causes compressor discharge pressure and power to exceed design. This can cause an unexpected shutdown, usually at a time when it can least be tolerated. Also, congested or unusual locations may create entering air temperatures higher than general ambient conditions."

    Ok who said that the system will not operate at 45°C (IN THREE DAYS) yes it will work with low efficiently due to the significant increase in Condensing Pressure. If someone says the high pressure cut out will be activated at this moment, ok its screw can be adjusted to some extent and sometimes by laymen’s term the technician Pour water on the Condenser to reduce the pressure. This is for cold room. However, in AC conditioning if the Condensing Unit seizes to work due to the high pressure cut out the users will understand that the system exerts its effort to work in very harsh scarce days but no luck with it and the user have to use only fan to adapt the problem. And this actually has been happened once day in my country 99 % of the Air conditioning is tripped off due to very harsh day. Even if the Condenser is designed with 45°c it may be occurred also the harshness lead to the system trip off.
    OK you have to pay someone to put water on the coil so it does not trip on high pressure.
    If you want to do that then I have no argument.


    It might be known that the decreasing one 1°C in Condensing Pressure can lower the energy bill with 2% and if I say to these users ur system will save to 20% (as the design of 35°C instead of 45°C) of electricity bill provided it will not working or stopped in 3 days from 362 day, what is his feeling?
    Right. If from the beginning proper condenser was selected then high pressure problem would not occur and therefore, no electricity bill penalty wold be encountered. Even for three days a year.

    To summarized this discussion I say the followings ;

    In order to have an efficient refrigeration system (high pressure point of view), the condenser must be designed according to the proper ambient temperature. If ambient air temperature goes above the designed value then condensing pressure would rise and therefore, compressor power consumption would be higher.
    Air cooled condensers are designed for maximum air dry bulb temperature. DC for condenser gives the required surface area for design condensing temperature.
    If low DC is selected then for a given capacity the condenser surface will be higher. If high DC is selected then smaller surface will give the same capacity.

    Cheers
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    Re: Evaporating / condensing temperatures

    Quote Originally Posted by lana View Post
    Hi Mohamed,

    .

    Reference ASHRAE handbook - HVAC Systems and Equipment 2004 - page 35-11. It states :

    " When selecting a condenser, determine the maximum air temperature entering the coil by referring to the weather data in Chapter 27 of the 2001 ASHRAE Handbook-Fundamentals. The 1% occurrence value is suggested for summer operation.
    The specific design dry-bulb temperature must be selected carefully, especially for refrigeration serving process cooling. An entering air temperature that is higher than expected quickly causes compressor discharge pressure and power to exceed design. This can cause an unexpected shutdown, usually at a time when it can least be tolerated. Also, congested or unusual locations may create entering air temperatures higher than general ambient conditions."
    In principle, thank you so much for a wealthy discussion... this is last thing in this issue it takes more than the required...Could you tell me what does it mean by "The 1% occurrence value is suggested for summer "?...As my knowledge that i got from the reference i cited today that means u take the air temperature which is occurred 0.01 of the year...i will give example in PhD thesis "ENHANCED FINNED-TUBE CONDENSER DESIGN AND OPTIMIZATION", by " Susan White Stewart"2003. I
    which is attached below.
    And this is minimum occurrence I found comparing to the references I cited today of 3% to 5 % occurrence. ok anyhow it does not make difference.


    Quote Originally Posted by lana View Post
    OK you have to pay someone to put water on the coil so it does not trip on high pressure.
    If you want to do that then I have no argument.
    Ok I don not have to hire or pay to anybody because the technicians of maintenance are around the cite.


    Lastly thank u for ur input.

    Best Regards

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    Re: Evaporating / condensing temperatures

    Also I thank you for good discussion Mohamed.

    Cheers
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    Re: Evaporating / condensing temperatures

    Quote Originally Posted by lana View Post
    Also I thank you for good discussion Mohamed.

    Cheers
    Oppossss...... i forgot to attach the example


    Cheers
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    Re: Evaporating / condensing temperatures

    The values you reference for 1%, 3% & 5% were revised sometime ago by ASHRAE. The new occurence values are 0.4%, 1%, & 2% and account for the percentage of annual hours where that value is exceeded.

    So, this equates to:

    0.4% X 8760 hours = 35.4 hours
    1.0% X 8760 hours = 87.6 hours
    2.0% X 8760 hours = 175.2 hours

    In some locations the dry bulb temperature may not vary much, in other locations it will.

    I'm late to this discussion, but I'm sure it has been lively with lana participating.
    If all else fails, ask for help.


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    Re: Evaporating / condensing temperatures

    Thank you US Iceman for clarifying the percentages.

    Cheers
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    Re: Evaporating / condensing temperatures

    Quote Originally Posted by US Iceman View Post
    The values you reference for 1%, 3% & 5% were revised sometime ago by ASHRAE. The new occurence values are 0.4%, 1%, & 2% and account for the percentage of annual hours where that value is exceeded.

    So, this equates to:

    0.4% X 8760 hours = 35.4 hours
    1.0% X 8760 hours = 87.6 hours
    2.0% X 8760 hours = 175.2 hours

    In some locations the dry bulb temperature may not vary much, in other locations it will.

    I'm late to this discussion, but I'm sure it has been lively with lana participating.
    Ok thank you for this data, i have seen it on http://www.microcool.com/cooling/bro...PreCooling.pdf.
    However, i am so happy that i was a reason of correcting the common perception of the fellows for using maximum ambient temperature occurred in the year as the design temperature for the condenser without looking to something named the frequency occurrence. Regardless of these new values i do believe as u mentioned each location has its specifications.

    Cheers

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    Re: Evaporating / condensing temperatures

    Hi there,

    In Persian Gulf area we have minimum of three months extreme temperatures between 45 to 55°C.

    3 months x 30 days x 24 hrs = 2160 hrs

    This is way more than 1% occurrence.

    Cheers
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    Re: Evaporating / condensing temperatures

    My data shows that the southern most part of Iran (Ahwaz, 31.33N) has 0.4% exceeding value of DBT at 47.5C and for 1% it is 46.5C. What you have to see is not the percentage occurance but percentage occurance crossing the given condition.

    If you design your cooling system for 1% values of 46.5C DB and corresponding MCWB of 22.6C, you will have trouble for 87.6 hours in a year.

    Mohamed,

    There is nothing wrong in selecting the final heat rejection devices (for which atmosphere is the sink) for the maximum possible conditions. Generally, for cooling towers, the maximum WBT is considered as a design criterion. It is prudent to consider the maximum DBT for an air cooled heat exchanger, IMHO, if this is for a compressor.

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    Re: Evaporating / condensing temperatures

    Quote Originally Posted by Ravi View Post
    Mohamed,

    There is nothing wrong in selecting the final heat rejection devices (for which atmosphere is the sink) for the maximum possible conditions. Generally, for cooling towers, the maximum WBT is considered as a design criterion. It is prudent to consider the maximum DBT for an Air cooled heat exchanger, IMHO, if this is for a compressor.
    Hi Ravi

    Ok firstly I greatly respect your opinion


    .

    Quote Originally Posted by Ravi View Post
    If you design your cooling system for 1% values of 46.5C DB and corresponding MCWB of 22.6C, you will have trouble for 87.6 hours in a year.
    Ok if u design 1% values of 46.5C dbt, u mentioned that there is a trouble for 87.6 hours in year, I think the calculation should tell a trouble (if there is actually trouble) of 35 hours (almost tow days) in a year and it is not the issue.

    Quote Originally Posted by Ravi View Post
    Mohamed,

    There is nothing wrong in selecting the final heat rejection devices (for which atmosphere is the sink) for the maximum possible conditions. Generally, for cooling towers, the maximum WBT is considered as a design criterion. It is prudent to consider the maximum DBT for an Air cooled heat exchanger, IMHO, if this is for a compressor.
    Well ….if u select the 0.4% occurrence design (corresponding to 47.5°C) and me select 1% occurrence (corresponding to 46.5°C) and let us say both of us select TDC = 15 K, therefore ur head pressure of ur condenser (give it a symbol of A), using R-134a, is 17.8 bar corresponding to condensing temperature of 62.5°C and head pressure of my condenser (give it a symbol of B) is of 17.4 bar corresponding to condensing temperature of 61.5°C. Now It is allowed for the two condensers to be incorporated in a specified system individually and let us imagine what will be happened for the system with condenser A and B. The fearing which will come up that for condenser B when it is subjected to ambient temperature of 47.5°C, simply the condenser pressure raises up to face this problem (not to the limit of high pressure cut out which is almost 10% to 15% of operating condensing pressure) and at this moment the system characteristic will inevitably impaired and the COP will drop by 2.79% (I take example using COOLPACK program) and you will save energy bill like follows

    Energy bill saving = (1/COPB-1/COPA)/COPB

    Therefore, ur condenser A will save around = 2.73% of the energy bill of B that I have to pay and u win this game for 2 days.


    Let jump this is harsh days (35 hours, 2 days) to the other days the year and see what is happened to the energy bill for the two system A and B. In fact the system B will be efficient more than A (without prejudice) along with the drop in ambient temperature because each 3 K as drop in condensing temperature is corresponding to 1 bar in the condensing pressure for condenser design (check it by CATT program). To ease the finding Let us assume there is a drop of 3 K in the ambient temperature followed by decreasing in condensing temperature in the rest of 363 day, therefore condenser A pressure is 16.8 bar and B is 16.5 bar, the enhancement of COP will be enhanced by 4.33% for Condenser B credit (check by COOLPACK). Let us say the energy bill saving

    Energy bill saving = (1/COPA-1/COPB)/COPA = 4.00% of the energy bill of A that you have to pay and I win this game for 363 days. Sorry for long post but I’d like to approach to u my idea. Anyhow as we summarized in the previous posts u can use the 0.4% occurrence if the temperature is this zone is much higher than the temperature of zone of 1% occurrence and this is only my point of view, u can reject or accept.

    BEST REGARDS

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    Re: Evaporating / condensing temperatures

    Dear Mehamed,

    We are all saying the same thing here, and I think there is a misunderstanding.

    I try to clarify things.

    What I say for design ambient temperature is to use the highest possible and therefore I would design a bigger condenser. This big condenser will give me lower condensing pressure at the worst ambient conditions which will save money, as you described.
    This is the same you are saying.

    If the whole idea is to save energy, then this can be done by selecting a condenser which gives us the lowest possible condensing pressure at the highest possible ambient temperature.

    I repeat we are saying the same thing.

    If smaller condenser is selected then for 3 days in a year this will cause the condensing pressure to rise and therefore, more energy will be wasted.

    Cheers
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    Re: Evaporating / condensing temperatures

    Quote Originally Posted by lana View Post
    Dear Mehamed,

    We are all saying the same thing here, and I think there is a misunderstanding.

    I try to clarify things.

    What I say for design ambient temperature is to use the highest possible and therefore I would design a bigger condenser. This big condenser will give me lower condensing pressure at the worst ambient conditions which will save money, as you described.
    This is the same you are saying.

    If the whole idea is to save energy, then this can be done by selecting a condenser which gives us the lowest possible condensing pressure at the highest possible ambient temperature.

    I repeat we are saying the same thing.

    If smaller condenser is selected then for 3 days in a year this will cause the condensing pressure to rise and therefore, more energy will be wasted.

    Cheers
    Hi Lana

    Yes it is correct, both of us are saying the same thing. Because as i deduce from previous posts u'd like to select the maximum temperature (corresponding to 0.4% frequency) and on the same time in this case u'd like small TDC. In contrary, me prefer to take not the maximum temperature (2% or 1% frequency) and i'd prefer large TDC in this case. Both of our sayings are almost the same. However, if the difference between the temperature in case of 0.4% and 1% or 2% is quite slight i think it makes sense to use the 1% or 2% typically as Ravi mentioned to me (46.5°C at 1% and 47.5°C at 0.4% ) the difference here 1K. The discrepancy between us in clod climate, u'd prefer take maximum temperature and large TDC (am right or wrong) and i agree i will take aslo maximum temperature (with the previous condition in the frequency factor) but i suggest small TDC (10 to 12 K) because the climate has a drastic variation in ambient temperature, ok this is my point of view. And i ask our colleagues who survive in these zones judge in this matter because they definitely are aware of that.

    Cheers

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    Re: Evaporating / condensing temperatures

    Quote Originally Posted by lana View Post
    Dear Mehamed,

    If smaller condenser is selected then for 3 days in a year this will cause the condensing pressure to rise and therefore, more energy will be wasted.

    Cheers
    Ok i forgot to tell u i clarified that in the example i gave to Ravi when i told him my design tends to consume a larger energy and of course more expensive energy bill in the three days but on the other days the design will be favorable in terms of energy consumption and bill. Kindly refer to this post and read it carefully.

    Cheers

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    Re: Evaporating / condensing temperatures

    Hi Mohamed,

    Again we are saying the same thing.

    Ambient = hot --TDc = small -- Cond. surface = big -- Cond. pressure = good -- Energy consumption = good

    Ambient = cold -- TDc = large -- Cond. surface = small -- Cond. pressure = good -- Energy consumption = good

    If your concern is energy consumption then you have to design a condenser which gives lower condensing pressure.

    Cheers
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    Re: Evaporating / condensing temperatures

    Quote Originally Posted by lana View Post
    Hi Mohamed,

    Again we are saying the same thing.

    Ambient = hot --TDc = small -- Cond. surface = big -- Cond. pressure = good -- Energy consumption = good

    Ambient = cold -- TDc = large -- Cond. surface = small -- Cond. pressure = good -- Energy consumption = good

    If your concern is energy consumption then you have to design a condenser which gives lower condensing pressure.

    Cheers
    Hi Lana

    Thanks Lana for this good demonstration

    Cheers

  41. #41
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    Re: Evaporating / condensing temperatures

    It's my honour to read your post,and I think maybe more guys should join in and write down their idea.

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    Re: Evaporating / condensing temperatures

    I red all the arguments among Lana & Mohamed Khamis. This was a very very interesting thread. Finally Mohamed agreed on your views Lana. Hats off to you Lana for your valuable presentation & detail explanation. I do not know what happened to the contractor who initiated this thread.

    Can anybody let me know, how many members viewed this thread. Is there any particular place to find out about this information?

    Suny

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    Re: Evaporating / condensing temperatures

    Suny, you have to go to the forum menu (a little bit higher in the root menu) and there you can see that more then 900 already have viewed this thread. http://www.refrigeration-engineer.co...splay.php?f=38
    It's better to keep your mouth shut and give the impression that you're stupid than to open it and remove all doubt.

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    Re: Evaporating / condensing temperatures

    It are all these great members who wants to share their experience that makes this forum so great, so useful.
    If you should pour all the knowledge in a big container, well, then I'm sure we should have the biggest refrigeration 'knowledge-container' in the whole world.
    It's better to keep your mouth shut and give the impression that you're stupid than to open it and remove all doubt.

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    Re: Evaporating / condensing temperatures

    As I have said many a times on the forum as well as outside the forum, that this is the best place to learn and educate your ownself. We have members who are not only learned but are a big help to everybody.

    I wish more refrigeration people joins this forum, so that we have more knowledge .

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    Re: Evaporating / condensing temperatures

    Thanks Peter_1. I am aware of the column. Every time a member or a viewer views the same thread more than once the counter will keep on adding. Thereby you will not know the actual count of the viewers who benefited from the thread. As such I propose you to consider having a new column which will give actual viewers count.

    Best regards
    Suny

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    Re: Evaporating / condensing temperatures

    Thanks to all.

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