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Sledge
19-06-2007, 05:56 AM
This ties into my other post, but thought it better as separate thread.

If I make my coil colder do I remove more humidity? The application is a single pass, fresh air makeup unit, supplying apartment building common areas.

MY thoughts on this are;
that coil temperature does not affect amount of humdity removed, as long as the coil temperature is below dew point. It doesnt matter how much below dew point. (The only stipulation about this would be that the coil maintains its temperature despite absorbing the heat necessary to condense).

I am thinking that the only situation where a colder coil will remove more humidity is where the air is recirculated across the coil. Each time the air passes over the coil, it becomes colder and drier. In order to continually remove humidity the coil will need to become progressively colder.

Am I correct in this way of thinking?

lana
19-06-2007, 06:50 AM
Dear Sledge,

As you know there are three humidity parameters for air.

1- Moisture content (kg/kg)
2- Relative humidity (%)
3- Percentage saturation (%)

Do you want to reduce moisture content?

Cooling with de-humidification (coil surface temperature below air dew point) increases relative humidity and percentage saturation, but decreases moisture content.

On the other hand lower evaporating temperature decreases moisture content more.

Because it is an air conditioning application you have to supply air which will maintain comfort situation.

There is a way for more moisture removal by using a cooling coil with two distributors after each other. It is called row split (see the attached picture). The best results is when you have two compressors supplying two distributors. Obviously the evaporating temperatures will be different because air passing through the second distributor is colder than air inlet to the first half of the coil.

This needs careful coil and system design.

Hope this helps.
Cheers

Sledge
19-06-2007, 11:05 AM
On the other hand lower evaporating temperature decreases moisture content more.



Thanks Iana...but I still am confused.

From the diagram that you have provided, I am understanding that the reason this coil will remove more moisture, is that the second pass is colder than the first...

Does this mean that a colder surface will condense more humidity, or,

Does this mean that the air passes through the first coil, becomes colder and drier, with a lower dew point, which now needs a colder surface, if we are to remove anymore moisture? The second pass provides that surface.

Can I acheive this without adding a second pass, or does it require a second surface, or
is the issue related to the time that the air is contacting the coil...slower moving air will condense out more humidity than fast moving air?

I guess the basic question that I am asking is; If I run the evaporator in my project, colder, will I remove a greater amount of moisture.

The system I am dealing with has no return air, it is a makeup air, so I need to cool/dehumidify to a comfortable level in a single pass. Am I going to have to consider reheat?

lana
19-06-2007, 11:45 AM
Hi Sledge,


From the diagram that you have provided, I am understanding that the reason this coil will remove more moisture, is that the second pass is colder than the first...

If you connect two compressors to two distributors, i.e. you have two separate cycles then you can design for two different evaporating temperatures.
If two distributors are supplied with one compressor then you get nearly same evaporating temperatures (TEVs the same) but in reality because the second half of the coil gets colder air then the evaporating temperature will be little bit lower than the first half.
In this case also more moisture will be removed.




Does this mean that a colder surface will condense more humidity, or,


YES.



Does this mean that the air passes through the first coil, becomes colder and drier, with a lower dew point, which now needs a colder surface, if we are to remove anymore moisture? The second pass provides that surface.


Again yes.



Can I acheive this without adding a second pass, or does it require a second surface, or


This depends on coil design and required air outlet conditions. Sometimes with one coil arrangement with one distributor it is possible.


is the issue related to the time that the air is contacting the coil...slower moving air will condense out more humidity than fast moving air?

Yes.


I guess the basic question that I am asking is; If I run the evaporator in my project, colder, will I remove a greater amount of moisture.

Yes, but there is limit. If the evaporating temperature is reduced in such a manner that the outlet air goes into the "Fog region" on the psychrometric chart then you have a problem and your AHU will blow out fog :confused:.
The best thing is to draw the cooling process on a psych. chart and see what the limits are. Draw the outside air condition on the chart as a point. Then choose a reasonable evaporating temperature. Add about 3°C to it and this will nearly be your coil surface temperature. Find this point on the saturation line. Connect the inlet air point to this coil surface temp. The air outlet conditions will be on this line. Where your outlet condition is, depends on the coil design and construction. Now you can see where you are and what to change to get the desired conditions.

For comfort air conditioning I don't think you need any complicated design.
I have seen this kind of design on precision air conditioning which controls humidity and temperature.



The system I am dealing with has no return air, it is a makeup air, so I need to cool/dehumidify to a comfortable level in a single pass. Am I going to have to consider reheat?


Reheat is used to increase the relative humidity. If your inlet air condition is very near the saturation line then if cooled it will go in the "Fog region", then reheat is needed. Other wise there is no need.

I suggest to give your design values here so I (or someone else) can be more specific and helpful.

Data needed :
1- Air inlet dry bulb
2- Air inlet wet bulb
3- Total air volume flow through the coil
4- Air outlet dry bulb required
5- Air outlet wet bulb required
6- Limits for Coil dimensions (L X H)

Cheers

mohamed khamis
19-06-2007, 11:48 AM
This ties into my other post, but thought it better as separate thread.

If I make my coil colder do I remove more humidity? The application is a single pass, fresh air makeup unit, supplying apartment building common areas.

MY thoughts on this are;
that coil temperature does not affect amount of humdity removed, as long as the coil temperature is below dew point. It doesnt matter how much below dew point. (The only stipulation about this would be that the coil maintains its temperature despite absorbing the heat necessary to condense).

Hi Sledge

The Capability of the coil to remove humidity is two coherent parameters:

1)- The coil surface temperature as u mentioned must be below the entering air dew point temperature. The lower surface temperature (apparatus dew point), the more possibility to remove a large amount of humidity and thus the lower evaporating temperature and system compressor power.

2)- The coil cooling capacity. The much cooling capacity the much removal condensate provided the coil surface temperature is below the dew point temperature. Sometimes u find out in spite of the coil temperature is below the dew point but the mount of removal condensate is small and this can be seen when the unit is undercharging or refrigerant leakage in hot humid country like Malaysia. Suppose like here dbt = 30°C and Rh= 75% this leads to 25°C dew point and normally the surface temperature in AC applications must below this value by a significant magnitude to provide at least sensible cooling. During undercharging case the coil has not the ability to condensate the required in spite of its temperature is under the required.

To sump up, this equation illustrates what I mean:

condensate flow rate = (1-SHF)*cooling capacity /{(entering air moisture content–outlet air moisture content)*2501 “Latent heat of removal water in the air’}

Note that the difference in moisture content in entering and outlet air is proportional to in the difference between the partial vapor pressure of entering air at its dry bulb temperature and the partial vapor pressure of outlet air at coil surface temperature (assuming perfect coil effectiveness ..zero bypass factor). Therefore, as it is shown in the previous equation if the surface temperature is low and on the same time the cooling capacity is also very low this leads to low condensate rate although the surface temperature is low (under the dew point). This equation will answer ur thought of recalculating the discharge air many times over the coil to get more removal condensate.


I am thinking that the only situation where a colder coil will remove more humidity is where the air is recirculated across the coil. Each time the air passes over the coil, it becomes colder and drier. In order to continually remove humidity the coil will need to become progressively colder.

Am I correct in this way of thinking?

You should note that the recirculation of the cold air yes it will make the coil temperature to drop responding to the drop in evaporating temperature and of course cooling capacity. That means the equation nominator is definitely decreased along with the recirculation times. On the other hand, we can not judge if the dominator will be decreased or increased. However, in practice the longer recirculation of the return air it becomes dry and has very low moisture content and on the same time the surface temperature continues to drop (not lower than corresponding cut out LP value) therefore the driving force is accounted for the dominator to be increased as a result of the drop in coil surface temperature and weak moisture content for entering air and thus the moisture removal is decreased alongside the recirculation times. Not only that if the preset LP cut out is small the surface can be covered up by frost as a result the latent heat availability of the coil does not find moisture content to remove therefore it turn its face to freeze the clogged droplets on the coil and coil is frosting, that is preferred in Japanese cars AC system. They take the advantage of frosting formulation on the coil when it covers certain value of the coil face area I don not remember exactly what it is, the system is shut off and air is recirculated to melt this ice. Sorry for long post and I wish it could help.

Cheers:)

mohamed khamis
19-06-2007, 12:22 PM
Hi Sledge,



If you connect two compressors to two distributors, i.e. you have two separate cycles then you can design for two different evaporating temperatures.
If two distributors are supplied with one compressor then you get nearly same evaporating temperatures (TEVs the same) but in reality because the second half of the coil gets colder air then the evaporating temperature will be little bit lower than the first half.
In this case also more moisture will be removed.




YES.



Again yes.



This depends on coil design and required air outlet conditions. Sometimes with one coil arrangement with one distributor it is possible.



Yes.



Yes, but there is limit. If the evaporating temperature is reduced in such a manner that the outlet air goes into the "Fog region" on the psychrometric chart then you have a problem and your AHU will blow out fog :confused:.
The best thing is to draw the cooling process on a psych. chart and see what the limits are. Draw the outside air condition on the chart as a point. Then choose a reasonable evaporating temperature. Add about 3°C to it and this will nearly be your coil surface temperature. Find this point on the saturation line. Connect the inlet air point to this coil surface temp. The air outlet conditions will be on this line. Where your outlet condition is, depends on the coil design and construction. Now you can see where you are and what to change to get the desired conditions.

For comfort air conditioning I don't think you need any complicated design.
I have seen this kind of design on precision air conditioning which controls humidity and temperature.



Reheat is used to increase the relative humidity. If your inlet air condition is very near the saturation line then if cooled it will go in the "Fog region", then reheat is needed. Other wise there is no need.

I suggest to give your design values here so I (or someone else) can be more specific and helpful.

Data needed :
1- Air inlet dry bulb
2- Air inlet wet bulb
3- Total air volume flow through the coil
4- Air outlet dry bulb required
5- Air outlet wet bulb required
6- Limits for Coil dimensions (L X H)

Cheers
Hi Lana

Welcome back…..the moisture removal does not relays only of the evaporating temperature but also the entering air moisture content. In ur schematized system which is so-called row-by-row control cooling coil or sometimes multiple-circuit AC system, What u will gain by right hand will be lost by left hand. The reason of that is yes the evaporating temperature is dropped as a result of entering cold air from the first circuit but the moisture content of this air is also decreased this leads to wane the driving force from partial pressure of entering air to partial pressure of saturated adjacent to cooling coil surface. The main purpose of ur system is used in the capacity control when the cooling load is dropped to let us say 50% one unit or circuit is off if the two units are identical. Ok if u draw as u mentioned by precise calculations for one big system at the same cooling capacity and evaporating temperature and for two identical two units with the same evaporating temperature the findings are almost the same. By the way, what about the clogged condensate droplets in the first stage when this units is off and the second is on, I guess that the relaxing droplets will evaporates (adiabatic humidification) and increases the entering air moisture content and the second stage coil has to pay the price of other faults.

I Suggest the proper solution if Sledge need more humidly capacity is decreasing the evaporating temperature to certain value and reheated the supply temperature again to the value of the design supply temperature (to avoid overcooling design). Reheater is needed for high latent load applications, to remove more moisture content of expense of decreasing the evaporating temperature then reheat the off coil air temperature (of course enomorous consumed power) . I wish it could help

Cheers

lana
19-06-2007, 12:49 PM
Hi Mohamed,


the moisture removal does not relays only of the evaporating temperature but also the entering air moisture content.

Of course.



In ur schematized system which is so-called row-by-row control cooling coil or sometimes multiple-circuit AC system, What u will gain by right hand will be lost by left hand. The reason of that is yes the evaporating temperature is dropped as a result of entering cold air from the first circuit but the moisture content of this air is also decreased this leads to wane the driving force from partial pressure of entering air to partial pressure of saturated
adjacent to cooling coil surface.


I don't understand what you mean.



The main purpose of ur system is used in the capacity control when the cooling load is dropped to let us say 50% one unit or circuit is off if the two units are identical. Ok if u draw as u mentioned by precise calculations for one big system at the same cooling capacity and evaporating temperature and for two identical two units with the same evaporating temperature the findings are almost the same.

Yes this coil design also can be used in capacity control systems where one system can be switched off.




By the way, what about the clogged condensate droplets in the first stage when this units is off and the second is on, I guess that the relaxing droplets will evaporates (adiabatic humidification) and increases the entering air moisture content and the second stage coil has to pay the price of other faults.


Yes, if one compressor is off then this problem can occur. Every system has its advantages and disadvantages. Personally I don't like this coil design. I avoid this design as much as possible.



I Suggest the proper solution if Sledge need more humidly capacity is decreasing the evaporating temperature to certain value and reheated the supply temperature again to the value of the design supply temperature (to avoid overcooling design). Reheater is needed for high latent load applications, to remove more moisture content of expense of decreasing the evaporating temperature then reheat the off coil air temperature (of course enomorous consumed power)


This is why I requested the design data to check if reheat is needed. As you said this is waste of energy and if possible this must be avoided.

Cheers

mohamed khamis
19-06-2007, 01:00 PM
Hi Mohamed,

I don't understand what you mean.Cheers


Kindly refer to my above post to reply Sledge and i wish it was written clearly to be readable and u understand what i mean. Thanx

Cheers

lana
19-06-2007, 01:15 PM
Hi Mohamed,

Thanks for clarifying.



....this leads to wane the driving force from partial pressure of entering Air (http://www.refrigeration-engineer.com/forums/glossary.php?do=viewglossary&term=17) to partial pressure of saturated
adjacent to cooling coil surface. :confused::confused::confused:


Cheers

mohamed khamis
19-06-2007, 01:41 PM
Hi Mohamed,

Thanks for clarifying.



Cheers

Ok the mass transfer phenomenon is typically such as the heat transfer depends on something named driving force named the temperature difference in case of the heat transfer and pressure difference for the mass transfer case. As we are taking about the humidity removal i.e. mass transfer the mass transfer as a result of the vapor pressure in the entering air is large than the vapor pressure on the saturated air layer which is so-called Interfacial adjacent layer to the coil surface. Note that this interfacial layer has almost the temperature of the coil surface (small boundary layer). Anyhow the decrease in surface temperature stimulates the vapor pressure of this saturated air to decrease. Thus the mass transfer will be increased in this case (lowering coil surf. or Tev) provided the entering moisture content is maintained at a fixed level. As u explained in ur system the off coil temperature from the first stage will be cold and thus the evaporating temperature in the next stage will also drop responding to the colder air and thus the removing moisture will be increased. Unfortunately, this will be true if the moisture content of the cold air stays constant but it also decreases with the decreasing in temperature. Thus the gain of lowering evaporating temperature (i.e. lowering vapor pressure in saturated air) is negated by reducing the vapor pressure in the incoming cold air. Thus if u draw pschromteric chart line for the two processes

One for the air with specified conditions over big coil without any splitting circuits and the second for the same air enters cooling coil with two circuits as u proposed the outlet conditions is almost the same for the two process. Hopefully it is clear

Cheers

lana
19-06-2007, 03:07 PM
Hi Mohamed,
Thanks a lot for your good explanation.

Cheers

mohamed khamis
20-06-2007, 03:56 AM
Hi Mohamed,
Thanks a lot for your good explanation.

Cheers

Hi Lana

You welcome at anytime and thanx for ur good participations in the forums

Kind Regards

Abby Normal
20-06-2007, 04:18 AM
I like Willis Carrier's approach on how a cooling coil works.

Air either makes contact with a tube or fin, or the air sails through without making contact.

If the 'average coil surface temperature' is at or below the dewpoint of the entering air, then moisture will condense out of the air that contacts the coil and this air will leave the coil saturated at that average coil surface temperature.

This average coil surface temperature is also known as the apparatus dew point.

The supply air condition leaving the coil is a mixture of the air which contacts the coil with that which does not contact the coil.

So keeping all things equal except the coil temperature being lowered, more moisture will condense out of the air with a colder coil, as the air making contact gives up more moisture.

Sledge
20-06-2007, 05:34 AM
Wow, Thanks guys!

Great explanations!

Abby Normal
20-06-2007, 07:37 PM
Besides running the coil colder, reducing the fraction of the air that bypasses the coil will increase moisture removal.

Use more fins per inch, have the coil be 'deeper' as in having more rows. 6 to 8 rows instead of 3 or 4.

US Iceman
20-06-2007, 07:52 PM
I seem to remember some simple forumla where you can use the air and coil temperatures to calculate an apparent bypass factor. Does anyone know what that might be?

It may be useful in troubleshooting...

Abby Normal
20-06-2007, 08:27 PM
Never use it myself Iceman, but will be in any reference book inspired by Carrier

mohamed khamis
21-06-2007, 03:56 AM
I seem to remember some simple forumla where you can use the air and coil temperatures to calculate an apparent bypass factor. Does anyone know what that might be?

It may be useful in troubleshooting...

Hi iceman

The bypass factor is the magnitude of coil deficiency, i attached to you file describe its definitions and foumla of its calculations.


Best regards:)

mohamed khamis
21-06-2007, 04:09 AM
Wow, Thanks guys!

Great explanations!

You welcome Sledge at any time and thanx for your courtesy

Best regards:)

US Iceman
21-06-2007, 04:24 AM
Thank you.;)

I believe this is what I remembered seeing somewhere before.

Best regards,
US Iceman

lana
21-06-2007, 05:05 AM
Hi there,

To add to Mohamed's description please see the attached file.

There is another definition called "Contact Factor" which is used instead of By-pass factor (I think in Britain :cool:).

The reference is "Air Conditioning Engineering" By Peter Jones, Fourth Edition page 272. (He was our professor at the UCL;))

Hope this helps.
Cheers

Abby Normal
21-06-2007, 05:12 AM
If you extend the cooling process line until it cuts the saturation curve at the ADP, you can compare the line segements to work out the bypass factor.

the length of line between the leaving coil condition and the ADP will be proportional to how much air bypasses.

Your leaving condition is a mixed air condition, between air at the ADP and air at the entering condition

mohamed khamis
21-06-2007, 05:57 AM
Hi there,

To add to Mohamed's description please see the attached file.

There is another definition called "Contact Factor" which is used instead of By-pass factor (I think in Britain :cool:).

The reference is "Air Conditioning Engineering" By Peter Jones, Fourth Edition page 272. (He was our professor at the UCL;))

Hope this helps.
Cheers

Hi Lana

Thank you. Could u please review the contact factor equation because I have a feeling of that this equation belongs to the coil effectiveness itself not contact factor. Just i have a doubt because this is the first time for me to hear about the contact factor and i don not have any reference for it. However, I deduced from the exponential formula that it is the break down of "number of transfer unit" {EXP (-NTU)} for heat exchanger with phase change material. That means this equation is valid for condenser and evaporator in HVAC systems only, Am I right or wrong. Once again, I appreciate ur input.

Best regards:)

Sledge
21-06-2007, 06:05 AM
I dont want to hi-jack my own thread but...

can anyone tell me where I can find an education program, either on-line or correspondance, that will cover this level of design and engineering for refrigeration. My Canadian college education simply doesnt reach these levels.

mohamed khamis
21-06-2007, 08:51 AM
There is another definition called "Contact Factor" which is used instead of By-pass factor (I think in Britain :cool:).

This sentence which got me confused. I think the contact factor is merely another terminology for the coil effectiveness and the bypass factor is (1 - coil effectiveness or contact factor). An i think our British fellows call the effectiveness as contact factor that is only the opposite meaning of bypass factor. Let us take example to discern these terminologies, normally NTU = 1 to 5, assuming a realistic heat exchanger NTU of 1.5, this accounts for effectiveness or contact factor (by using the equation cited by Lana) of 78% and bypass factor of 23%. normally the heat exchanger efefctivenss or now contact factor is normally in the range of 0.65 to 0.85 and in turn the bypass factor is now of 0.15 to 0.35. I think it is now clear

Cheers:)

mohamed khamis
21-06-2007, 08:57 AM
Thank you.;)

I believe this is what I remembered seeing somewhere before.

Best regards,
US Iceman

You welcome US Iceman at anytime

Best regards

lana
21-06-2007, 10:14 AM
Hi Mohamed,

Your deduction is correct.

I quote from the same reference :



It is unusual to speak of the efficiency of a cooler coil. Instead, the alternative terms, Contact Factor and By-pass factor are used. They are complementary values and contact factor, sometimes denoted by B (Beta) , is defined as :

B = (hi-ho)/(hi-hs)

Similarly, by-pass factor is defined as :

(1-B)= (ho-hs)/(hi-hs)

Where "i" denotes air inlet, "o" air outlet. "s" saturation at ADP.


I am used to contact factor. In reality they are the same thing with different approach.
Hope this helps.
Cheers

Abby Normal
21-06-2007, 04:49 PM
I dont want to hi-jack my own thread but...

can anyone tell me where I can find an education program, either on-line or correspondance, that will cover this level of design and engineering for refrigeration. My Canadian college education simply doesnt reach these levels.
The bypass factor is all Willis Carrier. The man was quite an innovator, too bad his company sucks now :)

Carrier corp offers training materials, maybe buy their courses on psychrometrics.

The entire air conditioning industry revolves around the pychrometric chart.

Trane sells a laminated chart on 11x17. On the back of the chart, they have some good examples of the basics.

ASHRAE was offering a course on psychromtrics also

Abby Normal
21-06-2007, 04:56 PM
This sentence which got me confused. I think the contact factor is merely another terminology for the coil effectiveness and the bypass factor is (1 - coil effectiveness or contact factor). An i think our British fellows call the effectiveness as contact factor that is only the opposite meaning of bypass factor. Let us take example to discern these terminologies, normally NTU = 1 to 5, assuming a realistic heat exchanger NTU of 1.5, this accounts for effectiveness or contact factor (by using the equation cited by Lana) of 78% and bypass factor of 23%. normally the heat exchanger efefctivenss or now contact factor is normally in the range of 0.65 to 0.85 and in turn the bypass factor is now of 0.15 to 0.35. I think it is now clear

Cheers:)
Always curious on how NTU, or LMTD works with the fact that moisture is condensing out with cooling. I can see it applying in an all sensible process, but something relying just on temperature be a little off in latent processes would it not?

mohamed khamis
22-06-2007, 03:10 AM
Always curious on how NTU, or LMTD works with the fact that moisture is condensing out with cooling. I can see it applying in an all sensible process, but something relying just on temperature be a little off in latent processes would it not?


Always curious on how NTU, or LMTD works with the fact that moisture is condensing out with cooling. I can see it applying in an all sensible process, but something relying just on temperature be a little off in latent processes would it not?


Yes, NTU and LMTD methods works effectively in heat exchangers those surface are bared from the moisture as in sensible cooling process. When there is condensation of moisture over the coil this produces a simultaneous occurrence of heat transfer and mass transfer as in cooling coil “DX evaporator or chilled water coil”. The reason of that the derivation NTU and LMTD methods are established on a basis of the temperature difference acting as the driving force only. However, there is an approximate solution to adapt these methods to be used in case of moisture condensation which is u deal with the coil typically as the dry coil except the heat transfer coefficient for the air-side will be modified by this :

ha = SHF/(ho* Eittaf), SHF = sensible heat factor “ sometimes taken as of 0.6 to 0.8” and some references took it as unity, ho = dry air heat transfer coefficient , and Eittaf = fin surface efficiency and it is normally in the range of 0.7 to 0.85

Therefore, u can deal easily with these methods by new modifications but if u need precise calculation in which numerical technique is used and the temperature difference in air-side is replaced by enthalpy difference named a method of Threlkeld. I wish it could help

Regards:)

Abby Normal
22-06-2007, 05:16 AM
some references took it as unity,

lol took as unity as in a sensible cooling process


The few cooling coil programs I have been through are either based on empirical data, a lot of them copied from "Bohn", or they are based on finite elements.

mohamed khamis
22-06-2007, 05:40 AM
lol took as unity as in a sensible cooling process


U can find that in "Wang, S - Handbook of Air Conditioning and Refrigeration 2nd Ed [McGraw Hill]", it take as unity as aforementioned. However, there is extra information in this link, it may help u, http://www.coolit.co.za/coilsim/coildesign.htm. By the way, there is a mistake in the definition i wrote in the previous post the 'ho" is the wet air heat transfer coefficient not the dry one and it is often 4 or 5 times greater than the dry one as in the fist above reference.

Cheers