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shsalameh
17-05-2005, 10:39 AM
Hi everybody;

I still awaiting for your help about my questions
How can I determine if I have right saturated evaporator and condenser temperatore in relation to evaporator leaving water temperature for evaporatore and in relation to ambient air temperature in air cooled condenser, or to water temperature in water cooled condensers
in other words what is the right evaporator and condenser approach fo different type of condensers and evaporators
and how this change if ***** is R22 or R134
Also what is the right discharge temperature

I hope some body help me

Sharif

Peter_1
17-05-2005, 08:07 PM
DT evaporator at least 5K (look in a pressure chart of teh used gas for the relation temperature/pressure), whatever gas you may use and DT for the condensor (air or water cooled, doesn't matter) the lower the better but I should say,...10 K is a good figure to start with.
Discharge temperature depends on your superheat, high pressure and can be plotted in a log/P diagram or calculated via special software ...if you know how to do this of course.

Sorry I have to ask but have you ever studied the real basics of refrigeration an more specific the pressure/enthalpy charts?

All the answers can be found in these P/E charts and you realy have to understand this before. I gave you the figures above but have you become wiser now? You have to know what will happen if superheat rises to 10 K or drops to 3 K.

shsalameh
18-05-2005, 09:20 PM
Thanks very much Peter for your reply
First I tell you that I know pressure - entalpy chart very well, and can plot any circuit and study it, I know that when suction superheat increase then same discharge superheat, can read that on p-i chart for any refrigerant
my friend, my question is actually about the perfrmance of the heat exchanger even if it is evap or cond, cause it is not the same when condenser for ex. is water cooled or air cooled, I mean the cond approach will not be the same, also for different *****s.
concerning disch supht or disch tmp I would like to know it related to cond sat. temp cause it change by suction supht and depend also on the sat temp of condenser
I had studied all the theory of refrigerant, but now and cause I start to work with operating and maintenance on site I am a little bit confused about these things
So when I start air cooled r22 recip chiller on site and I get water temp leaving evap for ex. 6 C is the sat evap temp of 0 C right value, this is a heat exchanger perfrmance matter, no ?
and depending on type of evaporator, also its not the same for all refrigerants.

Thanking your interest, and awaiting

Sharif

Gonzalo Arias
09-08-2005, 08:35 PM
Air condensers (Saturate condensing temperature – Outside air temp) 20 to 30F.

Water condensing (Saturate condensing temperature – Water in temp) 13 to 26F for ammonia y 20 F for halocarbons, also 10 to 20 F water rice for both

Water chillers No defined because they are intended to accomplish a duty of lowering the inlet water temperature to desired water out temperature.

Air evaporator The TD (room temperature – saturate evaporator temp) depends in the desired room humidity that depends on the storage product but roughly in refrigeration is 10 to 15, the higher the drier.

Best regards
Gonzalo

US Iceman
09-08-2005, 10:32 PM
Sharif,

There is a relationship between the actual data recorded for the system operation, the heat exchanger performance, and the compressor capacity.

I will try to explain some of these and see if this helps you. We also have to determine what type of heat exchanger is being used. Different types of heat exchangers use different selection processes.

First, we have to assume that someone designed the system correctly. The water flow rates, temperature differences, and the chiller, or condenser capacity have been correctly specified.

Second, is the condenser a shell & tube heat exchanger, or is it a finned-tube coil condenser using air?

Third, you need to have the component performance data; selection tables, performance curves, or computer programs. Otherwise all of this is a guess.

The easier items to discuss first are the air-cooled coils. For heat rejection, the coil capacity is selected by the amount of heat rejected in a specific time for a specific temperature difference. As Gonzalo stated, this temperature difference is the saturated condensing temperature minus the air inlet dry bulb temperature. In Imperial units, this would be BTU per hour per degree temperature difference.

If you know the condenser performance data and the entering dry bulb air temperature, you can determine the condensing temperature by calculation.

For shell & tube heat exchangers you need to know the original design conditions. If this is a direct expansion chiller, you can verify the flow rate by using the manufacturers chiller data.

For example, the flow rate can be verified by the pressure drop of the water through the chiller. The manufacturer should have performance curves that specifiy the pressure loss verus flow rate for any chiller configuration.

After you have measured the pressure drop of the water, the performance curve for your chiller will provide the water flow rate where this pressure drop will occur.

That confirms the water flow rate. Now you need temperatures for the entering and leaving water. This temperature difference is what we in the US call the temperature range.

After you have the water flow rate and temperature difference of the entering and leaving water temperatures (from the chiller), you can calculate the load being applied to the chiller.

Then you can check the compressor capacity to see if it is sufficient.

There is also another temperature difference called the approach temperature. The approach temperature is the saturated evaporating temperature minus the leaving water temperature. This is usually between 5F and 15F temperature difference.

Both of these temperature differences (range and approach) are used in calculating the log mean temperature difference. This is what you need when working with shell and tube heat exchangers. All of this is related to the performance of the system and the temperatures you record.

An important point to remember for direct expansion operation is the amount of superheat that is available for control of the thermostatic expansion valve.

If the leaving water temperature is only 5 degrees F warmer than the saturated evaporating temperature of the refrigerant, you will only be able to get 5 degrees F of suction superheat. This is all that is available from the chiller.

If the suction superheat is greater than 5 degrees F with a 5 degree F approach temperature, some othe problem is occurring.

Sorry for the long explanation. But this is a very complex discussion.

P-H charts or P-T charts can help, but you need to know the performance data of the system components and the original design conditions.

I hope this is usefull.

Best Regards,
US iceman

Au Yeong W. S.
06-08-2007, 12:59 PM
Hi everybody;

I still awaiting for your help about my questions
How can I determine if I have right saturated evaporator and condenser temperatore in relation to evaporator leaving water temperature for evaporatore and in relation to ambient air temperature in air cooled condenser, or to water temperature in water cooled condensers
in other words what is the right evaporator and condenser approach fo different type of condensers and evaporators
and how this change if ***** is R22 or R134
Also what is the right discharge temperature

I hope some body help me

Sharif
Reply:

Hi Sharif, I hope that this will settle your mind:

a. Of all the answers that came by, there is an underlying consideration, that is, cost vs performance.

b. Considering the 'right' discharge temperature will depends upon the 'acceptable' lubricant/crankcase temperature; and in most cases not more than 80 deg Celcius on your discharge as the limit before the lubricant begins the carbonising process.

c. What is right chilled water leaving temperature depends seriously on what is the air leaving temperature that your installation requires.

d. As a rule, the relationship between evaporator's water leaving and air off coil temperature, if this is what you are asking, is normally 1.6 to 2 deg Celcius, so that there is 'meaningful' heat exchange. Of course, one of the 'techniques' of getting more heat exchange capacity is to lower the cooling medium's entering temperature. The designing of heat exchangers in such relationships therefore goes beyond the realm of the PH diagram.

e. What are the 'changes' in respect to R134a and R22? This certainly centers on the refrigeration capacity, heat input, and power requirements and is best answered by plotting on the respective refrigerant PH charts. That's why someone asked you about your familiarity with these charts/diagrams.

I've tried to answer your multiple queries as best I could, and I might not be correct, and certainly stood to be corrected. I'm still learning. Who's perfect?

Best regards,
Au Yeong W. S.
Singapore