Evaporating and Condensing Temperatures

[AJS]

Hello everyone. I am a design engineer for ground source heat pumps and a greenie in refrigeration. I am curious what the design evaporating and condensing temperatures would be for the following ground loop heat pump system (water to refrig and then refrig to air).

The refrigerant is R410A, the unit uses the Copeland compressor ZPS51K4EPFV. This would be sized for 64,000 BTU output at 70 deg ground loop entering water and 80 deg house entering air temperature. According to my figures, and using the information supplied by ClimateMaster, Series: Tranquility 27 (TT) Model: 064, I get the following evap and condensing temperatures:

Evap temp: 50.5 (144 psi)
Cond temp: 106 (347 psi)
at 70 EWT and 80 EAT

My question is if 50 sounds reasonable for an evaporation temperature...?

It is a high efficiency unit, probably 18 SEER?? (although I don't know if the SEER rating directly applies to ground source heat pumps). The airflow is 1825 CFM.

Next, what is the normal temperature difference between the Leaving Air Temperature (cold air going to the house) and the refrigerant saturation temperature in the evaporator? (for a ground source heat pump?)

At low EAT conditions like 65 EAT & 41.5 Leaving Air Temp, I get a negative TD of around 4 deg. That would be 41.5 (LAT) - 45.5 (Sat Evap Temp) = -4 TD. Is this possible?

Meaning my LAT is colder than my saturated evaporation temperature...?

Thanks in advance for the help!!

And thanks all the veterans of this post :)... I have read through a number of items here and am quite impressed with the quality of this forum and the answers given and have found the topics covered here very helpful. Thanks!

Aaron

[mad fridgie]

Are you a designer of ground source systems or a designer of the heat pumps themselves

[tmm]

AJS, I think that you may have mistaken your units. Under ASHRAE 13256-1 (Testing standard for Ground Water Heat Pumps) the conditions are as follows:

Cooling DB/WB 27/19C
Entering Water Temp 25C

Heating DB/WB 20C
Entering Water Temp 0C

To calculate the suction pressure you will need the flow rate and the composition of the liquid. If the system runs the risk of freezing you will naturally dilute the water with glycol which effects the transfer constant.

Never forget that the superheated gas exiting the coil will cool the body of the compressor before being compressed within so that condensing can take within the ambient conditions.

So its very difficult to predict an exact suction pressure however given that 0C/R410a is @ 7Barg a figure between 9-10Barg would not be unrealistic

Hope this helps. tmm

[AJS]

Mad Fridgie,
I am both a designer of the loop system and heat pumps.

[AJS]

AJS, I think that you may have mistaken your units. Under ASHRAE 13256-1 (Testing standard for Ground Water Heat Pumps) the conditions are as follows:

Cooling DB/WB 27/19C
Entering Water Temp 25C

Heating DB/WB 20C
Entering Water Temp 0C


tmm,
You are right about the design conditions. However, since ClimateMaster submitted conditions for 70 EWT and 80 EAT, I would like to address the saturation temperature at those conditions (so I don't have to do any adjustments or figuring for the other "test" conditions).

[nike123]

Normal evaporator TD is 10K (10*(5/9) °F).
You should expect that evaporation saturation temperature is ≈20°F lower than evaporator water entering temperature.

Depending on Condenser air flow your condenser TD could be 10-20K and that mean that condenser saturation temperature should lie between 20-40 °F higher than condenser air temperature

For given conditions:

at 70 EWT and 80 EAT
that mean 50°F evaporation and 100-120°F condensation temperatures.
It look like your numbers fit in above rules.

[nike123]

At low EAT conditions like 65 EAT & 41.5 Leaving Air Temp, I get a negative TD of around 4 deg. That would be 41.5 (LAT) - 45.5 (Sat Evap Temp) = -4 TD. Is this possible?

No, it is not possible to have evaporator entering air colder than SET (Sat Evap Temp)!
Check your measuring instruments.

[AJS]

Normal evaporator TD is 10K (10*(5/9) °F).
You should expect that evaporation saturation temperature is ≈20°F lower than evaporator water entering temperature.

Depending on Condenser air flow your condenser TD could be 10-20K and that mean that condenser saturation temperature should lie between 20-40 °F higher than condenser air temperature


You have the evap and condensor flipped around...

The EAT = 80 deg F into evaporator coil (Air Conditioning)

EWT = 70 deg F into condensor coil

condensor is getting water, evaporator is in air... how does this affect it?

[mad fridgie]

Mad Fridgie,
I am both a designer of the loop system and heat pumps.
MMMMMMMMMMMMMMMMMMMM.
The object of efficiency is to keep, SCT as low as possible and the SST as high as possible whilst still give the desired end results, and ensuring sysytem reliability.
You need to check your basic fundementals.
If you are buying a package unit, then it is what it is.
Why would you even consider cooling your house if the air on the evap is 65f, thus the issue would never arise, but if did, no the air can be colder of than the SST

[nike123]

You have the evap and condensor flipped around...

The EAT = 80 deg F into evaporator coil (Air Conditioning)

EWT = 70 deg F into condensor coil

condensor is getting water, evaporator is in air... how does this affect it?


Than TD for water cooled condenser is 30°F
And that mean condensation saturation temperature of 100°F.


TD for air cooled evaporator is 30-40°F.
That mean evaporation saturation temperature of 40-50°F

[AJS]

Why would you even consider cooling your house if the air on the evap is 65f, thus the issue would never arise, but if did, no the air can be colder of than the SST

The situation is an odd one but we have some customers that insist on running their AC when the EAT is 65 degrees. They must really like it cold.... or perhaps also for dehumidication. Besides the unit should be able to handle 65 EAT but this would be about the lowest end of the envelope.

You say that the leaving air temp can be lower than the SST? And if that is what you meant, can you explain?

[AJS]

Than TD for water cooled condenser is 30°F
And that mean condensation saturation temperature of 100°F.


TD for air cooled evaporator is 30-40°F.
That mean evaporation saturation temperature of 40-50°F


How are you defining TD? Is it the difference between Leaving air / water and saturation temperature?

[mad fridgie]

The situation is an odd one but we have some customers that insist on running their AC when the EAT is 65 degrees. They must really like it cold.... or perhaps also for dehumidication. Besides the unit should be able to handle 65 EAT but this would be about the lowest end of the envelope.

You say that the leaving air temp can be lower than the SST? And if that is what you meant, can you explain?
Sorry about that pure "typo" "no" the air can not be colder than the SST (double negative I answer the question first:o)
Brain not ingaged first thing in the morning, after a Friday Night.
65F is really cold, your SST would drop to compensate for the lower EAT, equalibrium is always reached in a fridge system or trips on a safety device.
If you want to make this really efficient, why do you not put a pre-cooler before your evap.
Water is 70F, air is 80F, counter flow with right thermal length should be able to get reasonable close to approach

[AJS]

Sorry about that pure "typo" "no" the air can not be colder than the SST (double negative I answer the question first:o)
If you want to make this really efficient, why do you not put a pre-cooler before your evap.
Water is 70F, air is 80F, counter flow with right thermal length should be able to get reasonable close to approach

Okay, thanks for clarifying that for me! What would the approach temperature be between the leaving air temperature and saturated suction?

Thanks for the idea, i have read about liquid line heat exchangers... its something to keep in mind.

[nevgee]

I;m confused ... are you using the system as an air con unit, for comfort cooling? Or a ground source heat pump, for heating?

Or, are you trying to use a groundsource heat pump for cooling?

[AJS]

I;m confused ... are you using the system as an air con unit, for comfort cooling? Or a ground source heat pump, for heating?

Or, are you trying to use a groundsource heat pump for cooling?

Well actually both. I want to first get a good handle on air conditioning with the GSHP and then I want to look into heating.

[nike123]

How are you defining TD? Is it the difference between Leaving air / water and saturation temperature?
That is approach. TD is entering air/water and saturation temperature difference.

[nike123]

T They must really like it cold.... or perhaps also for dehumidication.


Than evaporator TD should be higher by lowering air flow across evaporator.

[nike123]

okay, thanks for clarifying that for me! What would the approach temperature be between the leaving air temperature and saturated suction?



http://i35.tinypic.com/io2rus.jpg

Note that sum of Delta t and approach is = TD

[AJS]

Great, thanks for the definition & temperatures chart! Much appreciated!

[AJS]

Nike123,
Correct me if I'm wrong but according to the table you gave,


Air conditioning (evaporator) :

Tai-Tao = 8 to 10 K Evaporator delta T (dt)

Tai-Te=16 to 20 K (evaporator dt) take low dt for high relative humidity
Tao-Te=8 to 10 K Evaporator approach

Gives a 4.4-5.5 degree F temperature drop of the air going through the DX coil for Air Conditioning (8*5/9=4.4). This table doesn't seem right to me as I think it should be around 18-25 degree drop in air temperature.

[nike123]

Nike123,
Correct me if I'm wrong but according to the table you gave,


Air conditioning (evaporator) :

Tai-Tao = 8 to 10 K Evaporator

delta T (dt)


Tai-Te=16 to 20 K (evaporator dt) take low dt for high relative humidity
Tao-Te=8 to 10 K Evaporator approach

Gives a 4.4-5.5 degree F temperature drop of the air going through the DX coil for Air Conditioning (8*5/9=4.4). This table doesn't seem right to me as I think it should be around 18-25 degree drop in air temperature.


That is the problem with conversions. You easily make mistakes.
If 1°F(temperature interval)= 5/9 K (temperature interval) than 1K = 1/(5/9)°F = 9/5°F
8K = 8*9/5= 72/5 = 14.4°F

[AJS]

That is the problem with conversions. You easily make mistakes.
If 1°F= 5/9 K than 1K = 1/(5/9)°F = 9/5°F
8K = 8*9/5= 72/5 = 14.4°F

Oops, thanks!