What is a typical Delta T over an air/refrigerant heat exchanger?

[rjsimmon]

Hi!

Sorry if this seems a silly question but I'm pretty new to this game.

I'm designing heat exchangers for an air-air source heat pump system. My problem is, I'm not sure what typical delta T values should be across the heat exchangers. Hot side and Cold.

I need to know to verify what I have designed is correct.

Condensing temp is: 50 degrees C.

Evap temp is: -10 degrees C.

Air flow over both heat exchangers is: 5 degrees C.

Cheers, Rob.

[Andy]

Hi!

Sorry if this seems a silly question but I'm pretty new to this game.

I'm designing heat exchangers for an air-air source heat pump system. My problem is, I'm not sure what typical delta T values should be across the heat exchangers. Hot side and Cold.

I need to know to verify what I have designed is correct.

Condensing temp is: 50 degrees C.

Evap temp is: -10 degrees C.

Air flow over both heat exchangers is: 5 degrees C.

Cheers, Rob.

Hi RJ

delta t or td. td is the difference between the air and the refrigerant 6k on the evaporator and 10k on the condenser.

dt would be the differance between the air on and off the heat exchanger. 1.5 to 2k on the evaporator(this is entirely down to the amount of tube rows and the humidity in the conditioned space required). On the condenser probably 8 to 10k, again subject to the number of tube rows.

Kind Regards Andy

[rjsimmon]

Cheers for the quick reply.

I did mean delta T (dt), although you had me questioning myself, lol.

So the difference between the air inlet temp and the air outlet temp of the condensor should be 8 - 10 Kelvin.

And the difference between the air inlet temp and the air outlet temp of the evaporator should be 1.5 - 2 Kelvin.

The number of tube rows in the flow direction is 4, if it helps. It's a pretty small!

Do you have a reference for this information or is it just rule of thumb?

Cheers, Rob.

[Andy]

Do you have a reference for this information or is it just rule of thumb?

Cheers, Rob.

Experience on site there are a few coil manufacturer with selection program that can be downloaded.

Maybe cool pack has some design information. Take a look at the software section of RE.

Kind regards Andy

[raviperumal]

Hi Andy,
Can you get us the link.

Best regards,
raviperumal

[Gary]

Hi RJ

delta t or td. td is the difference between the air and the refrigerant 6k on the evaporator and 10k on the condenser.

dt would be the differance between the air on and off the heat exchanger. 1.5 to 2k on the evaporator(this is entirely down to the amount of tube rows and the humidity in the conditioned space required). On the condenser probably 8 to 10k, again subject to the number of tube rows.

Kind Regards Andy

Hmmmm... So, if the return air is 24C (75.2F), the refrigerant should be 18C (64.4F) and the supply air should be 22-22.5C (71.6-72.5F)? That doesn't seem right.

This is a heat pump, so shouldn't we be talking about indoor coil and outdoor coil, instead of evaporator and condenser... and delta-T's/TD's in which mode?

[taz24]

This is a heat pump, so shouldn't we be talking about indoor coil and outdoor coil, instead of evaporator and condenser... and delta-T's/TD's in which mode?

I remember from an earlier post that he is only designing the heat pump side of it, So heating indoors and cooling outdoors.
In my oppinion a dt of only 2deg seems a bit low.
I would of thought it would be nearer 10deg.

taz.

[Gary]

I remember from an earlier post that he is only designing the heat pump side of it, So heating indoors and cooling outdoors.
In my oppinion a dt of only 2deg seems a bit low.
I would of thought it would be nearer 10deg.

taz.

Hmmmm... Designing the outdoor coil for 2K dT (big coil with lots of airflow) would be a very good thing in cold weather as it would minimize frosting the coil, provided there were some means to avoid overloading the compressor in mild weather. It would also minimize head pressure in cooling mode, thus minimizing energy consumption. Perhaps Andy is on to something here.

[lana]

Hi everybody,

I am in coil design and manufacturing. I should say that Andy is completely right.

Don't confuse refrigeration DX coil with air conditioning one.

Td (air in - air out) for air conditioning coil will be 10-20°C. and for refrigeration will be 1-3°C.

When you calculate the coil then you will get the same temp. but ofcourse with changing rows and face velocity you can get so many conditions.

For example with low face velocity (below 400FPM) you can increase Td in air conditioning coil.

I hope this clears some points.
Cheers

[NoNickName]

I would say 12 K across evaporator, and almost the same across condenser. More if 100% fresh air is treated.

[Andy]

I remember from an earlier post that he is only designing the heat pump side of it, So heating indoors and cooling outdoors.
In my oppinion a dt of only 2deg seems a bit low.
I would of thought it would be nearer 10deg.

taz.

It could be 10k, but then the evaporation would drop and the C.O.P.If it was you house and your electric which would you choose

We do a lot of refrigerant based heat recovery(recovery of heat off condensers) much like a VRF in heat recovery mode, thats where I get my figures from. I have a design now with evaporation at 18 deg c, condensing at 45 deg c, with a C.O.P at 6.

Kind Regards Andy

[rjsimmon]

Hi Guy's,

Only just read all the comments.

I've finalised my Evap and Cond designs with the data Andy originally provided.

delta T for the cond (inside) is about 10 degrees C, and about 2 degrees C for the Evap (outside).

I was more worried about the Evap due to ice build up, but I've got plenty of airflow over the evap which should hold back the icy problem.

Cheers again for all your help.

Rob.

[taz24]

It could be 10k, but then the evaporation would drop and the C.O.P.If it was you house and your electric which would you choose


Kind Regards Andy

Well if you put it that way!!

taz