Efficient Water-Source Condenser Design?
Hello All,
I want to design an extremely efficient geothermal heat pump system for my residence. I think I have quite a bit worked out, but I still have a few questions. Any input is welcome---including criticism.
First, I'm not a refrigeration engineer, tech, etc. I design electronic stuff. As I researched high efficiency options for cooling/heating my home I became more fascinated with the refrigeration cycle. I also came to the conclusion that packaged water-source heat pumps are designed for broad operating conditions limiting their efficiency for a given application.
I'll keep my day job, but this stuff is cool! Just when I think I have my arms around it, a new layer is revealed prompting more research. I've been on this task for about a month and this forum has been a gold mine---not to mention that the posters seem to be genuinely nice and knowledgeable people.
This project is not about saving money on the installation. It is about learning something new while trying to meet the challenge of lofty design criteria.
Climate
I live in central Texas where we have a design wet bulb of 79F. We have 3175 cooling degree-days and 1586 heating degree-days. For this reason I am mostly concerned with optimizing the system for best cooling efficiency.
Design Parameters- Water-to-water heat pump
- K5 scroll compressor using R410A
- 78F entering water-source temp at 3.5GPM/ton
- 45F evaporating temp
- Low as possible condensing temp
- Short refrigerant circuit-- the compressor, condenser and evaporator are packaged close to each other.
- No reversing valve in the refrigerant circuit. The evaporator and condenser heat exchangers never swap functions. The "reversing" occurs via valves in the water lines.
Maximize Cooling COP
When I first looked into maximizing COP I thought the secret was subcooling---lots of it. Then I learned that the p-h chart unveils the real culprit, compression ratio. So, to lower the compression ratio for a given evaporator temperature the condensing temperature must be lowered. Having said that, I know that some minimum amount of subcooling is required to insure 100% liquid to the TXV---more on that later.
As I understand it, to get the lowest possible condensing temperature I need to maximize the condensing surface area of the heat exchanger. In a typical condensing coil the top portion of the coil de-superheats the gas, the bottom area of the coil provides some amount of liquid subcooling, and condensing occurs in what's left in the middle.
Unless I'm already off base this is where my question comes in.
What is the best water-source heat exchanger arrangement for minimum condensing temperature?
1) One gigantic heat exchanger (HE) to insure plenty of condensing area?
2) Two properly sized HEs: one for de-superheating and condensing with a separate heat exchanger for subcooling/receiving to prevent flooding of the condensing HE?
3) Three HEs: one each for de-superheating, condensing and subcooling?
4) De-superheating via liquid refrigerant injection into the hot gas using arrangements 1 or 2 above.
Subcooling
For the sake of discussion, let's assume that I am able to get the approach temperature of the water source and condensing temperatures near zero. I would have no means to further subcool the liquid refrigerant via the water source, which would be bad for TXV performance.
Enter alternate subcooling techniques...
1) Use a liquid refrigerant pump (increased pressure at constant temp = subcooling)
2) Use a small heat exchanger between the output of the evaporator and the liquid line to the TXV (decreased temperature at constant pressure = subcooling)
Liquid Refrigerant Pump
This appears to be the easy way to do it. I am guaranteed enough subcooling at the expense of pumping energy. Another potential benefit is the ability to divert a fraction of this pump's output into the hot gas before it enters the condensing heat exchanger thereby quickly removing superheat and allowing more of the heat exchanger's surface to be used for condensing.
Suction Return Heat Exchanger
This is where I start to loose it (if I haven't already). On the surface it seems free since (I think) any enthalpy removed from the liquid line is transferred to the suction line. If this is true then all I have to worry about is maintaining enough pressure differential across the TXV? If the differential is marginal I could use an EEV in place of the TXV.
If I used this technique wouldn't it be best to place the TXV bulb on the suction line on the output side of the heat exchanger rather than the output of the evaporator? Assuming the net enthalpy gain/loss is zero, then maintaining ~5F superheat at the output of the subcooling heat exchanger means that more of the evaporator is wet --right? If so, doesn't this increase the capacity of the evaporator if not the system efficiency?
I have more questions about other areas of possible improvement, but I'll make those separate posts.
Thank you for your comments!
Ray
Re: Efficient Water-Source Condenser Design?
Hi there,
Some comments from me.
Quote:
K5 scroll compressor using R410A
If you have no experience in refrigeration then I would suggest another refrigerant because R410A is a high pressure refrigerant.
Quote:
Maximize Cooling COP
When I first looked into maximizing COP I thought the secret was subcooling---lots of it.
With water cooled condenser or air cooled one there is a limit for sub-cooling. You can not have liquid temp out of the condenser near the air or water inlet temp. (no HX is 100% efficient).
Quote:
Then I learned that the p-h chart unveils the real culprit, compression ratio. So, to lower the compression ratio for a given evaporator temperature the condensing temperature must be lowered. Having said that, I know that some minimum amount of sub cooling is required to insure 100% liquid to the TXV---more on that later.
Lowering the condensing temperature depends on the water or air inlet temperature. If you make a condenser with surface area of infinity :eek: , still you get the same condensing pressure. There is a limit. Remember there is no "Over Condensing".
If the condensing temperature is lowered too much then the TEV can not function properly.
Quote:
What is the best water-source heat exchanger arrangement for minimum condensing temperature?
1) One gigantic heat exchanger (HE) to insure plenty of condensing area?
2) Two properly sized HEs: one for de-superheating and condensing with a separate heat exchanger for subcooling/receiving to prevent flooding of the condensing HE?
3) Three HEs: one each for de-superheating, condensing and subcooling?
4) De-superheating via liquid refrigerant injection into the hot gas using arrangements 1 or 2 above.
Just use one properly sized condenser. In a condenser nearly 5 to 10% of the surface is used for de-superheating. 5-10% of the surface is used for sub-cooling and nearly 80-90% of the surface is used for condensing.
Quote:
Enter alternate subcooling techniques...
1) Use a liquid refrigerant pump (increased pressure at constant temp = subcooling)
2) Use a small heat exchanger between the output of the evaporator and the liquid line to the TXV (decreased temperature at constant pressure = subcooling)
Use the second method.
Quote:
Liquid Refrigerant Pump
This appears to be the easy way to do it. I am guaranteed enough subcooling at the expense of pumping energy. Another potential benefit is the ability to divert a fraction of this pump's output into the hot gas before it enters the condensing heat exchanger thereby quickly removing superheat and allowing more of the heat exchanger's surface to be used for condensing.
It is not easy believe me. Read the "Liquid Pressure Amplification" thread in this forum.
Quote:
Suction Return Heat Exchanger
This is where I start to loose it (if I haven't already). On the surface it seems free since (I think) any enthalpy removed from the liquid line is transferred to the suction line. If this is true then all I have to worry about is maintaining enough pressure differential across the TXV? If the differential is marginal I could use an EEV in place of the TXV.
This would not harm the TEV's function. The liquid sub-cooling is done with constant pressure.
Quote:
If I used this technique wouldn't it be best to place the TXV bulb on the suction line on the output side of the heat exchanger rather than the output of the evaporator?
I would not do that. Suction line heat exchangers are small and if you put the TEV bulb on the outlet of the HX then there is a possibility of liquid flood back.
I suggest you read a sub-forum called "Optimization" under "Industrial" forum.
Good luck.
Cheers
Re: Efficient Water-Source Condenser Design?
As Lana said, Just use one large one. I'll get my total system heat rejection+Compressor heat size the condenser then multiply it by 25% or if ambient is really toasty I over size it by 35% and use a fan speed control for when the weather gets cold and here in Canada it can do that to us ;)
Re: Efficient Water-Source Condenser Design?
But then again this is a morning and your case is water not air so a fan kit will do sod all for ya. You want to look into the Johnson's control V46-AB1 or AC1 Head modulated flow control valve.
Re: Efficient Water-Source Condenser Design?
Thank you for your comments.
Fortunately, I have a neighbor who is skilled in working with R410A to show me the ropes and keep me safe.
Ray
