Water sourced heat exchanger...
Hi everyone. Thanks for all the great info on this site. I am fairly new to the refrigeration cycle but I think I have a better than average (for a student) understanding of the cycle.
I am experimenting with installing a water sourced heat exchanger as a replacement to the typical residential air sourced condenser. I built a crude unit and I am not getting my expected results. I am hoping that someone could explain the results I am seeing.
The heat exchanger ("coil") is very basic. Appox. 30' of 1/4" copper tubing inside a 5/8" garden hose. 1/2" PVC pipe, tees and brass compression fittings on both ends isolate and seal the copper from the hose allowing water to flow around the copper. This is all rolled into a nice 12" coil. Crude, but it works. It's soaking up heat like butter in the Sun.
The current system is a 4 ton non-reversing air exchange system. And works perfectly.
My original installation design was to use a reversing valve to direct the high pressure off the compressor to the existing condenser or my coil based on a water flow switch. This would allow for a backup condenser in the event of a failed pump or a loss of water supply i.e. Lake dried up. After much thought, I made an quasi-educated guess that I might be able to avoid the valve and simply install the coil in series after the condenser (on the high side of course). My logic was that the compressor will only work as hard as needed to get the pressure high enough to throw off heat to the coolest medium. Apparently I erred in this logic.
Here's my data: all temps F.
Prior to coil installation: "baseline"
Working medium R22
Outside temp 90
Inside temp 79
high side 250psi
subcooling delta 10
plenum delta 20
compressor current avg. 4.2Amps per leg
After coil was installed but no water flowing all numbers appeared to be the same so my installation appears to be good. I mention this to emphasize that we are still working with that same baseline on a working system with my coil installed in series with the condenser but no water flowing. So the condenser is doing all the heat exchange exclusively.
Here's where I get unexpected results. Water source is from the tap and it enters at 80 degrees. When flowing at about 5gpm I see a delta on the water of only 1-2 degrees and a delta on the coil (freon) of 10+ degrees. This is great because it means the coil is working and taking away a lot of heat. What I expected to see was a high pressure drop, a compressor current drop, and the hot air coming off the condenser to nearly vanish. None occurred.
I don't currently have a pressure gauge after the coil. Later today I plan to install a schrader valve after the coil and I expect to see a pressure drop between the in and out of my coil. Currently I am reading the high side from the schrader after the condenser. I don't understand why I am still getting heat exchange at the condenser. I noticed that the line coming off the compressor is 3/8" and manifolds to 4 - 1/4" lines going through the condenser. It then comes out the condenser as a 1/4" line. I am thinking this is a restriction causing the pressure to build in the condenser. I may have to put the reversing valve after the compressor to control the the flow between the condenser and my coil. Is this the only way I am going to be able to get the pressures down? Also, will I need a restriction in my coil design as appears to be in a factory designed condenser? I thought the restriction occurs at that expansion valve or the capillary.
All compressor charts indicate as the medium, air or water, temperature decreases so too does the watts required to achieve the pressures needed to exchange the heat. Hence lower outside temps lower lower electric bills. Can someone help explain how to see empirical data that shows a energy saving by using 80 degree water over 90-100 degree air? I am currently not seeing the compressor current drop.
Since the refrigerant is still going through the air condenser first. Its still rejecting most of its heat at the regular condenser first.
You added 30' of copper to the liquid line, so you increased resistance, so you won't get a big drop in pressure.
The metering device is sized to the condenser based on operating as an air to air system, so you will never get a big drop in pressure, that still allows the system to work properly.
Drop teh head pressure too much, and you'll get flash gas in the liquid line before it gets to the metering device.