Any trick to sizing a coaxial condensor? - Page 2

jeff5may · 07-02-17, 12:35 PM

For your setup, water loop or well and coaxial HX outside and DX window unit HX indoors, you could use the existing cap tube in the window unit if it's not plumbed with a reversing valve. The cap tube will be matched to your indoor coil, and the water side of the cycle will do a superb job at dissipating your waste heat. The thing to do in this case is to watch your high side pressure and the water entering and leaving temperature. The water flow should be sufficient to heat the discharge into the range of body temperature or higher to keep the indoor coil from frosting up. This will get you good dehumidifying action and sensible cooling. Too little water flow and your head pressure goes through the roof, killing efficiency.

When and if a reversing valve goes in, you can run the thing with the original cap tube for simplicity. If your water temperature doesn't drop too much in winter, the cap tube may not try to freeze the coaxial HX. Again, watching the pressure and water temps will let you know what's going on with the refrigerant cycle. Measuring the suction line temperature leaving the coaxial HX will allow you to calculate the superheat. Some superheat is good, but too much is not good. Window unit design targets the superheat to never fall below about 15 degF. Sometimes it rises into the 30's. With a txv you can do better, but with extra performance comes extra plumbing.

digiz · 07-08-17, 03:52 PM

Make one.
Lauterbach Verfahrenstechnik GmbH - Software for thermal and hydraulic design of heat exchangers

http://ecorenovator.org/forum/geothe...html#post51805

---After getting a couple of geothermal holes drilled, I'd like to start testing/experimenting with some actual heat transfer. My initial thoughts were to replace the condenser of a window unit with a coaxial condenser and pump the water though while measuring temps. The question is, If I start with 12000btu window unit, do I just replace the air condenser with a 1 ton coaxial condenser? Is it this simple?
---Then I start thinking why only set this up for summer? Why not pickup a 1 ton portable heat pump to use year round? What gets me here is that they are 12000btu in cooling mode and 10000btu in heating mode. Does this still mean to use a 1 ton coaxial condenser?

jeff5may · 07-09-17, 12:30 AM

To clear up the original question concerning the discrepancy between cooling mode capacity vs. heating mode capacity, I will explain the HX manufacturers' methods. In the standard industry testing, they are required to run their products in both modes of transfer. The higher heat transfer value is obtained in counter-current flow, where the water flows the opposite direction from the refrigerant. The lower heat transfer value is obtained in co-current flow, where the water and refrigerant flow in the same direction through the heat exchanger. They are required to test their products this way mainly because the refrigerant reverses direction with heating vs. cooling mode of operation, while the water is normally pumped in the same direction (regardless of mode).

This is an important decision to make when building a heat pump: which mode (heating or cooling) wins over the other? Or, more importantly: which flow mode should be avoided in certain conditions and situations? The answer depends not only on raw heat transfer. Things like pressure drop, vapor and liquid distribution through the plumbing, oil carry-over, and temperature pinch points or limits can make lots of difference as to the effectiveness and longevity of the rig over time.

For a more in-depth and concise view of this topic, SWEP has a section about reversible heat exchanger operation here:

7.5 Reversible systems - SWEP

While you're there, wrap your brain around more of their material. They refer to specific models and lines of what they build (brazed plate heat exchangers) in the handbook, but the physics work the same for coaxial heat exchangers as well. They don't dive deep into the math or theoretical side of the subjects they cover, so the material is an easy read and super useful. Just click on the "Navigation" button to bring up the table of contents, then pick a chapter and have at it.

07-02-17, 12:35 PM	#11
jeff5may Supreme EcoRenovator Join Date: Jan 2010 Location: elizabethtown, ky, USA Posts: 2,428 Thanks: 431 Thanked 619 Times in 517 Posts	For your setup, water loop or well and coaxial HX outside and DX window unit HX indoors, you could use the existing cap tube in the window unit if it's not plumbed with a reversing valve. The cap tube will be matched to your indoor coil, and the water side of the cycle will do a superb job at dissipating your waste heat. The thing to do in this case is to watch your high side pressure and the water entering and leaving temperature. The water flow should be sufficient to heat the discharge into the range of body temperature or higher to keep the indoor coil from frosting up. This will get you good dehumidifying action and sensible cooling. Too little water flow and your head pressure goes through the roof, killing efficiency. When and if a reversing valve goes in, you can run the thing with the original cap tube for simplicity. If your water temperature doesn't drop too much in winter, the cap tube may not try to freeze the coaxial HX. Again, watching the pressure and water temps will let you know what's going on with the refrigerant cycle. Measuring the suction line temperature leaving the coaxial HX will allow you to calculate the superheat. Some superheat is good, but too much is not good. Window unit design targets the superheat to never fall below about 15 degF. Sometimes it rises into the 30's. With a txv you can do better, but with extra performance comes extra plumbing.

07-08-17, 03:52 PM	#12
digiz Lurking Renovator Join Date: Jun 2016 Location: EU Posts: 7 Thanks: 0 Thanked 0 Times in 0 Posts	Make one. Lauterbach Verfahrenstechnik GmbH - Software for thermal and hydraulic design of heat exchangers http://ecorenovator.org/forum/geothe...html#post51805 ---After getting a couple of geothermal holes drilled, I'd like to start testing/experimenting with some actual heat transfer. My initial thoughts were to replace the condenser of a window unit with a coaxial condenser and pump the water though while measuring temps. The question is, If I start with 12000btu window unit, do I just replace the air condenser with a 1 ton coaxial condenser? Is it this simple? ---Then I start thinking why only set this up for summer? Why not pickup a 1 ton portable heat pump to use year round? What gets me here is that they are 12000btu in cooling mode and 10000btu in heating mode. Does this still mean to use a 1 ton coaxial condenser?

07-09-17, 12:30 AM	#13
jeff5may Supreme EcoRenovator Join Date: Jan 2010 Location: elizabethtown, ky, USA Posts: 2,428 Thanks: 431 Thanked 619 Times in 517 Posts	To clear up the original question concerning the discrepancy between cooling mode capacity vs. heating mode capacity, I will explain the HX manufacturers' methods. In the standard industry testing, they are required to run their products in both modes of transfer. The higher heat transfer value is obtained in counter-current flow, where the water flows the opposite direction from the refrigerant. The lower heat transfer value is obtained in co-current flow, where the water and refrigerant flow in the same direction through the heat exchanger. They are required to test their products this way mainly because the refrigerant reverses direction with heating vs. cooling mode of operation, while the water is normally pumped in the same direction (regardless of mode). This is an important decision to make when building a heat pump: which mode (heating or cooling) wins over the other? Or, more importantly: which flow mode should be avoided in certain conditions and situations? The answer depends not only on raw heat transfer. Things like pressure drop, vapor and liquid distribution through the plumbing, oil carry-over, and temperature pinch points or limits can make lots of difference as to the effectiveness and longevity of the rig over time. For a more in-depth and concise view of this topic, SWEP has a section about reversible heat exchanger operation here: 7.5 Reversible systems - SWEP While you're there, wrap your brain around more of their material. They refer to specific models and lines of what they build (brazed plate heat exchangers) in the handbook, but the physics work the same for coaxial heat exchangers as well. They don't dive deep into the math or theoretical side of the subjects they cover, so the material is an easy read and super useful. Just click on the "Navigation" button to bring up the table of contents, then pick a chapter and have at it.