EcoRenovator - View Single Post

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.

07-09-17, 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.