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Old 09-25-17, 02:59 PM   #11
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If after reading the previous post if you still don't have a understanding of how this could work read Jeff's #15 post again.

It helped me to understand that the theory that it works is actually a fact.

Originally Posted by jeff5may View Post
Ok, here's my take on what you're figuring. The compressor uses maybe 1000 Watts up from the power line. Some of this is radiated from the compressor shell directly and some superheat is added to the cool refrigerant as it passes by the motor coils. The rest is used up as work of compression. As the preheated and compressed refrigerant condenses, all of the added superheat is dissipated before the gas condenses. So there's your standard resistance heating component. The fans don't count, as they are not using lots of power, and are either both adding to indoor heating or subtracting heat from each other.

As the refrigerant condenses, it gives off lots of heat. It is then subcooled a few degrees. Due to the high surface area and high airflow, heat recovery is effective. Lets say that x amount of airflow heats up 9 degrees F in one pass through the condenser. The warm air rises naturally away from the unit. Between the compressor radiation and forced air convection, there's your 14K BTU of raw heat gain.

On the other side, the room air is sucked through the evaporator at a slower rate and is cooled by the evaporating refrigerant. Let's say that x/3 amount of airflow is cooled 30 degrees F. This cold air is exhausted outside. Let's say 12K BTU makes it outside. So you lost 2K BTU in leakage between everything everywhere. That's 12K BTU NET HEAT GAIN.

This arrangement works well because the x/3 amount of infiltration (make-up) air is not always as cold as the exhaust air, especially in spring and fall. When it is warmer outside than your exhaust air, let's say 35-40 degrees F, the system is gaining BTU from your make-up air. With a 1-pipe setup, the delta T is always going to be low, as both heat exchangers are being fed room temperature air. As a result, the evaporator is not going to frost up on you (more latent energy rejection down the drain as condensed humidity), and the unit COP remains high. Even when it is frosty outside, the recirculated BTU (drawn into the refrigerant in the evaporator and rejected into the house) is much more than the power drawn from the power line. The gradients from (the make-up air minus exhaust air) and (outdoor air minus exhaust air) subtract from the overall refrigerant heat transfer.

With a 2-pipe setup, the evaporator is fed cold outdoor air. The delta T works against outdoor temperature, and so does COP and condenser discharge temperature: as outdoor temperature drops, they all get worse. As outdoor temperature approaches freezing, the evaporator can frost up fast, then the unit loses its advantage. Efficiency really depends on lots of stuff that is usually not included in these rigs, like variable expansion valves and hot gas defrost. That's why the makers discourage 2-pipe use below about 40 degrees F: passive defrost using ambient airflow to thaw out the evaporator is slow, plus the compressor is shut off.


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