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Old 12-20-13, 04:10 PM   #10
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Both Daox and Stevehull make good points here concerning flow. Parallel flow is the way you want to go with the evaporator side. Assuming the same outdoor temperature at the two units, the flow will tend to be equally divided between the two units when plumbed in parallel. With this divided flow, the refrigerant will spend more time in each evaporator at a lower speed, and heat transfer will increase in both units simultaneously. With similar expansion devices in each unit, balancing the heat transfer is a simple matter. This is how the manufacturers optimize the heat transfer in multi-circuit exchangers like the one in AC's post. Defrost happens faster, but requires a sensor on each unit.

With series-connected units, the flow is constant in both units, and with the added surface area, this flow will increase. The increased flow causes pressure drops which add up to less effective drop in each unit, causing less delta-t in each at a higher flow rate. The heat transfer is still greater than with only one unit, but the units will tend to find equilibrium where one will transfer a lot more heat than the other. Usually, the first unit after the expansion valve will transfer most of the heat, and the other will be left with whatever is left. As temps approach freezing, the first unit will frost up and the load will be transferred by the second unit until it frosts up. Defrost cycles will be enormous.
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