Some questions came up in a meeting of some home inspectors yesterday.
1: The level of humidity affects the temp split.
Some were arguing the higher the humidity, the higher the split, arguing that the condensation on the condensor fins increases cooling.
I have always thought that the higher the humidity the lower the split because of the increased amount of energy available being devoted to dehumidification
The dehumidification contributes to perceived lower temps, of course, but as measured with a thermometer, I believe the actual split will be lower with higher humidity.
2. The outdoor temp affaects the split.
I believe the cooler it is outside, the higher the split. Some argue the opposite.
Any expert thoughts here?
Replies
What do you mean by "split".
From from what you wrote I am guess the temp drop for the air across the coil, but I am not sure.
Yep, I was taught to call the temp difference across the coil as the "split" or "delta."I guess it might be a regional thing
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Sojourners: Christians for Justice and Peace
Now my background is in electrical engineering. And while I had did take a course or 2 in thermodynamics back them it was studying the melting rate of ice and how much sawdust that you needed to keep ice for a whole sumemr.
"I have always thought that the higher the humidity the lower the split because of the increased amount of energy available being devoted to dehumidification"
I agree. Lots of energy is "used up" condensing the moisture.
"2. The outdoor temp affaects the split.
I believe the cooler it is outside, the higher the split. Some argue the opposite."
Not real sure about this one.
1. Yes and no. The level of humidity in a small range will affect the temperature drop across the evaporator coil. The coil at a specific air flow has a capacity typically stated in total cooling and s/t (sensible/total) ratio. If the humidity is in the range where the coil can provide all the latent cooling then the delta T will increase at lower humidity levels, all else being the same. If the humidity is greater than the latent capacity of the coil, no changes will occur. At high humidity, the coil removes as much moisture as the surface area and drainage can handle, and the remaining cooling capacity(sensible) is seen as drop in dry bulb temp.
2. Also, yes and no. In a txv controlled system, the txv will limit the refrigerant mass flow rate though the evaporator coil based on the superheat conditions of the refrigerant leaving the coil. This is an energy saving feature and WILL NOT result in colder coil or better dehumidification, and as such will not affect the "split".
In an orifice or "piston" based control, the head pressure (i.e. the discharge pressure of the refrigerant leaving the compressor) will vary based on outside air temperature, which will affect the mass flow rate through the evaporator coil and potentially, the delta T across the coil. At lower temperatures, within the range of the capacity of the system, the suction temperature of the evaporator coil can be lower and thefore require absorption of more energy to evaporate and cause a slight increase in the split, but at a lower delta P, less mass is flowing. This is almost a self-cancelling set of phenomena.
Until OAT reaches a level where capacity is lost, the temperature drop across the coil, all other things being equal, is minimal.
Thanks!
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Sojourners: Christians for Justice and Peace