How does high humidity affect suction pressure & discharge pressure?

[fourthindiana]

TL;DR Summary

How does high indoor humidity affect suction pressure & discharge pressure of an A/C?

How does high outdoor humidity affect suction pressure & discharge pressure of an A/C?

In Chapter 40 (titled Typical Operating Conditions) of my textbook Refrigeration and Air-Conditioning Technology, the authors list several different objectives for readers at the beginning of the chapter. One of the stated objectives is for the reader to be able to describe how humidity affects equipment suction and discharge pressure. I read the entire chapter, and I don't fully understand how humidity affects equipment suction pressure and discharge pressure. Some of what I have read in the chapter seems to support the idea that humidity affects suction pressure and discharge pressure of an A/C, and other parts of the chapter seem to support the idea that humidity does not affect suction pressure and discharge pressure of an A/C.
Chapter 40 of my textbook is divided into subsections. Subsection 40.1, titled Mechanical Operating Conditions, says the following:
"...AHRI rates condensers on the basis of 95 degree fahrenheit air passing over them. With 95 degree fahrenheit air passing over it, a new standard-efficiency condenser will condense refrigerant at about 125 degrees fahrenheit. As the condenser ages, dirt accumulates on the outdoor coil and the efficiency decreases. The refrigerant will then condense at a higher temperature, which can easily approach 130 degree fahrenheit, a value often found in the field. However, the unit uses a 125 degree fahrenheit condensing temperature, assuming that equipment has been properly maintained. It is not easy to see the change in load conditions on guages and instruments used in the field. An increase in humidity is not followed by a proportional rise in suction pressure and amperage. Perhaps the most noticeable aspect of an increase in humidity is an increase in the condensate accumulated in the condensate drain system."

Subsection 40.2, titled Relative Humidity and the Load, says the following: "The inside relative humidity adds a significant load to the evaporator coil and has to be considered as part of the load. When conditions vary from the design conditions, the capacity of the equipment will vary. The pressures and temperatures will also change."

I highlighted what I consider to be the most important parts of the text in green.

In a way, the two statements I highlighted in green seem like they might contradict each other. The first green statement says that an increase in humidity is not followed by a proportional rise in suction pressure and amperage. The second green statement says that the inside relative humidity adds a significant load to the evaporator coil and has to be considered as part of the load. If humidity adds a heat load to the evaporator, wouldn't an increase in heat load increase the suction pressure and discharge pressure?

Perhaps the first green sentence only is discussing an increase in outdoor humidity, not indoor humidity.

How does high indoor humidity affect suction pressure and discharge pressure on an air-Conditioner?

How does high outdoor humidity affect suction pressure and discharge pressure on an air-conditioner?

 

[AZFIREBALL]

Here are some things to consider relative to the subject.
I would expect high inside humidity to reduce the evaporator’s efficiency due to the fact additional cooling is needed to support the phase change as the humidity is converted to liquid water during condensation; thus increasing the evaporator heat load.

High outside humidity would seem to make the condenser coil more efficient in removing heat from the refrigerant as more BTU’s are absorbed in cooling wet air than dry air.

 

[fourthindiana]

Here are some things to consider relative to the subject.
I would expect high inside humidity to reduce the evaporator’s efficiency due to the fact additional cooling is needed to support the phase change as the humidity is converted to liquid water during condensation; thus increasing the evaporator heat load.

The second quote that I put in boldfaced green font says that. But how would that affect the suction pressure and the discharge pressure?

My intuition tells me that high inside humidity would result in high discharge pressures and higher suctions pressures because I think that increasing the evaporator heat load would result in higher discharge pressures and higher suction pressures.

High outside humidity would seem to make the condenser coil more efficient in removing heat from the refrigerant as more BTU’s are absorbed in cooling wet air than dry air.

I don't understand what you're saying here. The hot refrigerant in the condenser warms the ambient outside air. The hot refrigerant in the condenser does not cool the outside air.

 

[AZFIREBALL]

You are right! The statement should have read:
'High outside humidity would seem to make the condenser coil more efficient in removing heat from the refrigerant as more BTU’s are absorbed in heating wet air than dry air. '
Sorry about that.
I agree with you regarding the pressures.

 

[fourthindiana]

You are right! The statement should have read:
'High outside humidity would seem to make the condenser coil more efficient in removing heat from the refrigerant as more BTU’s are absorbed in heating wet air than dry air. '
Sorry about that.
I agree with you regarding the pressures.

There is still one matter that we have not discussed. How would high outdoor humidity affect suction pressure and discharge pressure on an air-conditioner?

On this, my intuition tells me that high outdoor humidity would likely decrease the suction pressure and decrease the discharge pressure. I associate more efficient rejecting of heat in the condenser with a lower discharge pressure because more efficient rejecting of heat would cool the refrigerant in the condenser. If you cool the refrigerant in the condenser, i would think that that would make the discharge pressure lower. The lowering of the discharge pressure will lower the suction pressure. Am I correct on this?

 

[AZFIREBALL]

Yes. I think you are correct.

 

[PeterDonis]

I highlighted what I consider to be the most important parts of the text in green.

Let's take them in order:

An increase in humidity is not followed by a proportional rise in suction pressure and amperage.

Remember that here we are talking about the humidity of the air passing over the condenser. The heat transfer at the condenser adds heat to the air. No phase change in the air is involved, because if the air contains water vapor (i.e., humidity), the water vapor gains heat; it would have to lose it for a phase change to be involved. So the only difference humidity makes is an increase in the heat capacity of the air (because water vapor has higher heat capacity than dry air does)--i.e., the difference between the air temperature just before the condenser, and air temperature just after the condenser, will be smaller (same amount of heat rejection requires less temperature change because of higher heat capacity).

Now, what effect will this have on the refrigerant inside the condenser? Basically none. It will still be condensing at the same temperature and giving up the same amount of heat to the air, since the key things that drive those quantities are the temperature of the air just before the condenser, the flow rates of refrigerant inside the condenser and air over the condenser, and the geometry of the condenser coils, and none of those things have changed.

So we would indeed expect that an increase in humidity will not change any of the parameters at the condenser.

"The inside relative humidity adds a significant load to the evaporator coil and has to be considered as part of the load.

This is true; but what does "considered as part of the load" actually mean in practice? Look at the very next sentence:

When conditions vary from the design conditions, the capacity of the equipment will vary. The pressures and temperatures will also change."

In other words, the inside relative humidity needs to be considered when determining the capacity of the equipment, i.e., what kind of unit you install in your house. It needs to be capable of enough heat transfer at the evaporator to handle not just the desired temperature change, but the required condensation of excess water vapor associated with that temperature change (because the air can hold much less water vapor once it's cooled, and the excess gets condensed out), and how much excess water vapor there is will depend on the relative humidity of the air before it gets cooled.

If all this is done correctly and your house's A/C unit has enough capacity to handle the relative humidity inside your house during the summer, then you won't notice anything unusual about the pressures and temperatures in your system, because its capacity will have been properly matched to the expected peak load. But if this is not done properly, and the system does not have enough capacity to handle the expected peak load, then what will happen? Evaporator out superheat will increase (because there is more heat transfer required at the evaporator than can be handled by the phase change, since the system's capacity is insufficient), so suction pressure will increase, so discharge pressure will increase.

 

[fourthindiana]

Thank you, Peter Donis, for that excellent explanation. We're lucky to have you at Physics Forums.

 

[PeterDonis]

You're welcome!