How does a walk-in freezer with R-22 get temperatures as low as -15F?

[fourthindiana]

Before today, I always thought that walk-in freezers had far colder evaporators than residential air-conditioners because of only two reasons: 1# the evaporators of walk-in freezers were in small, insulated confined spaces and 2# the refrigerants used in walk-in freezers were more suitable to having colder evaporators than the refrigerants in residential air-conditioners. However, today I noticed that the walk-in freezer at the trade school I attend has R-22 refrigerant. R-22 is a refrigerant that is frequently used in residential air-conditioners. When R-22 is used in residential air-conditioners, the normal temperature of the refrigerant in the evaporator will range from approximately 35-45 degrees. The walk-in freezer with R-22 refrigerant at my trade school get down to around minus 15 degrees fahrenheit.

To me, it does not seem plausible that the fact that the evaporator in the walk-in freezer at my trade school is in a confined, insulated space can explain ALL the temperature difference in the evaporator between a -15F walk-in freezer and a residential air-conditioner with R-22 with a 40 degree evaporator temperature.

I noticed that the walk-in freezer at the trade school with R-22 has a fairly large (and presumably powerful) reciprocating compressor. The residential air-conditioners with R-22 typically have scroll compressors, in my experience.

Other than the fact that the walk-in freezer has its evaporator in a small space, why is a walk-in freezer with R-22 able to have such a far lower evaporator temperature than a residential air-conditioner with R-22?

 

[Tom.G]

why is a walk-in freezer with R-22 able to have such a far lower evaporator temperature than a residential air-conditioner with R-22?

Because the air conditioner is designed so there is no ice build-up on the evaporator. If the air filter on the air conditioner is clogged due to lack of maintenance, the reduced air flow allows the evaporator to get below freezing and ice builds up on it, just like in the refrigerators of the 1950's that needed periodic manual defrosting.

These days, refrigerators, and your walk-in freezer, have automatic defrosting as a design feature. After a certain number of run-time hours, it is assumed that defrosting is needed. The defrost cycle is often to turn off the compressor and turn on an electric heater at the evaporator, melting the ice. In large freezers the melt water is sent down the drain. In home refrigerators, it flows to a catch pan near the condenser fan and is evaporated back into the environment.

Here is a link to the physical properties of R-22. It shows that -15F is achievable if the evaporator pressure is below about 13 psi gauge pressure.
http://www.refrigerants.com/pdf/R22_LINK.pdf
Hope this helps.

Cheers,
Tom

 

[fourthindiana]

Because the air conditioner is designed so there is no ice build-up on the evaporator.

Did you mean to say that the freezer is designed so there is no ice build-up on the evaporator? The reason I ask that is that your very next sentence mentions a scenario is in there is ice build-up on the evaporator of an air-conditioner. Also, further down the thread you mention how walk-in freezers have automatic defrosting.

If the air filter on the air conditioner is clogged due to lack of maintenance, the reduced air flow allows the evaporator to get below freezing and ice builds up on it, just like in the refrigerators of the 1950's that needed periodic manual defrosting.

These days, refrigerators, and your walk-in freezer, have automatic defrosting as a design feature. After a certain number of run-time hours, it is assumed that defrosting is needed. The defrost cycle is often to turn off the compressor and turn on an electric heater at the evaporator, melting the ice. In large freezers the melt water is sent down the drain. In home refrigerators, it flows to a catch pan near the condenser fan and is evaporated back into the environment.

Here is a link to the physical properties of R-22. It shows that -15F is achievable if the evaporator pressure is below about 13 psi gauge pressure.
http://www.refrigerants.com/pdf/R22_LINK.pdf
Hope this helps.

It's certainly not the type of answer that I was expecting, but it does help. I can see how having a defroster on the evaporator coil can help the evaporator coil get even colder.

Am I correct in my supposition that the fact that the evaporator of a walk-in freezer is in a small, confined, insulated space is part of the reason that a walk-in freezer with R-22 can get down to -15F while a residential air-conditioner with R-22 cannot?

Does the difference in power of the compressor of a walk-in freezer with R-22 with the power of a scroll compressor in a residential air-conditioner have anything to do with why a walk-in freezer with R-22 can get down to -15F while an air-conditioner with R-22 cannot?

 

[PeterDonis]

why is a walk-in freezer with R-22 able to have such a far lower evaporator temperature than a residential air-conditioner with R-22?

Just knowing the evaporator temperature is not sufficient to know how the system operates. We also need to know the evaporator pressure. The obvious way to lower the temperature in the evaporator is to lower the pressure in the evaporator, since saturation temperature varies with pressure. So if the walk-in freezer system operates at a lower evaporator pressure than, say, a typical home air conditioner, the evaporator temperature will be lower.

 

[fourthindiana]

Just knowing the evaporator temperature is not sufficient to know how the system operates. We also need to know the evaporator pressure. The obvious way to lower the temperature in the evaporator is to lower the pressure in the evaporator, since saturation temperature varies with pressure. So if the walk-in freezer system operates at a lower evaporator pressure than, say, a typical home air conditioner, the evaporator temperature will be lower.

PF member TomG already posted a link in post #2 that said that the corresponding pressure to -15F for R-22 is about 13 psig.

 

[PeterDonis]

PF member TomG already posted a link in post #2 that said that the corresponding pressure to -15F for R-22 is about 13 psig.

Yes, I know, but that doesn't tell us what the actual operating pressure is in the evaporator of the walk-in freezer you mentioned. Finding out what that actual operating pressure is seems like an obvious thing to do to answer your question.

 

[fourthindiana]

Yes, I know, but that doesn't tell us what the actual operating pressure is in the evaporator of the walk-in freezer you mentioned. Finding out what that actual operating pressure is seems like an obvious thing to do to answer your question.

If the temperature of the R-22 refrigerant in the evaporator is -15F, the only reason that the pressure of the refrigerant would not be about 13 psig is due to the superheat; correct?

When I go back to school on Monday, I am not allowed to just turn the walk-in freezer on and take pressures. My instructor might let me do it. I don't know.

Don't you know the answer to my question already though? You're an expert on this stuff. I thought you would know the answer to my question without needing any further information from me.

 

[PeterDonis]

If the temperature of the R-22 refrigerant in the evaporator is -15F, the only reason that the pressure of the refrigerant would not be about 13 psig is due to the superheat; correct?

That's what I would expect, yes. But I think it would be a good idea to try to confirm it.

Don't you know the answer to my question already though?

Didn't I already give you an answer? I only cautioned you about confirming it because you should always confirm hypotheses with actual data, no matter how confident you are in them.

 

[PeterDonis]

You're an expert on this stuff.

Actually, all of my HVAC experience is with automotive systems. Their operating parameters are generally similar to household systems (although R-134a was the most common refrigerant when I was working on them), so I feel reasonably confident about analyzing household systems. But walk-in freezers have very different operating requirements.

 

[fourthindiana]

Didn't I already give you an answer? I only cautioned you about confirming it because you should always confirm hypotheses with actual data, no matter how confident you are in them.

I'm assuming that your hypotheses that you are referencing is the following statement you made:

So if the walk-in freezer system operates at a lower evaporator pressure than, say, a typical home air conditioner, the evaporator temperature will be lower.

If the walk-in freezer operates at a lower evaporator pressure than a typical home air-conditioner with R-22, why would the walk-in freezer operate at a lower evaporator pressure than a typical home air-conditioner? Is it because the compressor on the walk-in freezer is designed to have weaker suction pressure than the compressor on a typical residential air-conditioner with R-22?

 

[fourthindiana]

Actually, all of my HVAC experience is with automotive systems. Their operating parameters are generally similar to household systems (although R-134a was the most common refrigerant when I was working on them), so I feel reasonably confident about analyzing household systems. But walk-in freezers have very different operating requirements.

You're too modest.

 

[PeterDonis]

why would the walk-in freezer operate at a lower evaporator pressure than a typical home air-conditioner?

Um, because it needs to have a lower evaporator temperature?

Is it because the compressor on the walk-in freezer is designed to have weaker suction pressure than the compressor on a typical residential air-conditioner with R-22?

The compressor isn't really designed for a particular suction pressure. It's designed for a particular compression ratio. The discharge pressure has to be high enough for the saturation temperature in the condenser to be higher than the ambient air pressure (remember that was a key point in a previous thread), and preferably a fair bit higher to get decent heat transfer. So for a given ambient air temperature, if you need a lower evaporator pressure, that means you need a compressor with a higher compression ratio.

For a given compressor discharge pressure, what actually controls the evaporator pressure is the expansion device: it has to be designed to create the appropriate pressure drop. So other key parameters you would want to find out for the walk-in freezer at your school are what the ambient air temperature is at the condenser (in particular, is the condenser outside the building or inside, in an air conditioned space) and what kind of expansion device it has and what pressure drop is expected.