Unit cooler rating factor

[nh3simman]

I have noticed that the equipment manufacturers give a de-rating factor for evaporators that is based on the suction and the temperature difference.

However, according to the heat transfer theory, there should be no de-rating for low suction.

Manufacturers who I have asked, can not give me a satisfactory answer about this factor.

After much searching, I did find an indirect reference to this. Dossat and Horan "Fundamentals of Refrigeration". Problem is, it is a bit obscure and contradicts info in the text of the book.

Essentially, the low suction de-rating is due to frost build-up on the coil surface. The factors are in the range of 0.75 for -25degC.

My question is:

How can you give a fixed de-rating for an effect that starts out as f=1 (clean dry coil) and ends up as f=0 (completely blocked by ice)?

[US Iceman]

I can't say I have seen this before. The industrial coils manufactured here have only several correction factors.

One rating for wet coils (above freezing and considered frost-free)

One rating for frosted coils (which is based on a "uniform" thickness of frost = 1/16" (~1.6mm).

One for fin spacing.

Sometimes there is one for DX use with NH3 (and almost always no ratings provided when the operating evaporating temperature is below 0F (-17.7C). And, sometimes this de-rating factor has mutiple corrections depending on the actual evaporating temperature before you get to 0F (-17.7C.

And sometimes you will see a correction factor for material (stainless steel compared to galvanized steel)

Each manufacturer is a little different, but in essence they all tend to do the same thing. Usually, if the information is important it's buried in the fine print. And, all of the corrections are done to the rated coil capacity, not the TD.

The only think I can think of for the rating correction you describe is they are trying to correct for the insulating effect of ice (higher resistances).

Can you provide a quote from the book please? It might help to shed some light on the topic.

And, if you can provide a link to the manufacturers website for further information.

Note: when you post the link just type out the link without the www. or http://www. in front. You will not be able to post a link for a while until your post count rises. This is a safety check for new members to prevent spamming and not directly related to you.

[Gary]

On each downstroke of the piston, refrigerant vapor is pushed into the cylinder to be compressed on the upstroke. How much vapor is pushed into the cylinder depends upon how much pressure is pushing it in there. With a lower suction pressure/temperature, the compressor is pumping less vapor and therefore has less pumping capacity.

IOW, anything that lowers the suction pressure lowers the capacity.

[nh3simman]

I can't say I have seen this before. The industrial coils manufactured here have only several correction factors.

One rating for wet coils (above freezing and considered frost-free)

One rating for frosted coils (which is based on a "uniform" thickness of frost = 1/16" (~1.6mm).

One for fin spacing.

Sometimes there is one for DX use with NH3 (and almost always no ratings provided when the operating evaporating temperature is below 0F (-17.7C). And, sometimes this de-rating factor has mutiple corrections depending on the actual evaporating temperature before you get to 0F (-17.7C.

And sometimes you will see a correction factor for material (stainless steel compared to galvanized steel)

Each manufacturer is a little different, but in essence they all tend to do the same thing. Usually, if the information is important it's buried in the fine print. And, all of the corrections are done to the rated coil capacity, not the TD.

The only think I can think of for the rating correction you describe is they are trying to correct for the insulating effect of ice (higher resistances).

Can you provide a quote from the book please? It might help to shed some light on the topic.

And, if you can provide a link to the manufacturers website for further information.

Note: when you post the link just type out the link without the in front. You will not be able to post a link for a while until your post count rises. This is a safety check for new members to prevent spamming and not directly related to you.

Hi US Iceman,
There are 4 major unit cooler manufacturers in south Africa. All have printed catalogs that show nominal duty and a 2-D table giving a de-rating for DT and Suction temp.

The lower the suction, the lower the factor. I assume that this can only be due to the ice layer as you point out.

The only technical reference can find for this is Dossat and Horan "Fundamentals of Refrigeration".

Problem is, the explanation in the book is a misleading.
I have made an enquiry to the publisher and will post the authors response for discussion...

[US Iceman]

The lower the suction, the lower the factor. I assume that this can only be due to the ice layer as you point out.

Another possibility may be this is a method of correcting for pressure loss in the coil circuits, IF a standard coil circuit design is used for all applications.

For low temperature applications the gas volume increases, in doing so the pressure losses of the coil circuit would increase.

By correcting the TD at lower evaporating tempratures these factors may be a way to compensate for circuit pressure losses by reducing the actual coil capacity and hence, the circuit losses.

This is the only thing I can think of for why this might be done.

Manufacturers who I have asked, can not give me a satisfactory answer about this factor.

Now that's interesting. A manufacturer who can't explain their own ratings and correction factors.

It seems the major difference between the rating methods used in our respective countries try to accomplish the same thing.

We use the correction factors against the unit basic capacity, while the other method corrects the actual TD used. In practice they both achieve the same result of lowering the actual capacity of the unit by some means.

Any other comments than the ones above would be an attmpt to over-reach or out-guess on my part.

You have started some very good discussions lately nh3simman. Keep up the good work!