hotgas and crank case pressure regulator

[abbasi]

hi there
How do we select the hot gas bypass for specific model (say 8-ton capacity). Is there any ratio between unit capacity and hot gas bypass capacity coz we normally take 75% of unit capacity as hot gas bypass capacity also what impact will small/large size selection make?
Which is preferred hot gas bypass or crank case pressure regulator?

[US Iceman]

If you have capacity reduction ability on the compressor, the hot gas capacity is based on the last step of capacity reduction.

Let's say the compressor has 50% capacity reduction. In other words, the compressor is able to operate at 100% (fully loaded) or at 50% of the full capacity.

The hot gas regulator would be sized to provide the volume of gas required at 50% compressor capacity. You will also need some means of making sure the suction gas is not too hot, because this can overheat the compressor.

If the compressor does not have any capacity reduction, then the hot gas regulator would be sized for about 100% of the full load. This way any decrease in the heat load would cause the suction pressure to reduce.

Crankcase pressure regulators are used to prevent a high suction pressure from overloading the compressor motor or condenser. Using a crankcase pressure regulator for your application would not provide any benefit.

[abbasi]

can u give me some example involving figures for say 10 ton unit?

[abbasi]

Comply with local codes. ASHRAE/IESNA Standard 90.1, Energy Standard for
Buildings Except Low-Rise Residential Buildings, precludes the use of hot gas
bypass for HVAC systems larger than 7.5 tons unless they are designed with
multiple steps of unloading or continuous capacity modulation. The maximum
amount of HGBP allowed by the standard varies with system size: 25 percent of
total capacity for systems larger than 20 tons, 50 percent for systems ranging
from 7.5 to 20 tons.
Standard 90.1 (which excludes) process applications and low-rise residential buildings) grants an
exception to systems smaller than 7.5 tons. Be sure to consult the standard for specific requirements.

[Peter_1]

This is how we have done it once in a market truck +/- 15 years ago.

It's still working this way.

The liquid reciever is right on top, left bottom suction accumulator, hot gas injection (T junction to the air coold condensor) , and TEV to cool these gasses (bulb attached on the right side coming out the accumulator)

[US Iceman]

What is perfect about general comments? This was not meant to be a dissertation on the use of hot gas bypass.

I would be glad to entertain the discussion though. I have quickly read the article in the link. Pretty decent job of explaining it.

[US Iceman]

No problem with your comments at all. You are right. I did not explain all of the details.

We do indeed have several issues that contribute to the total suction volume flow; the hot gas which is bypassed and the flash gas due to desuperheating.

[abbasi]

Although what US Iceman says in general is correct, his statement quoted above is not perfectly correct.

Here is a link to an article I wrote on the topic back in about 1998. It provides a few illustrative numbers as well.

http://www.fridgetech.com/miscellany/capacity%2Dcontrol/

well quite useful link
i think we should not generalise by saying that HGBP be 50% of total capacity but it should abide some calculations

[US Iceman]

i think we should not generalise by saying that HGBP be 50% of total capacity but it should abide some calculations

I am not saying we should generalize when it comes to system design. You need to do the calculations for all applications.

The point I was attempting to make was that the percentage of hot gas required for bypass will be approximately equal to the minimum step of capacity reduction available on the compressor(s).

The reason for the ASHRAE standard for energy use in buildings is due to the wasted energy to provide part load control with hot gas bypass. The hot gas only provides a false load to keep the compressor operating at some suction pressure.