# Thread: low back pressure/medium back pressure question

1. ## low back pressure/medium back pressure question

Can someone explain the difference between low back pressure/medium back pressure/high back pressure.
See highlighted portion of document for example. Thanks

2. The higher the temperature of the gas, the more dense it is, the more BTU the compressor is pumping.

3. Low temp=LBP, med temp=MBP. The compressor example you used can be used as a low temp or med temp, but it looses capacity as a med temp.

4. .

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Higher ambient. Makes it harder to get rid of the heat. Crappy way to show capacity. Copeland's charts are the way all mfg should do it.

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licen,

This chart makes no sense. No matter what you call it if you have the same evaporating temp then you should have the same suction pressure. If you have the same suct press and same head press then your capacity should be the same, yet their chart shows different. I would call engineering at the mfg and have them explain it. Please post your answer back so we can all learn

Thanks

MIke

7. The chart is normal...

It shows BTU/h at different saturated temperatures.

Which is, normal.

8. Originally Posted by Phase Loss
The higher the temperature of the gas, the more dense it is, the more BTU the compressor is pumping.

Check that.

The higher the pressure of a gas, the more dense it is. The higher evaporator pressure allows for more heat to be transferred by the refrigerant. Therefore, the higher capacity.

Also, the more heat introduced into refrigerant in an evaporator, the higher the suction pressure. Higher pressure gas in the evaporator indicates heat in the refrigerant, hopefully imparted to the refrigerant while it is IN the evaporator.

9. Originally Posted by timebuilder
The higher the pressure of a gas, the more dense it is.
The higher the pressure of the gas, the higher the Saturated Temperature

10. Originally Posted by Phase Loss
The higher the pressure of the gas, the higher the Saturated Temperature
Are you missing the point on purpose?

Making a gas hotter does not increase its density. Making it colder, OR raising its pressure do that.

In this situation, the gas is more dense because of the increased pressure, not because of the associated increased temperature. We can have an increase in temperature AND an increase in pressure because of the nature of the system which is containing the gas.

A lower temp evap has less capacity because of the low pressure at which it operates, and the less dense gas means there are fewer gas molecules per cubic volume that are present to carry away the heat being introduced into the evaporator.

I just don't want folks to think of an increase in density being associated with an increase in temperature, because if the pressure remains the same, they are inversely, and not directly, proportional.

11. Originally Posted by timebuilder
Are you missing the point on purpose?

Making a gas hotter does not increase its density. Making it colder, OR raising its pressure do that.

In this situation, the gas is more dense because of the increased pressure, not because of the associated increased temperature. We can have an increase in temperature AND an increase in pressure because of the nature of the system which is containing the gas.

A lower temp evap has less capacity because of the low pressure at which it operates, and the less dense gas means there are fewer gas molecules per cubic volume that are present to carry away the heat being introduced into the evaporator.

I just don't want folks to think of an increase in density being associated with an increase in temperature, because if the pressure remains the same, they are inversely, and not directly, proportional.

I deleted my post #4 because I was not up for a HEATED discussion. Simply put, what happens to the gas when you cool your cylinder ? You get more liquid.

12. Correct. When you remove heat from a refrigerant, it changes to a liquid, which is more dense.

Think of our atmosphere. At sea level, the pressure is 29.95 in hg. If you find land that is below sea level, such as death valley, the pressure is greater. The higher pressure dictates a greater air density. The Molecules are literally packed closer together. More molecules in the same space means greater density.

As we go higher in the atmosphere, pressure and density both decrease until we leave the atmosphere behind, and we have no pressure and no density.

You could say that every discussion about refrigeration is heated...

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Originally Posted by mikeweber3
licen,

This chart makes no sense. No matter what you call it if you have the same evaporating temp then you should have the same suction pressure. If you have the same suct press and same head press then your capacity should be the same, yet their chart shows different. I would call engineering at the mfg and have them explain it. Please post your answer back so we can all learn

Thanks

MIke
Timebuilder,

I agree with everything you have said, however my point was .................... If you look at the -30 column (or any column for that mater) it shows a differrent BTU capacity for LBP than MBP. With the same refrigerant and assuming the same condensing temp, and same evap temp, why would the BTUs be different?

Thanks
Mike.

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