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Quiz

to those great test question you used to ask Professor?
I sure do miss them.

How about some question on HOC,HOR,RCR,RF.

Why is it when suction vapor is less dense that it take more compressor cylinder volume to pump the same mass flow?

Comments

  • Eugene SilbersteinEugene Silberstein Member Posts: 1,380
    In response to your question

    ...about less dense refrigerant requiring more compressor cylinder volume...

    As the suction pressure of the refrigerant drops, assuming other conditions remain relatively constant, the specific volume of the suction gas increases. This means that it takes more cubic feet of vapor refrigerant to make up one pound of refrigerant. In other words, the density of the refrigerant decreases.

    The Net Refrigeration Effect of the system is the number of btu that one pound of refrigerant can hold at the given operating conditions of the system. The NRE is in the units of btu/lb.

    In order to get one ton of refrigeration, we must move 200 btu/min, so 200 btu/min = 1 ton.

    So, we get:

    (200 btu/min/ton) / NRE(btu/lb) = lb/min/ton

    So, if we divide 200 by the NRE we get the required mass flow rate in pound per minute to give us one ton of refrigeration.

    Example:

    Let's say that the refrigerant at the inlet of the evaporator has a heat content of 40 btu/lb and the refrigerant at the outlet of the evaporator has a heat content of 100 btu/lb. The NRE for the evaporator is 60 btu/hour (heat content at the outlet - heat content at the inlet)

    This means that, in order to get one ton of refrigeration, we need to circulate 3.33 lb/min of refrigerant through the system.

    Now, since the specific volume of the refrigerant increases as the suction pressure of the refrigerant drops, it will take more cubic feet of refrigerant gas to make up each pound of refrigerant.

    For example, let's say that the specific volume of the refrigerant is 2 cubic feet per pound. If we needed to move 3.3 lb/min (as in our previous example) we would have to move 6.6 cubic feet of refrigerant per minute. Now, if the specific volume increased to 3 cubic feet per pound, we would now have to move 9.9 cubic feet of refrigerant per minute to move the same 3.3 pounds...

    Get it?

    We do really neat stuff like this at the seminar and the cool thing is that you get a disc with all of the formulas on it so all you have to do is plug numbers in and the programs spits all the answers back at you.

    How neat is that?
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