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Discharge temps

Can of worms...thats good.LOL!

Back to discharge temps.Why is it that goodman has you charge their system using discharge temps?

They make claim that at six inches away from the comp at the discharge line you add 126 degree plus ambient temps and
you will come close to a fully charge unit.

Comments

  • don_185
    don_185 Member Posts: 312
    Discharge temps? Professor.

    Professor..could you please explain how discharge temps could be use as another tool in the field.

    Example..R22 system.I was always taught that below 160 degree is flooding back to the compressor.
    Above 212 you're aking for trouble.

    Also why is it that when you go to a system that appears to be low on charge the discharge temps are high?Other times,
    when the system is overcharge the discharge temps are low?

    What about vapor density?






  • Eugene Silberstein
    Eugene Silberstein Member Posts: 1,380
    Can of Worms

    Here you go.

    Since compressors (reciprocating, mainly) are cooled by suction gas, an undercharged air conditioning system will often operate with higher-than-normal compressor discharge temperatures. Since the evaportor superheat will be higher, the temperature of the refrigerant entering the compressor will be higher and there will be less of a cooling effect on the compressor. Higher temperature refrigerant in, higher temperature refrigerant out.

    Your questions regarding the low compressor discharge temperatures and the vapor density of the refrigerant bioth refer back to the lines of constant entropy on our pressure enthalpy diagram. For those not in-the-know at this point in time, constant entropy relates to a constant change in heat content (btu/lb) per unit change in absolute temperature.

    With all other system conditions remaining the same, lower compressor discharge temperatures are typically the result of low evaporator superheat. In addition, a well-insulated suction line will result in very little heat being added to the refrigerant from the point it leaves the evaporator to the point where the refrigerant enters the compressor.

    For example, if an R-22 evaporator has refrigerant boiling at 40 degrees F and has refrigerant leaving the evaporator at 40 degrees F, the coil is operating with no superheat. Let's also say, for arguments sake, that the inlet of the compressor is very close the outlet of the evaporator and the very short suction line is insulated very well. From this, let's say that the temperature of the refrigerant entering the compressor is 42 degrees F. Since we only have 2 degrees of system superheat, the amount of heat added to the refrigerant once it has completely boiled off in to a vapor is very low.

    This will put the point at which the refrigerant enters the compressor dangerously close to the saturation curve on the pressure-enthalpy chart. The result will be a low compressor discharge temperature.

    Another word on compressor discharge temperature. If you are working with tin can type compressors, the compressor dischage temperature cannot be accurately measured at the discharge port on the tin can itself. These compressors are referred to as low side devices, as the shell is filled with suction gas. The "compressor" itself is located within the shell and the discharge line carries refrigerant to the outer shell of the device through copper tubing. As the refrigerant flows through this tubing, a good deal of heat is transferred from the discharge gas to the suction gas that is filling the shell. Since there is so much more suction gas (by volume) than discharge gas, the resulting temperature increase of the suction gas is not that drastic.

    So, if you are attempting to plot a system on a pressure-enthalpy diagram and you are using a tin-can reciprocating compressor, you are better off taking a temperature reading at the inlet of the compressor and working your way up the lines of constant entropy as opposed to taking a reading at the "discharge" port and working your way down.

    Now, about the specific volume issue.

    With all other conditions the same, an increase in the compressor discharge temperature will result in an increase in the specific volume of the refrigerant entering the compressor. This means that the refrigerant is becoming less dense and more cubic feet of vapor will be required to make up one pound of refrigerant. Since the compressor is a vapor pump and the amount of refrigeration depends on the number of pounds of refrigerant that are moving through the system, the compressor will have to move more refrigerant to obtain the same refrigeration effect. In plain English, the higher the compressor discharge temperature, the lower the system efficiency.
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