Tech method lesson 1b.txt

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Tech method lesson 1b.txt
2013-05-24 06:49:49


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    • author "me"
    • tags "Cooling 101"
    • description ""
    • fileName "Tech method lesson 1b"
    • freezingBlueDBID -1.0
    • Subcooling measurement
    • When you remove heat from a vapor, it condenses into a liquid.
    • too much refrigerant in the high side will cause refrigerant to back up in the condenser limiting the ability of the condenser to reject heat.
    • This quickly raises the condensing temperature and allows more time for the liquid too cool before reaching the outlet of the condenser.
    • Resulting in high subcooling.
    • The higher the subcooling the more refrigerant that is contained in the high side of the system.
    • Some subcooling is required to ensure a steady flow of liquid to the metering device however too much will reduce the capacity of the condenser.
    • Cold air flowing through the condenser coil increases its ability to reject heat and increases its capacity.
    • Under cold ambient conditions it is necessary to simulate design ambient temperature by blocking off part of the coil to achive a condensing temperature of 100 - 110 degrees.
    • Most gauges have both temperature and pressure scales get in the habit of reading the temperature instead of pressure think in terms of condensing temperature not head pressure.
    • In most systems anything over 15 degrees subcooling is considered excessve consult the manufactures specifications.
    • Excessive subcooling is caused by;
    • Noncondensables,
    • Overcharge,
    • Restriction.
  1. Noncondensables
    • Air or other noncondensables add pressure to the system without raising the temperature which distorts the pressure/temperature relationship.
    • Pump the system down to ensure all the refrigerant is in the condenser coil then disable the compressor and run the outdoor fan till air in/out of the coil are the same when compare the pressure to a PT Chart if there is a higher pressure than the temperature (5 degrees) then there are non condensable in the system.

    • Symtoms of non condensables in the system.
    • Low side = normal side pressure and saturation temp. Normal superheat.
    • high side = high discharge pressure, high subcooling
  2. Excess refrigerant
    • If subcooling is high and the system does not contain non-condensable then we have excess refrigerant in the high side of the system.
    • The system may be overcharged or the excess refrigerant has been borrowed form the low side (restriction)
  3. Over charge
    • If the subcooling is excessive we must check superheat to dtermine if the system is over charged or restricted.
    • Excessive subcooling with a high superheat would indicate a restriction.
    • Xcessive subcoolng with a low superheat (fixed metering device) or normal superheat (txv) would indicate an over charge.
  4. Condenser airflow
    • Measure the air temperature entering the condenser and the air temperature leaving the condenser the difference should be no more than 20 degrees.
    • The slower the air moves across the condenser coil the more time the air has to absorb heat.
    • Causes;
    • Dirty coil, improper fan blade or position in shroud, wrong RPM motor or wrong rotation motor.

    High efficiency unit is not more than 15 degrees dfference.