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Checking system charge on start up

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  • TGO_54
    TGO_54 Member Posts: 327
    Checking system charge on start up

    Hi Professor,

    When starting up a new installation with a fixed bore metering device, and the indoor air temp is very high (85 to 90) what is the best method to check the system charge?

    What about with a TXV?

    How much will humidity affect the readings?

    Thanks in advance

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  • Eugene Silberstein
    Eugene Silberstein Member Posts: 1,380
    System Charge

    Here we go! (Are you sitting down?)

    There are a number of factors that are taken into account and/or calculated when the refrigerant charge on an air conditioning system is evaluated, either on initial startup, general service or after a repair. These factors include relative humidity, evaporator superheat, condenser subcooling, temperature split across the evaporator coil (referred to as the delta-t across the evaporator coil), temperature difference between the return air temp and the evaporator saturation temperature (referred to as the delta-t of the evaporator coil), ouside ambient temperature, system efficiency rating and the type of metering device. Whew!

    Depending on the type of metering device being used, the system factors that must be monitored in order to ensure that the refrigerant charge is correct will vary.

    Now, to adress the first part of your post.

    If the system is utilizing a fixed-bore metering device, we use evaporator superheat instead of condenser subcooling. The main problem with using evaporator superheat under high load conditions is that the refrigerant will boil off in the evaporator vey quickly, causing the evaporator superheat measurement to be excessively high. This may cause you to add too much refrigerant to the system. Be careful. However, there are other factors that, along with the superheat, will drasticaly increaseyour chances of getting it right the first time. Isn't that what we want anyway?

    Take a deep breath.

    Since the system is utilizing a fixed bore metering device, it is relatively safe to say that the system is not a high efficiency system. As PART of our evaluation, we can estimate that the condenser saturation temperature will be in the range of 30 to 35 degrees higher than ambient. If the indoor temperature is 90 degrees, it is also safe to assume that the outdoor temperautre is also in the 90-degree range. The condenser saturation temperature should therefore be in the range from 120 to 125 degrees.

    In addition to this, the temperature differential ACROSS the evaporator coil should be in the range from 17 to 20 degrees, assuming that the relative humidity is about 50%. If the humidity is higher than 50%, expect the delta-t across the evaporator coil to be a little lower than the 17 to 20 degrees just mentioned. If the humidity is lower than 50%, the delta-t will be a little higher.

    In addition to the in addition, the delta-t OF the evaporator coil should be about 35 degrees. As an exaple, if the temperature of the return air is 85 degrees, the evaporator saturation temperature should be about 50 degrees, which is 35 degrees lower than the return air. (The numbers are different for R-410a systems). For an R-22 system, this equates to a suction pressure of about 84 psig.

    HERE'S THE IMPORTANT PART (not that the other stuff isn't important)!!!

    Never charge a system by looking at only one of the above factors! A suction presusre that is lower than the desired 68.5 psig at design conditions does NOT necessarily mean that the system is undercharged. On the same note, a pressure higher than design does NOT mean that the system is overcharged. You must evaluate as many factors as possible to ensure that the charge is correct.

    If the system is operating with a TXV, use condenser subcooling onstead of the evaporator superheat. THe reason for this is because the only thing that a TXV is responsible for is maintaining evaporator superheat. THE TXV HAS A ONE-LINE JOB DESCRIPTION, WHICH IS TO MAINTAIN CONSTANT EVAPORATOR SUPERHEAT! Therefore, we can't use superheat as a reliable method to check the refrigerant charge on systems that are equipped with TXV's.

    So, use the condenser subcooling, which should be between 15 and 20 degrees (Of course, if the manufacturer recommends different numbers be sure to use them!), the delta-t of the coil, the delta-t across the coil and the other goodies that were mentioned above.

    Hope this helps (And I know it will!)


  • TGO_54
    TGO_54 Member Posts: 327
    Thanks Professor

    The system in question is rated at 12 Seer, I was suprised to see a piston type metering device but I verified that the system componets were matched and correct.

    You mentioned that the condensor saturation temp should be 30 to 35 degrees higher than outside air temp in a non high eff system.

    What is the cut off point for high eff?

    How much will the condensor saturation temp change as the SEER goes up?

    Thanks
    Tom

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  • Eugene Silberstein
    Eugene Silberstein Member Posts: 1,380
    High efficiency systems

    On higher efficeincy systems, the condenser saturation temperature can be as low as 10 degrees higher than the outside ambient temperature. For example, if the outside ambient temperature is 85 degrees, it can be expected that the condenser saturation temperature will range from 95 to 105 degrees.

    Higher efficiency systems can also be identified by larger-than-usual condenser coils. The purpose of the oversized coil is to increase the heat exchange surface area, which results in the decreased operating temperatures and pressures.

    Not to get too technical, but the EER rating of an air conditioning system is 3.413 times the Coefficient of performance of the system. The COP is defined as the net refrigeration effect, in btu/lb, divided by the heat of compression (HOC), in btu/lb. By decreasing the condenser saturation temperature, the HOC will drop, resulting in a rise in system EER.

    If this last paragraph is totally confusing, stay tuned for information about upcoming air conditioning seminars that will address the hot topic of pressure-enthalpy.
  • don_144
    don_144 Member Posts: 27
    Will we

    be discussing kinetic energy,and the french engineer....
    Sadi Carnot?

    Can I get the seminars on a cd please.



  • Eugene Silberstein
    Eugene Silberstein Member Posts: 1,380
    If all goes well...

    If all goes well, we'll be doing them all over this great land. Where are you located Don? No plans for seminars on CD, yet.
  • don_144
    don_144 Member Posts: 27
    We're

    in the tidewater area of Virginia.And if you do get close give us a shout,will look forward to meeting you.



  • Eugene Silberstein
    Eugene Silberstein Member Posts: 1,380
    Duly noted

    Thanks.
  • Eugene Silberstein
    Eugene Silberstein Member Posts: 1,380
    Springtime is a possibility

    springtime (mid to late May) is a possibility. Where in the great southwest are you?
  • Bob Sweet
    Bob Sweet Member Posts: 540
    New Mexico

    Eugene, Coversions from Evaporative coolers to A/C is about the hottest thing going out here next to the 98* temps in the shade. We're doing conversions at about 4 per week(this includes major duct rennovations to accomodate the difference in static press,air flow etc. between Evaps and A/C) as are most hvac company's. It would be a GREAT place for someone of your exp. to educate a VERY fast growing and sometime's ill-informed workforce. Hope to see you, if not here someplace nearby.
  • Eugene Silberstein
    Eugene Silberstein Member Posts: 1,380
    E-mail

    Bob,

    Can you please e-mail me with your contact information so I can add it to my list to keep you updated about dates, locations, etc of any future seminars.

    Thanks for your interest and keep up the good work!
  • Bob Sweet
    Bob Sweet Member Posts: 540
    Eugene, Thank you

    I will forward info. asap.
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