General Perimeters for Typical A/C
What should it be? I hear that question quite often. But the reality is, there is no simple answer. If you look over my posts, you’ll see I rely heavily on AHRI, design data, and pressure-enthalpy charts because manufacturers can build just about whatever they want so long as it functions as stated and meets regulations.
There are some typical aspects of air-conditioning design that are dictated by materials of construction and good practice such as line size and sub-cooling. For instance, lines are sized to keep pressure drop low (larger lines) but promote oil return (smaller lines). The sub-cooling will probably fall around 9 – 15 degrees on a split system because they can’t control how much liquid line (added pressure drop) the installer will add. But on a package unit, where they control the entire design, it may operate just fine with 5 degrees of sub-cooling, which reduces the total charge and cost of the unit.
Here I’m going to outline a few aspects of typical air-cooled single stage air-conditioners that won’t get you into too much trouble. (heh, heh, heh).
EER rating: This is actually a pretty useful piece of information if you find it on the unit. The EER is the ratio of the Btu per hour to the Watt per hour energy it takes to achieve capacity. If you know it’s a 5000 Btu/hour unit, and has an EER of 3, then 5000/3 = 1666.7 Watts are needed, divide by 120 volts, 1666.7/120 = 13.9 amps. You now know the total amp draw of the unit.
The EER was tested when the condenser was seeing 95 F air, and the evaporator was getting 80 F air at 50% relative humidity. If your conditions are different you will have a different current draw, but it will be close.
The EER is also NOT the SEER, seasonal energy efficiency rating. The SEER is an average Btu/Watt hour for an air-conditioning season of 125 8 hour days of who knows where. The SEER is used to sell units and appease regulators.
AMPS: For the compressor allow 1 horse power per ton (12000 Btu/Hour). Multiply the horse power by 746 Watts, then divide by the appropriate single phase voltage (120 or 220 volts). For three phase, multiply the volts by 1.27 first, then divide. (it’s 1.73 X .92 X .8 for the factor, efficiency and power factor.)
For total unit, 1.5 horse power per ton of cooling may be more appropriate.
Superheat: System with a thermal expansion valve, TXV, will run 7-10 degrees of suction super-heat. Units with electronic expansion valves, EXV, will run about 4-5 degrees.
If the unit has a capillary tube or fixed orifice it can be anywhere between 5 and 45 degrees depending on the operating conditions. Use the manufacturer tables or a generic superheat chart at the very least.
Sub-cooling: The sub-cooling varies with design but will fall somewhere between 5 and 15 degrees. Check manufacturer data. Usually on one of the panels of the unit you will find a chart like the one below. Use your phone to snap a picture and begin a library of these charts by model, you’ll soon be prepared for the unit that the chart has faded.
Condenser/discharge pressure: For air-cooled units, take the outdoor air temperature and add 30 degrees. Say its 90 outside, add 30, to get 120 degrees, look this valve up in the pressure temperature chart for the refrigerant in the system. For R-22 its 260 psig.
For water cooled units, measure the inlet and exit water temperatures and add them together, divide by 2, then add 10 degrees. For instance, the inlet water is 68 F, and the exit water temperature is 81 F, 68+81 = 149, divide by 2, 149/2 = 74.5, then add 10 to get 84.5 degrees, for R-22 that’s about 155 psig.
Suction/low-side pressure: Measure the air leaving the evaporator. Take several measurements and average (add them up and divide by the number of readings you took). Subtract 15 degrees from the average and look up the pressure from the temperature you got. Say the average was 57 degrees, subtract 15 degrees to get 42 degrees. For R-22 that pressure would be about 69 psig.
Air flow: Air flow should be about 350 – 450 cubic foot per minute per ton of cooling, with 400 cfm per ton being the most common rating when the fan is set to run on medium-high speed (blue tap). So, a 3-ton system will come set up to deliver 1200 cfm at about .3 inches of static pressure.
All that said, none of this will hold true on an inverter drive system unless its operating at 100% speed.
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I worked for over thirty years in the HVACR industry. I have designed, installed, serviced, and trouble shot units of various types throughout the years. The posts here are information based on that experience, I hope you find them useful. If you have a different experience, please comment.