Cap Tube Charging

Questions on cap tube charging (one of which most recently appeared in the April 7, 2003 Hotline) have been kicked around for a long time. So I would like to set the record straight on how to charge a cap tube/fixed orifice system. I believe you will find all manufacturers will support this method.

The best way is to measure (weigh) in the known amount. If you do not have an electronic scale for one reason or another, I suggest you look at another career. Charging by superheat is the most convenient method. To do this properly, you need two temperatures: entering condenser air drybulb temperature and entering evaporator wetbulb temperature.

With the manufacturer’s superheat information, sometimes provided within the unit, you would charge to the required superheat. Also, when charging to the required superheat, if the other operating information (such as pressures, subcooling, condenser air delta T, evaporator air delta T) is not falling into place, you have problems other than the system charge.

I know of one manufacturer that provides entering condenser drybulb temperatures from 115 degrees down to 55 degrees; entering evaporator wetbulb temperatures range from 76 degrees down to 50 degrees. If you try charging outside of these ranges or try to affect the entering condenser air, it is one big guess.

On Contactors

I read the Feb. 3, 2003 Hotline question about the single-pole contactor and thought the following information may be helpful.

On most single-phase compressors with a single-pole contactor, there is a solid bar on one side of the contactor instead of having two contacts. One leg of power passes down the bar to the capacitor and to the run winding. It passes through the run winding of the compressor and from common back to the contactor. It lands on the load side of the normally open contact of the contactor. At off cycle, line voltage will be read from one side of the normally open contact to the other: Line 1 on the line side and Line 2 on the load side via the windings.

The crankcase heater will be attached to the contactor with one leg on the load side and one on the line side of the normally open contacts. At off cycle it will have line voltage applied to it and will heat the crankcase. Once the contactor is energized, both the line and load side of the normally open contacts become one. Since you cannot feed a load with one line of power, the crankcase heater will not work again until the contactor opens. This is a simple way to turn the crankcase heater off during the on cycle and to turn it on at the off cycle. Exercise caution when checking this type of setup because power is always present on the load side of the contactor.

COMMENT:
By Neal Broyles
HVACR Instructor
Rolla Technical Institute
Rolla, Mo.

An answer concerning contactors stated that a “neutral leg” can be opened. It should be noted that in a single-phase condensing unit, there are two hot legs and no neutral. Breaking one leg still leaves potential at the compressor connections and windings through the other leg. The 208 VAC will be measured across the open contacts. Breaking both legs is more of a safety issue than one of performance.

If you have a technical question, fax it to 847-622-7266 or submit it online on The Hotline page.

Publication date: 05/05/2003