Semi-hermetic compressors are frequently used on light commercial air conditioning and refrigeration equipment. With the cast iron body and bolted head, oil pan, oil pump, and end cover that opens the electric motor section for service, certain field repairs can be made. Only two types of repairs should be attempted in the field. Replacement of valve plate assembly and/or the oil pump, if the unit has separate pump. No other type of repair should be attempted in the field.
The semi-hermetic compressor has many of the same component parts as an automobile. To rebuild a compressor thoroughly, special tools and test instruments are needed. If a compressor is not pumping properly, the head bolts and head should be removed carefully from the block. Looking into the cylinders, you can turn the crankshaft and see if all of the pistons move up and down. If there is a broken piston or connecting rod, put the head back on and replace the whole compressor. Some say that a compressor with the above condition would make a lot of noise. In some cases, this is so; however, I’ve come across many compressors that had broken rods or pistons and didn’t make any noise.
If the pistons are alright, examine the valve seats and the valve reeds. Also, examine the head gaskets. There is one gasket between the block and the valve plate and another gasket between the valve.
■ Check the settings on the thermostat. Is unit turned on and is the temperature high enough to be calling for the unit to operate.
■ If unit fails to come on, turn fan control switch of the sub-base to the constant run position. If the fan comes on, low voltage is present in the system. If it doesn’t come on, there is no low voltage in the system. Check at the transformer for secondary voltage. If not present, check the primary.
No primary voltage — Check fuse that supplies the unit.
No secondary voltage — Bad transformer.
Both primary and secondary voltage present – Check at sub-base of thermostat for 24 volts.
Voltage present at sub-base — Replace sub-base and thermostat.
Voltage not present at sub-base — Control wiring from transformer to thermostat defective.
Depending on the unit you are working on, service valves vary and will be different. Figure 26-1 shows some different service valves. Service valves are devices that allow access to the sealed refrigeration system. The most economical way this is done is by crimping. This procedure is done at the factory. A small length of copper tubing is attached to the suction side and high side of the system. When the proper amount of refrigerant charge has been placed into the system, the tubing is crimped and soldered closed. When this is completed, there isn’t a way to get back into the system. In Fig. 26-2 a crimped access is shown.
The next type service valve is called a line-tap-valve. Figure 26-3 shows one of these. This valve is used on soft copper and doesn’t require soldering. It is installed with the use of screws. One of the disadvantages of this type of valve is its potential to leak. A small rubber “0″ ring seals the valve to the refrigerant line and is the weak link in the chain. If used on a discharge line, the valve has a limited life due to the temperature variations being applied to the rubber “0″ ring.
The most common service valve used on small equipment is the Schraeder valve. This valve is pictured in Fig. 26-4 and is very familiar to all for it is used on both the low and high sides of the refrigeration system. Located in the hoses of your service manifolds are indenters. These push the core of the Schraeder valve open when the ‘/4-inch fitting is tightened onto the valve. The actual piece that holds the pressure is the valve core. The valve cores are replaceable with a valve extractor. Caution should be used when this type of valve is used on a liquid line. Liquid refrigerant has a tendency to spew when these valves are opened and closed. Burns will result if refrigerant touches your skin. If a crimped type of seal is being repaired, the Schraeder valve can be used. This valve can be acquired with a small length of copper tubing soldered to it. These access valves are soldered into place instead of crimped in the line.
The packed, angle-type valve is found in more expensive types of refrigeration equipment and air-conditioning systems. As shown in Fig. 26-5 it is entirely different from any of the previous valves mentioned. This valve has an actual valve stem that moves back and forth to open and close a valve from its seat. The valve stem passes through a gland nut on the outside of the valve. The packing within this gland nut keeps the refrigerant from leaking out of the system. A small wrench with a ratchet called a refrigeration service wrench is used to open and close this valve. On most semi-hermetic compressors this type of valve is used as the service valve and king valve on the receiver. Some versions of this valve require an Allen wrench, instead of a service wrench, for adjustment.
This type of valve is the safest to use since you have constant control of the refrigerant.
For the isolation test, you will need four line access Schraeder valves with pig-tails, a drum of nitrogen, and a nitrogen regulator. One feature about the inert gas nitrogen is the fact it is not affected by temperature changes as are refrigerants. The advantage of this in testing is that the pressure placed on the system will be the same when it is checked a day or two later, regardless of what the temperature is. In Fig. 25-1 the system is divided into four segments. Each segment is sealed with an access service valve hard soldered to it.
Nitrogran is then administered to each of the four segments with the use of your service manifold and gauges. Place the exact amount of pressure required in each segment. As stated before, do not exceed the specified pressure testing amounts that the manufacturer had placed on the unit. In an R-22 system, 350 psi pressure is sufficient; for R-12 system, 250 psi pressure is sufficient. The important thing to remember about this test is that the pressure amounts should be the same in the four segments. The nitrogen should be left in the system for several days, the longer the better.
When you return to the unit, place your manifold gauges on each segment and check the pressure. The unit with the leak will naturally have the least pressure. This isolation test is a costly one, but it definitely confirms which section of the system has the leak. Many times, this method reveals the leak to be in the condensing unit that you checked in the beginning. This test eliminates tearing things apart for nothing. When separating the system for this test, remember you have to put it together again, so don’t destroy piping where joints will be needed to place the system back together. If the leak is found to be in one of the lines connecting the condensing unit with the evaporator unit, a new line might be run instead of trying to repair the leak in the original one. This test gives you that option by eliminating any doubt as to where the leak is.
Tests like this one will detect a leak in an evaporator coil or condensing coil. You might then remove it from the unit and seal its ends so that it may be dipped in water, just as an inner tube is tested to find a leak. A leak in a coil might be located directly under one of the fins that is attached to the tubing. Water bubbles will show exactly where the leak was. Then the fins can be cut out of the way to make the repair.
You will be dealing with copper, steel, or aluminum when making your repair. You should be familiar with the three materials. In addition to silver solder and flux, it is advisable to carry a couple of brazing rods. A can of flux or pre-fluxed rods can be used if you store them in a dry place. Many steel parts are being used in the
field such as condenser coils, receivers, accumulators, oil separators, to name a few. A rust hole or small crack can be repaired easily
with the use of a brass brazing roa. The secret in any molten metal repair is to have the surfaces of the piece to be repaired clean and dry. Many coils are being epoxy coated at the factory to extend their service life. This must be sanded from the area to be repaired very thoroughly. Aluminum solder is used extensively in the field today. It is a special solder that needs very low heat. It is expensive, yet in comparison to the cost of a evaporator coil or condensing coil, the price of the solder is cheap. The use of this solder takes practice and is difficult to teach with a written word. Practice on an old coil first. Remember, the surface must be very clean, fluxed, and low heat must be moved constantly over the repair area. The use of the oxygen-acetylene torch is a must in this industry, and you must practice to be proficient at it.
Other means of repairing aluminum have appeared on the market, some work well. A compression type of fitting is being used where a joint of steel to aluminum is made. The device works fairly well. Another type of repair is coupling two pieces of aluminum with the use of a sleeve that is sealed in the line once in position by an epoxy. This adhesive is activated by placing heat on the fitting.
The solenoid valve is an electrically-operated valve. It uses an electromagnetic field to raise a pilot valve. The gas pressure raises the main valve allowing flow. The flow will continue as long as the pilot valve is held off its seat. The valve in Fig. 12-8 is typical. It can be used with liquid as well as vapor or high-pressure gases. In the condensing unit, it can be used for a pump-down liquid-line valve or an anti-slug valve. If used for a pump-down valve, the unit must have a low-pressure control to stop the compressor operation. For use as an anti-slug valve, the valve is wired to close as soon as the compressor stops. It prevents any liquid in a vertical liquid line from dropping down into the compressor. When mounting a solenoid valve, it is good practice to orient the valve for operation in a vertical plane. This would require the inlet and outlet of the valve be in a horizontal plane. The electromagnetic coils are available in 24, 120, and 240 volts.
This type of switch is used to monitor the pressure differential on both sides of the evaporator coil. The sail switch has a small plastic sail that is counter weighted with a small spring to keep the sail off center. When this unit is placed in the supply air stream, air velocity keeps the sail on center. If the velocity drops, the spring pulls the sail off center and opens the switch. This condition can happen if the filter becomes clogged, the fan isn’t moving enough air, or the coil is dirty. The control can be wired to set off an alarm system to alert the occupants, or it can be wired to turn off the system.
A pressure switch works with a diaphragm having a tube sensing pressure on each side of the coil. This switch can be wired just as the sail switch is. Some residential units are equipped with a switch like this which is wired to an indicator light on the thermostat. The device helps avoid refrigerant slugging and decreases the running time of the compressor by telling the owner there is a restriction. Figure 18-3 illustrates these components.
Driers should be used on both the liquid line and the suction line. The driers are designed differently and should be used for the purpose designated. The liquid line drier filters as well as dries the refrigerant as it leaves the condensing unit. The drier contains a desiccant and a strainer or sieve. The refrigerant flowing through the drier gives up any moisture it may be carrying to the desiccant. Any particles of a solid will be trapped and held in the drier. Most of these units are designed with a two-pound pressure drop across it. When a drier starts to become clogged, the pressure drop increases as a first indicator. Frost might form on the drier when it becomes clogged. The primary purpose of the liquid line driers is the protection of the metering device against dirt and moisture. No more than one liquid line drier should be installed in the liquid line at one time. If two or more are placed in series, the pressure drop could be severe enough to impede refrigerant flow. It is recommended that driers be installed on a vertical plane. The reason is that if the refrigerant charge becomes low, it would only flow over a small amount of the desiccant if the drier were mounted in the horizontal plane. In Fig. 12-4 a drier is shown. Notice the directional arrow that designates the direction it should be installed in the liquid line. If the drier is a bi-flow type, the directional mounting is not important because it will function in both directions. This type of bi-directional flow drier was designed for the use with heat pumps due to the reverse flow of the liquid when in cooling or heating mode. The suction line driers are bigger and usually have a Schraeder-type valve on the inlet side of the drier. By taking the pressure at the suction of the compressor and at the inlet of the drier, the pressure drop is easily found. The basic construction of this filter drier is the same as the liquid line. These filters are used to protect the compressor from contamination by acid, metal filings, etc.
Pulsations created by the pistons of a reciprocating compressor can create objectionable noises. When noise level is an important consideration, or hot gas lines are long, mufflers can be used to minimize the transmission of “hot gas pulsations.” Figure 12-3 is an illustration of a hot gas muffler. This component is installed on the discharge line as close to the compressor as possible.
■ Use ohmmeter first to check unit continuity ground.
In checking for a ground in either section, power supply disconnects should be open. This prevents any accident from occurring due to a feed back of power. There are times when only one fuse opens letting electricity flow through the good fuse. With disconnect open, check from the load side of the circuit being tested. In the case of the condensing unit, check the compressor contactor. If a ground is indicated, all of the circuits must be isolated including the compressor, condenser fan motor, crankcase heater, and any other electrical device that may be wired to the unit. This also applies to the evaporator section when it is tested.
At times, a ground can develop in the supply circuit itself that would cause the circuit to open. In cases like these, your responsibility is over. The electrician should be notified that you are to be informed when the power supply circuit is repaired.
The third reason for power supply to open is an overloaded circuit. The demand exceeds the rating of the protection device.
Mitsubishi AC MS-18NN Outdoor Unit Wiring Diagram