The controls applied to any airconditioning system, including those found on railway cars, must be carefully designed, not only to supply cooling on demand, but to protect the compressor from compressing condensed liquid coolant (called slugging), from running short of oil, and against high and low pressure conditions.
The one method most commonly employed to prevent slugging is called single pumpout. This technique uses the solenoid valves on the evaporator to shut off the coolant flow to the evaporator while the compressor is allowed to continue running. This causes the compressor to suck most of the coolant out of the return side of the system before it shuts off from low pressure, which means that, when it eventually does shut off, there won't be enough coolant left in the return line to condense and form a slug of liquid.
A crankcase heater is often installed to prevent the compressor from running out of oil upon startup. How does this work? If the compressor sits for a long period of time and the oil in the sump is allowed to cool off (this is especially possible if the compressor is riding along in a nice, cool stream of air underneath a railway car), it can pick up quite a bit of dissolved coolant in the oil (you might notice the oil level rise substantially when this happens). When the compressor starts up and immediately pulls a partial vaccuum on the oil in the sump, the coolant outgasses (much like a bottle of soda does, when opened quickly) and whips the oil up into a foam in the process. This foam is easily sucked out of the compressor sump and pumped into the system piping, causing the compressor to be starved of oil. Keeping the oil hot with a heater forestalls this problem.
Head pressure controls are employed to protect the compressor from excess pressure or too low pressure. The former might occur because a solenoid valve sticks shut, while the later might occur because of a coolant leak. In the first case, there is a very real danger of blowing the heads off of the compressor, which is an extremely dangerous situation (I've seen a valve blow out on an R-22 system while a car was standing in a station -- the people on the platform were running for cover and rightly so -- there are very high pressures inside airconditioning systems and they want to get out with a vengance). In the second case, losing the coolant will take the charge of oil with it and the compressor will quickly destroy itself from lack of lubrication. Either way, head pressure controls are very important.
Finally, a good system will allow for changes in cooling demand, through a split evaporator with two solenoid valves and pressure unloaders on one or more of the compressor cylinder heads. This will allow part of the evaporator to be closed off to reduce cooling (in times of low demand) while still providing a measure of dehumidification (sometimes more important than cooling).
The drawing Electric_ACCtrl can be downloaded as a PDF. This drawing shows the electrical hookup for the compressor, condenser fans, solenoid valves, etc.
In the drawing, 240V line power (L2) is connected to TB3, while the other side of the line power (L1) is connected through a differential pressure switch in the plenum, downstream from the blower, so that it closes only when the blower is on. This powers the airconditioner only when the blower is running so that it is not possible to run it with no air and, as a result, freeze up the evaporator.
Power from L2 is also fed through a line voltage thermostat to TB2. Applying power to TB2 will start the airconditioner. Note that a time delay relay should also be connected across the thermostat, such that it will cause the compressor to run for a minimum of eight minutes to prevent short cycling.
When the thermostat applies power to TB2, relay RY6 closes, removing power from the crankcase heater. Power is applied to CT8, which is latched closed through its holding contacts, and one or both of the evaporator solenoid valves, thereby admitting liquid coolant into the evaporator through the expansion valves. The main contacts of CT8 apply three phase power to the evaporator fans and the compressor and cooling begins.
Note that there is a momentarily closed time delay relay (TD5), whose contacts bypass the pressure switch. Set at a 30 second interval on delay, this relay cuts the pressure switch out until the compressor has enough time to build up pressure past the minimum pressure cut out. After this time delay relay opens, any drop in pressure will remove power from CT8 and shut down the system.
When the thermostat stops calling for cool or the minimum 8 minute cycle time runs out, power is removed from TB2 and relay RY6 opens. This causes the evaporator solenoid valves to close immediately and the crankcase heater to turn on.
The compressor continues to run, since CT8 is held closed through its holding contacts. Since the solenoid valves are closed, no liquid coolant can enter the evaporator. The compressor begins to evacuate coolant from the evaporator and suction line. When the pressure therein drops sufficiently, the pressure switch opens and shuts down the compressor. By that time, only a small amount of gaseous coolant remains, not enough to liquify and form a compressor slug.