Safety controls are designed for the protection of the component parts of a refrigeration or air conditioning system. The basic controls such as, fuses, circuit breakers, and overload relays were covered in earlier chapters.
The amount of protection a system receives depends upon the purchase price of the unit. In order to remain in a competitive market, many manufacturers have a basic stripped model without safety controls. The more protection a unit has, the more expensive the retail price. Many of these units can be seen in multiple family structures throughout the country. The purchaser is very price conscious when signing a contract buying a home. As long as the air conditioning cools, they are happy. Unfortunately, this equipment is in the out-of-sight, out-of-mind category.
In this chapter, I’ll cover some of the safety controls that you will encounter on some of the units you service, or you might want to install a safety device on a unit that doesn’t have one. Regardless of how complex a control looks to you, remember, there are three variables that can cause it to operate. Pressure, temperature, and humidity, or a combination of these are used to activate a safety control.
I am placing these safety controls into two categories: those that protect the system electrical components, and those that protect the system mechanical components. Only those used on residential and light commercial units will be covered.
Figure 9-1 shows the typical overload relay used for the protection of the windings in the compressor. This type is fastened to the compressor body in a way that heat is transferred from the steel body to the overload sensor. In this way, the Klixon can sense excess heat in the steel compressor shell. There are heaters within the Klixon that generate heat when the current drawn by the compressor reaches a pre-determined amount. In this way, the Klixon overload relay will interrupt the current flow if either condition is sensed. There are slight lag times built into these relays. This eliminates many pre-mature trips upon start up. There are specific sizes to be used with specific compressors. In general, the control will carry an overload for about six to 16 seconds in those that are rated under 100-amp service protection. Relays that service loads over 100 amps would have a trip time of about two to six seconds. This is just another rule-of-thumb to give you an idea of the type of protection that can be expected from this type of control.
Fig. 9-1. Overload relay, clamp-on type.
There is a big difference between this overload relay and the one mounted to the compressor. This type would be found more commonly on the larger tonnage units. I will describe one here in the event you encounter one installed on a smaller unit. Figure 9-2 shows a typical remote overload relay. Notice the size is comparatively larger than the Klixon. This device in most cases can be acquired with either a manual reset or an automatic reset. This kind of overload is called the hydraulic-magnetic type while the Klixon is called the disc type.
Fig. 9-2. Overload relay, remote (simplified view).
The time delay relay has become a very popular safety device. It is a control that is directly responsible for extending the life of the compressor. Its function is to keep the compressor from trying to start until a certain time has lapsed from the time it stopped operating. This gives the refrigerant within the sealed system time to equalize. By doing so, the pistons do not have excessive pressures against them when the compressor restarts. An electric motor trying to start with both a heavy normal load plus high head pressures placed on the pistons causes excessive heat in the wire windings, that eventually result in the insulation breaking down and the windings grounding out, or opening, or cross phasing. The time delay lets the pressure equalize avoiding this problem.
There are many examples that can be cited. Those people that never seem to get the thermostat set just right. They are constantly trying to make it hotter or cooler. All that they are accomplishing is the murder of their compressor. A time delay prevents this. In areas where brown-outs are common, that is when the power company cuts power from one part of the grid to supply another, it is a good idea to install time delay relays. An instant power outage can occur during the switching procedure and cause the compressor almost to stop and then to try to restart. A time delay provides useful protection to the unit in these situations. In areas where there are extremely severe thunderstorms and the lightning strikes cause frequent, short outages, it is also desirable to use time delay protection.
There are solid-state electronic time delays that range in price from inexpensive to expensive, depending what the application is to be. There are mechanical types that include an electric clock that interrupts the control circuit. The clock is coupled to switches that accomplish this. After the designed amount of time is passed, the clock activates a switch that completes the control circuit and allows the compressor to start. In many instances, when time delay relays are installed, it is a favorable condition to wire them into the circuit allowing the condenser fan motor to operate while the compressor is being timed out. The reason is, air movement in the compressor area and across the condenser usually helps to cool the refrigerant, which in turn drops the pressure. Figure 9-3 shows both types of time delays. The electronic device is easier to install and takes less time.
Fig. 9-3. Time delays, electronic and mechanical.
In Fig. 9-4 the typical high pressure controls are shown. One of these controls is primarily used in residential units while the other is used in commercial units as well as some refrigeration (commercial) equipment. The main difference is that one is factory adjusted, while the other may be field adjusted for whatever application it is being used for.
Fig. 9-4. High pressure controls, commercial and residential.
The control was designed to open the control circuit when a preset pressure is reached. This condition of high pressure exceeding factory specifications can be due to inoperative condenser fan motor, defective condenser fan blade, restriction in the discharge line of the compressor, or just an overgrowth of vegetation due to owner neglect. Regardless of the cause, the control opens and stops the compressor from becoming damaged. This type of switch can have a capillary tube that directs the high pressure to a remote switch, or it can be located directly in the discharge line as a diaphragm switch.
Due to Murphy’s Law (if something can go wrong and break down, it will), a back up is needed in the event the high-pressure control malfunctions. A relief valve can be used in commercial units but isn’t found often in light commercial or residential units. Figure 9-5 shows the relief valve. This device is costly and would raise the price of a home system. For this reason, other pressure relief devices were introduced. In Figure 9-6 a typical relief plug is shown. The center of the plug has a hole machined into it. This hole is then filled with solder that melts at the temperature reached when the refrigerant pressure exceeds the specified limits. When the solder melts, the refrigerant is released from the system. In some units, one of the factory soldered joints is made with soft solder. All of the other joints are hard soldered with some type of silver content. In the event of excessively high pressures, the joint will rupture, open, and release the refrigerant from the system before anything is damaged. In commercial systems, where the amount of refrigerant lost would be quite expensive to replace, the costly pressure relief valve is used. This valve only opens long enough for the pressure to drop within specified limits. In this way, only a small amount of refrigerant is lost. The cost of the valve is offset in the savings made by not having to recharge the complete system.
Fig. 9-5. Pressure relief valve
Fig. 9-6. Pressure relief plug, with fusible plug.
You must remember that ample refrigerant has to be in the system so that the compressor has proper lubrication and cooling. The refrigerant is the oil carrier through the system. Each time a compressor starts, it pumps a large amount of oil out of the crankcase. It usually takes several minutes of operation for the oil to return to the crankcase. Also the suction gas returning from the evaporator cools the motor windings of the compressor. So what happens to a compressor that operates without refrigerant? You’re right, it will destroy itself. It is for this reason that a low pressure control is used on some systems. A preset pressure opens a switch when it is reached. Again there are both factory preset and adjustable types. Many of these open when a low pressure about 15 pounds is reached. This control, like the high pressure control, can have a capillary tube or a direct mount diaphragm. The high and low pressure controls look alike. One way to tell them apart is by the size of their diaphragm. The high pressure control has a small one and the low pressure control has a large one. Plus the scale of pressure will be different. In commercial applications, a dual-pressure control can be used. This is a combination of both controls enclosed in a common housing. The added advantage to this control is that it will stop the machine when it still has a positive pressure within the refrigeration system. This prevents air from entering the system and introducing moisture to it. The service technician is usually called before atmospheric pressure is reached inside the system.
This is a small bimetal limit switch that opens on temperature fall. It can be located in the outdoor condensing unit. This switch can be wired to prevent the compressor from operating on those days when the outside temperatures are low.
This bimetal switch is wired to electric strip heaters to prevent them from energizing on temperate days. Both of these devices became popular during the energy crisis. I’m telling you about them if you choose to use them, or if you should come in contact with a system that uses these devices.
This control can be one of the most important in some geographical areas, especially the cooler latitudes. In order for an air conditioning system to operate properly, the condensing temperatures are operating within certain limits. If the limit is exceeded on either side of the scale, too high or too low, the efficiency of the unit is lowered. For his reason, the temperature of the condenser can be controlled by cycling the condenser fan motor. Some commercial units of higher tonnage may also use a damper device to prevent a cold prevailing wind from blowing across the condenser coil causing the head pressure to remain too low. The damper is actuated by the rise and fall of the head pressure. In some residential units, a two-speed condenser fan motor is used to attain better control. The fan-cycling is accomplished with one control. The fan is either off, low speed, or high speed.
I’ve been asked, “why don’t they just open the windows?” In certain circumstances, perhaps they would, however in many cases it would be impractical. This type of control holds true for low ambient operation of refrigeration equipment such as walk-in-freezers and ice making equipment.
This control is found on heat pumps. The device has either a temperature or pressure-actuated clock that places the condensing unit of a heat pump into a defrost cycle when the ice buildup on the condenser inhibits its performance. The cycle is terminated with the same control when a high limit is reached. This control has a switch on it that also might energize the auxiliary heat strips located in the electric furnace of the evaporator section. This is discussed in detail in the chapter covering heat pumps.
The air conditioning manufacturers usually have several models to be price competitive throughout the world. Depending on the model you own or are repairing the number and quality of the control devices will vary. Many manufacture some of their own controls, but for the most part, there are a few companies that manufacture controls for the different air conditioning units. I’ve found that in certain areas, some brands of controls are more dominant than others. Remember, they all do the same thing, basically. There are many other controls on the market today, and if you look over a control catalogue, you can learn how to add safety devices to a system that has none.
