Air Conditioning on a 172?
By Greg Napert
July / August 1998
ant to surprise a pilot next time you see them stepping out of a Cessna 172 with sweat dripping off their forehead? Ask them if they want you to install an air conditioning system.
Although it's been tried before, a system has finally been developed for installation at the Cessna factory for new order airplanes, or as an STC'd kit for installation by any certified technician.
According to Mike Fitch, sales manager for Keith Products, the Cessna 172 air conditioning installation weighs 58 lbs and shifts the center of gravity aft about one inch. With this little of an effect on the aircraft, there is no change in the aircraft weight and balance limitations or significant performance degradation in the airplane. "Obviously," says Fitch, "you have to decrease the amount of baggage you can carry by what the evaporator weighs, which is about 18 lbs. This is because the compressor, mounted out in front by the engine, balances out a portion of the installation weight and the resultant change is approximately 18 pounds."
The air conditioning installation is fully integrated. Fitch says the company ran engine tests with the compressor installed. Additionally, there is no cowling modification needed to accommodate the compressor.
The only change in appearance to the aircraft is a condenser exhaust cutout and a condenser cooling air intake towards the rear of the aircraft.
The interior modifications include ductwork along the floor; a small reduction in space in the baggage compartment (about 1 1/2 cubic feet), to accommodate the evaporator and evaporator blower; and a few changes to the pedestal to accommodate cooling ductwork and two air conditioning outlets. The evaporator in the baggage compartment is designed with a cover that allows you to stack baggage right on it, and to protect the evaporator coil and cooling fan. There is no thermostat in this system; it is either "on" or "off," Fitch says. "The reason for that is the addition of a thermostat just increases the installation time, the kit complexity, and the kit cost, and we're trying to keep the price down to where the owner operator can have the entire system installed in the $10,000 to $11,000 range.
Fitch says the system works exceptionally well in the 172 as there is a relatively small amount of space to cool. "It immediately starts blowing air that is about 30 degrees lower than the ambient air temperature," he explains. "Even with the engine at idle, the unit maintains a 30 degree drop in temperature at the outlet." Additionally, as the unit continues to run, it recirculates the cabin air to continue to drop the temperature. "In fact, you typically have to turn the unit off after about 30 minutes even at very hot temperatures," Fitch says.
In operation, Fitch explains that the unit cools you down as you taxi down the runway — then you turn the unit off before take off, which disengages the clutch on the compressor; roll down the runway and take off, and, as you're on climb out and firmly established in the climb, then you can re-engage the air conditioning.
Fitch comments, "Typically, one of the first questions we get asked regarding this installation is "Where is the scoop?" That's because many people who associate air conditioning with general aviation aircraft, remember the days when some manufacturers would have a scoop that lowered down into the air stream which allowed air to flow over the condenser coil. Well, that's fine and good, except that when you lower a scoop into the airstream, you're adding quite a bit of drag to the airplane, to the extent that you would lose five to seven knots of your cruise speed. That is not a good situation, particularly on climb out, when performance is marginal on many small aircraft.
"In our installation, we have no scoops, so there is really no performance degradation, relative to cruise speed," he says.
Keith's condenser cooling is accomplished with a fan/blower placed in the tail of the aircraft.
To review the air conditioning cycle — the compressor compresses the refrigerant to a high temperature gas, and it's passed through the condenser coil where it condenses to a high pressure liquid, and heat is carried away from the coolant. The liquid is pumped through the receiver/dryer bottle, where the refrigerant is filtered and moisture is extracted. At that point, it passes through the expansion valve on the evaporator where it expands back to a gas, and that's where you get the cooling effect from. Air is then drawn through the evaporator and blown into the cabin, via the air outlets mounted on the instrument panel and the center ducts.
According to Fitch, "The air conditioning installation is a fully STC'd kit, so any A&P technician can install it. It can either be ordered directly from Keith, or from a Cessna authorized dealer. The kit includes a complete set of installation drawings, a copy of the STC, the weight and balance information, continued airworthiness information, and general service and maintenance manual so that you know what kinds of things to check, how often to check them, and how to ensure the system is going to continue to perform well years on down the road."
Fitch goes on to say, "One of the really nice things about today's aircraft air conditioning is that if it's working, you don't really don't have to do anything to it. The evaporator blower and the condenser blower do not have a brush inspection criteria because it's a sealed motor and it's Ôoperate until failure.' Incidentally, the mean-time-between failures on these motors is something between 900 and 1,200 hours."
Speaking on the safety of the system, Fitch says "The system is also very safe to operate as failure of any of the components means that the system simply quits working. For example, if the evaporator blower stops working, the system still functions, but you won't get any airflow out of it. In the case of a condenser blower failure, you are still going to have airflow, but it's going to be hot air, because there's no way for the condenser blower to exhaust all that heat away from it, so it starts to build up heat and the refrigerant temperature never goes down. The greatest danger there would be that you would see increasingly high compressor discharge pressure. What will most likely happen in that case is there is a binary high/low pressure switch in the system that disengages the compressor in the event the condenser blower stops working, so that you don't have any damage to any components. A backup safety feature that is incorporated with all of our systems is what's known as a fuse plug, or blowout plug in the receiver/dryer bottle that gives way in the event of escalation of compressor discharge pressure. If this plug blows, the R134A refrigerant is simply vented overboard."
He explains, "In the event the blowout plug blows, you would simply need to determine why you had the high pressure situation, and, if in fact it's a condenser blower failure, you know you have to replace the condenser blower, and the only other components you would have to replace is the receiver/ dryer bottle and then recharge the system."
In terms of what you need to install the system, Fitch says "Certainly you have to have some sheet metal capability. The installation of the doubler requires you to cut holes in the aircraft according to the installation drawings and riveting in of doublers that are included in the kit. The kit is designed to fit into the aircraft without any modifications, but in some instances there may be some trimming and fitting that is required."
Fitch states "Additionally, you have to have some basic electrical knowledge to wire the air conditioner into the aircraft electrical system according to the electrical schematics. The electrical hookup is very simple — you've got two wires on the pressure switch, you've got two wires on the evaporator blower, you've got a couple of wires on the condenser blower and you wire them up to the switch panel. The switch on the switch panel includes a two-speed fan switch, and a vent switch, which allows you to turn the evaporator blower on so that it's just blowing fresh air out of the same outlets that you use for the air conditioning. There is no modification of the existing electrical system as it is quite capable of handling the additional four or five amps required. There are some restrictions for operation, however. For example, you can't operate the air conditioning with the pitot heat on — but then, why would you want to?" says Fitch.
Maintenance and servicing of air conditioning systemsJuly / August 1998
Michael Fitch, sales manager for Keith explains, "There are essentially three major components in a vapor cycle air conditioning system that make the cold air: the compressor, condenser, and evaporator. You have the gaseous refrigerant that is compressed by the compressor, sent through the condenser so it condenses to a high pressure, low temperature liquid. The refrigerant then circulates through the receiver/dryer which has three distinct purposes: It is a filter (it helps to clean and filter the material), it acts as a dryer (actually absorbs moisture that accumulates in the system), and it acts as an accumulator (where it builds up liquid refrigerant to meet demand). "The liquid then travels downstream from the dryer bottle into the evaporating unit where it evaporates and gives up all of its heat."
Fitch adds, "Major components of the evaporator unit consist of the blower motor, the evaporator coil, and the expansion valve (a very significant part of the system). You get the high pressure liquid flowing into the expansion valve, and this orifice is what measures how much is going to flow into the evaporator coil, which is where the refrigerant expands back to its gaseous state, absorbs heat, and you get very cold air."
On expansion valves, Fitch explains, "With regard to the expansion valves, there are two different types of expansion valves: pressure-regulated valves and temperature-regulated valves. A temperature-regulated valve has a small capillary tube with a sensing bulb that's attached to the header tube of the evaporator. And this is where the low side, low pressure refrigerant flows back to the condenser coil. This measures the temperature of the refrigerant as it leaves. You want to make sure the sensing bulb makes good full contact with the header tube of the evaporator, that it is well insulated, and that the insulation isn't torn off. If it's dangling in space, it's going to provide a false reading to the expansion valve, and it won't be able to react appropriately to what the temperature of the refrigerant is."
Fitch adds,"A pressure-regulated valve does essentially the same thing, only it has no capillary tube. It's got an internal valve spring that senses the pressure of the refrigerant and acts accordingly. The reason these sensors are important is because you need to restrict the flow of refrigerant as the evaporator gets colder and colder in order to prevent icing of the evaporator coil from the flow of too much refrigerant. If you get ice buildup on the evaporator coil, that restricts the airflow across the coil and you lose efficiency of the system. Frosting or icing of the evaporator coil is an indication there is something wrong with the expansion valve or the sensing bulb, or that the capillary line is ruptured, faulty, or has come loose. Also, you will see a very high suction pressure on the low side with a correct or below normal discharge pressure."
Fitch says, "You normally get a lot of condensation on the coil due to the expansion of the gases, so there are drains provided which must be kept clean and free of debris.
Once you have a properly serviced system, there is no need to add or check oil level unless you suspect that oil has been lost from the system from a major leak." "Another item to inspect regularly, is the drive belt,"advises Fitch. "Check it for wear and replace as necessary. As a general rule, the belt should be inspected within five hours of installing new, and then every 100 hours or three months afterward."
Fitch says, "All components should be looked at every 100 hours or three months for any obvious signs of damage. The evaporator and condenser coils especially should be checked for dirt, blockage, or damaged fins, which can reduce the efficiency of the system. Fins can be straightened by using a coil or fin comb. The combs are available from air conditioning repair shops and have different fin spacings to accommodate various fins. Additionally, between 750 and 1000 hours of operation, you can inspect the compressor motor for brush length by removing the shroud and fan blade. You don't have to remove the motor to do it."
Fitch says "For all our compressor motors, we have overhaul kits that either consist of just brushes or brushes and bearings. We also have brush inspection manuals to provide guidance on inspection and replacement of these brushes."
"Finally," he explains, "check the sight glass when necessary to inspect for the proper refrigerant level. One of the good things about air conditioning is that it's basically one of those systems that if it's not broke, don't fix it. In other words, if you're getting cold air out of the system and don't see any degradation of performance from season to season, it's a good idea to leave the system alone and don't do anything until you have to. Certainly, preventative maintenance is a good idea, and it's always a good idea to do a visual inspection, checking drive belt wear, for example, but not much else is required."