Heat and Heat Transfer

Condensation from AC either on the coils themselves or somewhere along the ductwork can be the source of mildew odors in forced-air heating and cooling systems and can be a real pain to locate and cure.

All matter contains heat, a form of energy. Heat causes the molecules in the matter to move: The more heat, the greater the movement of the molecules.

The form matter takes-solid, liquid, or vapor-is dependent on the amount of heat it contains. For example, when enough heat is added to water, its state changes to vapor, or steam.

Even cold matter contains some heat. This is how a home heat pump is able to operate.

Though the outside air feels cold to us in winter, a heat pump can remove heat from this air and release the heat into the home.

Heat is removed from the air enters the passenger compartment and is released into the atmosphere via the condenser in front of the radiator in an A/C system. Heat always moves from a warmer to a cooler environment.

Evaporation and Condensation

Unlike the coolant in a car’s radiator, boiling the refrigerant in the A/C system is a good thing.

In fact, when the system is operating, refrigerant is constantly boiling in the evaporator and condensing back to liquid in the condenser.

This is desirable because of an amazing thing that happens when liquid changes to a vapor state: It absorbs a tremendous amount of heat. Conversely, when vapor condenses into liquid, it gives off a great deal of heat.

Pressure, Temperature, and Changes in State

If any given liquid boils and condenses at the same temperature, how can heat be removed from one location and then disposed of in another?

The answer is in the relationships between pressure, temperature, and changes in state.

When the pressure acting on liquid increases, so does the boiling point of the liquid, and if the pressure acting on a liquid is lowered, the boiling point is likewise lowered.

For example, putting an automobile’s engine cooling system under pressure increases the coolant’s boiling point substantially over its boiling point at atmospheric pressure.

Conversely, water boils at a lower temperature on top of a mountain, where air pressure is lower. Pressurizing a vapor also increases its temperature.

This is an important factor in the condensation stage at the condenser. The higher temperature increases the differential between the vapor and the ambient air, aiding condensation from ac and heat transfer to the atmosphere.

Two Sides to the Story

An A/C system is split into two sides: the low-pressure side and the high-pressure side.

The low pressure, cold side works to remove unwanted heat from the passenger compartment, and the high pressure, hot side releases this heat into the atmosphere.

The high-pressure side begins with the compressor output, continues through the condenser, and terminates at a flow restriction.

At the flow restriction, the low side begins. The low side includes the evaporator and it continues up to the suction side of the compressor.

High-side lines are generally smaller than low-side lines. The high-pressure lines will be warm or hot to the touch, while the low sidelines will be cool and may collect frost or water droplets on hot, humid days.

Frost buildup not in proximity to the evaporator may indicate a line restriction on the upstream side of the frost.

A/C System Components

Before we trace the refrigerant flow through the system, let’s take a closer look at some typical components of an A/C system. First on the list is refrigerant.

There are currently two refrigerants used in different A/C systems: R-12 and R-134a. R-12, or Freon, was used for many years and was efficient and inexpensive, but due to environmental concerns, it is being phased out.

Vehicles manufactured before 1992 use R-12.Vehicles were made in 1995 and later used R134a. During the transition years, both of the systems were used.

The two refrigerants are not interchangeable. An identification label can be located usually on the compressor or elsewhere under the hood.

To prevent contamination of systems and service equipment, the service valve design was changed for the R-134a systems. R-134a system service valves are either quick to connect fittings or metric-threaded.

Most R-12 systems can be retrofitted for R-134a, so be on the lookout for converted systems.

If an R-12 system has sustained significant collision damage or another component failure, it may be a good candidate for retrofitting. R-12 is becoming prohibitively expensive as available quantities dwindle.

For a refrigerant to be efficient, it must have a very low boiling point. The boiling point for R-12 at sea level is -21.70 F, and the boiling point for R-134a is -15.1’F, making them good heat transfer mediums.

Beware of refrigerant blends and hydrocarbon-based refrigerants, such as OZ-12 or HC-12a. In addition to being extremely flammable, they may damage system components. Only R-12 and R-134a are approved for use in their respective systems.

Additional system components include:

Refrigerant Oil-A/C systems carry a charge of oil to lubricate the moving parts in the compressor and certain control valves.

The oil combines with the refrigerant and freely circulates throughout the system. R-12 systems use mineral oil, while most R-134a systems use a synthetic oil called polyalkaline glycol, or PAG.

Other oils may be used in some applications. The oils for R-12 and R-134a systems are not interchangeable or compatible.

Compressor: Many types of compressors are used, including various piston-driven designs, rotary vane types, and scroll types. Regardless of the type, the belt-driven compressor’s job is to pressurize the high side and pump the refrigerant through the system.

Condenser: The next component in line is the condenser, which is a heat exchanger. Seated in front of the radiator, the condenser transfers the absorbed heat from the refrigerant to the ambient air.

The condenser is a long tube, folded back and forth several times, usually made of aluminum. Like a radiator, it has fins to help dissipate heat.

Evaporator: Like the condenser, the evaporator is a heat exchanger. The evaporator is usually mounted on the right side of the vehicle, in proximity to the heater core and blower housing.

The evaporator receives air discharged from the blower. where heat from this air is removed.

Flow Restriction: Different systems use different types of flow restrictions, but in all cases, a restriction is used to isolate the high side from the low side and to meter the correct amount of refrigerant into the evaporator.

Located at or near the evaporator inlet, many systems vary the restriction as a means of directly controlling the refrigerant flow. These systems use a thermostatic expansion valve (TXV) as the restriction to control the evaporator temperature.

Another common system uses a fixed orifice tube to meter refrigerant into the evaporator. Most orifice tube systems cycle the compressor on and off according to evaporator pressure (and thus, temperature).

Some systems use a variable-displacement compressor, which may not cycle except for protection from adverse conditions, such as pressure being too high or low.

Receiver-Drier/Accumulator: Drier-A receiver-drier is used to remove moisture from the system and acts as a reservoir for the refrigerant.

Receiver-driers are used on expansion valve-type systems and are usually found on the system’s high side before the flow restriction. The receiver-drier contains a desiccant bag to remove moisture from the system.

The desiccant is a drying agent, like the silica gel contained in those little packets labeled “do not eat” that are found in many new product packages, such as electronics or shoes.

Some receiver-driers have a sight glass mounted on top of the unit, permitting a quick check of the refrigerant level in the system.

A low level is indicated by bubbles in the sight glass during operation. Oil streaks indicate an absence of refrigerant, and a milky appearance is evidence of excessive moisture in the system.

An accumulator: drier is primarily used on orifice tube-type systems. It performs the same functions as a receiver-drier, but it is normally located on the system’s low side at the evaporator outlet.

It performs the additional function of separating refrigerant vapor from liquid to protect the compressor from damage. Some systems, such as those using a suction-throttling valve to control refrigerant flow from the evaporator, may have both a receiver-drier and an accumulator.

Modern A/C systems may have many more components, different system controls, and protective switches and circuitry. Some examples of components that may prevent the system from operating include:

Ambient Temperature Switch: This prevents compressor operation during very cold weather.

Low-Pressure Cutout Switch: This prevents compressor operation and damage when system pressure drops below a certain point.

Wide Open Throttle Switch: This shuts off the compressor for better rapid acceleration.

Some systems include a muffler installed on the compressor discharge line t, reduce compressor noise. Of course, all systems include refrigerant hoses and metal lines. 0-ring seals and/or flared tube ends are commonly used to prevent leaks.

Now that we’ve looked at the basic components of an A/C system, let’s follow the flow of refrigerant through a cycle and see just what’s going on.

Refrigerant Flow in an A/C System

Driving your car on a warm afternoon, the cabin begins to feel a little stuffy and you decide you would like some cool refreshing air. You turn the A/C control on, and the system springs to life.

The compressor engages, and the blower begins delivering a stream of frigid air from the dash vents to your moist brow.

When you switched on the A/C, liquid refrigerant under pressure from the high side began to pass through the flow restriction into the bottom of the evaporator.

When this liquid enters the lower pressure of the evaporator, it boils immediately and draws heat from the evaporator surfaces, making them very cold.

Absorbing more heat from the air flowing around the evaporator coils, the refrigerant becomes a warmer, low-pressure vapor on its way to the compressor.

The evaporator temperature may be from about 26 deg F to 45 deg F, with a corresponding pressure of 22 psi to 40 psi.

The compressor pulls the low-pressure vapor in and compresses it, producing a high-pressure, high-temperature vapor. The hot vapor is pumped to the top of the condenser and begins flowing through the coils.

On its way through the condenser at a higher pressure, the refrigerant condenses back into a liquid and gives up its heat. As the cooler ambient air flows around the condenser coils, heat is carried away.

The condenser temperature may be from about 112 deg F to 160 deg F, at a pressure of 150 psi to 300 psi.

The refrigerant flows from the bottom of the condenser as a high-pressure, medium– temperature liquid on its way back to the flow restriction, ready for another cycle.

A/C System Service

In the automotive paint and body repair business, damaged components are obviously the most common problem.

Smashed condensers and cracked or kinked lines often cause leaks or restrictions. These systems are frequently discharged because of damage before they come into the shop.

If a system has been discharged and opened to the atmosphere for more than a few hours, the receiver– drier/accumulator-drier should be replaced.

It should also be replaced if an internally caused system restriction is indicated, physical damage to the unit is evident, or another component replacement is necessary on a vehicle 5 years old or older.

A ruptured desiccant bag can be a source of trouble in the system, clogging valves and lines. Some manufacturers have approved procedures for flushing the A/C system. Others do not.

When replacing components, check the service materials for the amount of oil normally contained in the component. A condenser may hold 1 oz.-to-2 oz., for example.

Some conditions may cause insufficient cooling from a system that seems to be functioning properly otherwise. A restriction to airflow through the condenser (and radiator) or evaporator can reduce A/C performance.

Many systems use a vacuum to operate control valves and doors. Check for cut, loose, or missing vacuum lines, and any conditions that could cause a loss of vacuum.

Thames notes that some repairs, such as the replacement of certain baffles or upper tie bars, may require the discharging and servicing of the A/C system, even when it is not damaged.

To ensure a quality repair, he also recommends replacing O-ring seals at connections whenever they must be opened and the use of an electronic/ halogen leak detector for best results in finding refrigerant leaks.

Leaks

The first thing to do is conduct a thorough visual inspection of the system, checking for obvious sources of leaks, such as a punctured condenser and cracks or cuts in lines and hoses.

You can perform a static pressure test to determine if the system has lost its charge.

Connect a pressure gauge to the high side with the engine off. The reading should be about 50 psi. A lower reading indicates that the system is not fully charged, and you should check for leaks.

If the system is discharged and there is no obvious cause, you will need to add about 1 lb. of refrigerant to check for leakage. Large leaks will be indicated by a hissing sound or bubbling of oil from the source of the leak.

To locate smaller leaks, you will need to use a solution of soapy water or an electronic leak detector. Check around connections, service ports, and system components. Pressure switches are susceptible to leaks.

On a hot day, high side pressure may exceed 300 psi. It may be difficult to detect high-pressure leaks in cool weather.

Ultraviolet light-sensitive tracer dyes may help find small leaks. It can take a day or up to a week for a small leak to show. R-12 and R-134a systems use different dyes, and some newer systems contain dye from the factory.

Considerations for a High-Quality, trouble-free A/C repair include:

Clean all dirt, moisture, and other contaminants from fittings before disconnecting them. Immediately seal the rest of the system from the atmosphere with tape or other suitable means.

Promptly make the repairs and service the system.

Replace caps on all service ports. Up to 1 lb. of refrigerant fluid per year can be lost if the caps are missing.

Replace hoses and O-rings with the proper type. The molecules that makeup R-134a are much smaller than those of R-12: refrigerant can seep through hoses if the proper barrier type is not used.

Torque all fittings carefully to avoid leaks and prevent damage to aluminum components.
If the dashboard indicator lamp glows, a scan tool may be used to pinpoint the problem.

Refrigerant Service and Recovery

It is against federal law to discharge any automotive refrigerant into the atmosphere. For this reason, refrigerant recovery equipment must be used when servicing A/C systems.

This equipment is used to recover and recycle refrigerant and to evacuate and recharge the system. Evacuation is extremely important for proper service.

Air and moisture are enemies of A/C systems and must be removed. During the evacuation, the system is held under a vacuum, which vaporizes moisture and removes it, along with air and contaminants. Evacuation will also indicate major leaks before refrigerant is installed.

Exposing the eyes or skin to the refrigerant can cause freezing and injury. When working with refrigerant, observe safety precautions. Follow equipment safety guidelines.

With the changes in A/C systems and the many different locations of service ports today, the old benchmark manifold pressure gauge readings are less valuable for diagnosis than they once were.

It may be necessary to compare the readings to manufacturers’ specifications. Also, it is more critical than ever to be sure the system is charged with the proper amount of refrigerant. Most manufacturers list a tolerance of 2 oz. or less.

Is AC Condensation Safe for Plants?

Moisture forms during the use of an air conditioner and are usually removed by a drip line or hose outside the home. When temperatures are high, condensate production can range from 5 to 20 gallons (23-91 L.) per day.

This water is pure, drawn from the air, and free of the chemicals found in municipal water.

Combining air conditioner water and plants is an effective way to conserve this valuable and costly resource. Unlike tap water, AC water does not contain chlorine or other chemicals.

It is formed when the unit cools warm air, resulting in condensation. This condensation is discharged outside the unit and can be safely redirected into plants.

Irrigating with AC water can water just a few pots or an entire bed, depending on how much your unit runs and the temperatures.

Many large institutions, such as college campuses, are already harvesting their air conditioning condensate and reusing it in water-wise landscape management.

Watering plants with air conditioner water not only conserves and thoughtfully reuses this resource but also saves a lot of money.

Watering Techniques Using AC Water When using AC condensation for plants, no filtering or settling is required. Collecting water in a bucket outside the house is one of the simplest ways to harvest it.

If you want to get some use, you can direct the drip line into nearby plants or pots. The average home generates 1 to 3 gallons (4-11 L.) per hour.

That’s a lot of free water to go around. A simple afternoon project involving PEX or copper pipe can result in a consistent, dependable water source that can be distributed wherever it is required.

It is probably a good idea to divert runoff to a cistern or rain barrel in hot, humid areas where there will be a lot of condensates.

The Drawbacks of Using AC Water for Irrigation

The lack of minerals in air conditioning water is the most serious issue with watering plants with it. The condensate, which is essentially distilled water, is corrosive.

That is why the water is routed through copper pipes rather than steel. The corrosive effect only affects metals and has no effect on organic materials such as plants.

Air conditioning water is also extremely cold when it comes out of the tubing or pipe and, if applied directly, can harm plants.

This can be avoided by directing the piping to the soil rather than the plant’s leaves or stems.

The water also lacks minerals, which can deplete the soil, particularly in container situations. Mixing it with rainwater will help balance the minerals and keep your plants happy.