A clear explanation, with diagrams, of how a central air conditioner cools a house by cycling refrigerant through its system. A central air conditioner has a primary appliance such as an air handler or furnace located in an out-of-the-way place such as a basement or attic. This appliance pumps chilled air throughout the house through a system of air ducts—often the same system utilized by a forced-air furnace during the heating season. One or more thermostats in the house turn the cooling system off and on as room temperatures rise and fall. A central AC runs on electricity and removes heat from air with basic refrigeration principles. When the thermostat signals the air-conditioning system to lower air temperature, a whole sequence of events begins. The air-handling unit kicks on, drawing room air in from various parts of the house through return-air ducts. This air is then pulled through a filter, where airborne particles such as dust and lint are removed—in fact, sophisticated filters may remove microscopic pollutants as well.

Then the air is routed to air-supply ductwork that carries it back to the rooms. But how does the evaporator coil get cold in the first place? That is where refrigeration principles come into play. Every air conditioner has three main parts: a condenser, an evaporator, and a compressor. With a typical “split system,” the condenser and the compressor are located in an outdoor unit; the evaporator is mounted in the air-handling unit, which is often a forced-air furnace. With a “package system,” all of the components are combined in a single outdoor unit that may be located on the ground or on the roof.) Refrigerant such as freon circulates through copper tubing that runs between these components. This refrigerant receives and releases heat as it raises and lowers in temperature, changing from liquid to gas back to liquid. The refrigerant is especially cold when it begins to circulate through the indoor coil. As the air handler pushes warm air across the coil, the refrigerant absorbs so much heat from the air that it turns into vapor.

As a vapor, it travels to a compressor that pressurizes it and moves it through the outdoor coil, which jettisons the heat. A fan also helps to dissipate the heat. The refrigerant then passes through an expansion device that converts it to a low-pressure, low-temperature liquid, which returns to the indoor coil. And so the cycle goes. This FREE service will help you find a qualified local AC professional. Call for free estimates from local pros now: We provide useful tips and information for homeowners, facility managers and contractors looking to improve their current HVAC system. Since the minimum efficiency regulation changed to 13 SEER in January 2006, most OEM systems now incorporate a thermostatic expansion valve (TXV) style metering device as the standard for air conditioning systems. It is now extremely important for the HVAC technician to understand the design and operation of this type of valve. The thermostatic expansion valve (TXV) is a precision device, which is designed to regulate the rate at which liquid refrigerant flows into the evaporator.

This controlled flow is necessary to maximize the efficiency of the evaporator while preventing excess liquid refrigerant from returning to the compressor (floodback). One of the design features of the TXV is to separate the high pressure and low pressure sides of an air conditioning system. Liquid refrigerant enters the valve under high pressure via the system’s liquid line, but its pressure is reduced when the TXV limits the amount of this liquid refrigerant entering the evaporator.window ac unit water The TXV – What It Does Dowindow ac unit full of water The thermostatic expansion valve controls one thing only:  the rate of flow of liquid refrigerant into the evaporator. ac unit not blowing heatContrary to what you may have heard, the TXV is designed to control:

Trying to use the TXV to control any of these system variables will lead to poor system performance – and possible compressor failure. How the TXV Controls the System As the thermostatic expansion valve regulates the rate at which liquid refrigerant flows into the evaporator, it maintains a proper supply of refrigerant by matching this flow rate against how quickly the refrigerant evaporates (boils off) in the evaporator coil. To do this, the TXV responds to two variables: the temperature of the refrigerant vapor as it leaves the evaporator (P1) and the pressure in the evaporator itself (P2). It does this by using a movable valve pin against the spring pressure (P3) to precisely control the flow of liquid refrigerant into the evaporator (P4): TXV Pressure Balance Equation P1 = Bulb Pressure (Opening Force) P2 = Evaporator Pressure (Closing Force) P3 = Superheat Spring Pressure (Closing Force) P4 = Liquid Pressure (Opening Force) Energy Transfer in the TXV

Here is a closer view of the TXV in operation. The flow of the liquid refrigerant is restricted by the valve pin. As the flow is restricted, several things happen: The pressure on the liquid refrigerant drops A small amount of the liquid refrigerant is converted to gas, in response to the drop in pressure This “flash gas” represents a high degree of energy transfer, as the sensible heat of the refrigerant is converted to latent heat The low pressure liquid and vapor combination moves into the evaporator, where the rest of the liquid refrigerant “boils off” into its gaseous state as it absorbs heat from its surroundings. The pressure drop that occurs in the thermostatic expansion valve is critical to the operation of the refrigeration system. As it moves through the evaporator, the low pressure liquid and gas combination continues to vaporize, absorbing heat from the system load. In order for the system to operate properly, the TXV must precisely control the flow of liquid refrigerant, in response to system conditions.