ACHRNEWS

The Role of the Metering Device

September 30, 2000
Different types of metering devices have different ways of charging. A thermostatic expansion valve (TXV) is charged to the subcooling of the liquid line leaving the condenser. A fixed orifice is charged to the superheat of the suction line leaving the evaporator.

Understanding why this is so requires that you understand the physical properties of the refrigeration cycle.

The four main components of the refrigeration cycle are:

1. Compressor;

2. Condenser;

3. Metering devices; and

4. Evaporator.



COMPRESSOR

The compressor compresses a low-pressure superheated gas into a high-pressure superheated gas. If the suction gas is not superheated, the compressor can be damaged.

The compressor pulls the refrigerant out of the evaporator and pushes it through a condenser. The act of compression is performed by any one of the following six types of compressors: reciprocating piston, rotary, scroll, screw, centrifugal, and sonic. Of these, the reciprocating and scroll compressors are the two most frequently found in residential air conditioning systems.

The mass flow rate produced by a compressor is equal to the mass of the suction gas pulled in by the compressor. The compressor’s output is equal only to its intake because the mass flow must be equal.

The process of compression, through mass flow, raises the temperature and pressure of the refrigerant. The result of the temperature increase is superheat. Pressure and temperature of the refrigerant must be higher than the condensing temperature. The refrigerant temperature must be higher so heat can flow into the condensing medium.

This process explains the necessary relationship between the increased pressure and the rise in temperature. If the pressure and temperature are not increased through compression, there is no heat transferred from the refrigerant to the condensing medium. The compressor has a maximum inlet temperature of about 70°F and outlet temperature of about 225°. Inlet refrigerant gas cools the compressor motor.

Desuperheating (heat leaving the refrigerant gas) of refrigerant begins as it is discharged from a compressor and pushed into a condenser.



CONDENSER

The condenser removes heat and changes a high-pressure vapor into a high-pressure liquid. As the superheated (high-pressure) gas is pushed into the condenser, it is desuperheated. The temperature is reduced to saturated pressure-temperature.

The refrigerant does not start to change state until the temperature reaches saturated pressure-temperature. The only variable that can change the temperature is a pressure change (Table 1).

At the saturation pressure-temperature point, the change of state becomes latent heat (invisible or hidden heat). Latent heat is a lack of rise or fall of temperature during a change of state (saturation). When the temperature does not rise or fall, it is at saturation and the change-of-state process begins.

Refrigerant continues to change state at one pressure-temperature. The only variable that can change a temperature is a pressure change. If a temperature change occurs, a pressure change occurs. If a pressure change occurs, a temperature change occurs. At the change of state the refrigerant liquid and vapor are at the same temperature. This is defined as equilibrium contact.

The temperatures of the liquid and vapors will stay the same until the temperature of the refrigerant starts to drop. The temperature of the refrigerant starts to drop once 98% to 99% of the refrigerant becomes a liquid. This is called subcooling.

Subcooling is a temperature below saturated pressure-temperature (Table 1). Subcooling is a measurement of how much liquid is in the condenser.

In air conditioning, it is important to measure subcooling because the longer the liquid stays in the condenser, the greater the sensible (visible) heat loss. Low subcooling means that a condenser is empty. High subcooling means that a condenser is full. Overfilling a system increases pressure due to the liquid filling of a condenser that shows up as high subcooling.

To move the refrigerant from the condenser to the liquid line, it must be pushed down the liquid line to a metering device. If a pressure drop occurs in the liquid line and the refrigerant has no subcooling, the refrigerant will start to revaporize (change state from a liquid to a vapor) before reaching the metering device.



METERING DEVICE at work

A metering device provides a pressure drop point. This device has two jobs:

1.It holds refrigerant back in a condensed state; and

2.It feeds refrigerant into the evaporator.

When high-pressure liquid enters a metering device, pressure starts to drop as the temperature remains the same until it reaches saturation pressure-temperature. At this time, both the pressure and temperature continue to drop to the evaporator pressure-temperature (Table 2).

Low-pressure liquid that is leaving the metering device is boiling at saturated pressure-temperature. The process of a refrigerant changing its state (from a liquid to a vapor) in the metering device is called flash gas. Flash gas is what cools the refrigerant liquid in the metering device. A system with no subcooling has more gas that is flashed and less capacity.

Publication date: 10/02/2000