Table of Contents
Introduction
The purpose of the expansion device is to rapidly reduce the pressure of the liquid refrigerant produced inside the condensing unit in the refrigeration cycle. This also allows the refrigerant to rapidly cool before entering the evaporator.
Thermostatic expansion valve
The thermostatic expansion valve (TEV) controls the amount of liquid refrigerant injected into a system’s evaporator. It bases on the evaporator outlet temperature and pressure (superheat).
TEV with internal pressure equalization
The TEV strives to maintain a stable level of superheating inside the evaporator under all conditions by adjusting the mass flow of refrigerant in response to the evaporator load. This is achieved by a membrane inside the valve housing, which compares the temperature before and after the evaporator. The TEV is connected by a capillary to a thermostatic element, which acts as a temperature sensor. The pressure difference between the pressure at the inlet to the evaporator and the pressure inside the thermostatic element is balanced by a diaphragm inside the valve head. This type of TEV valve does not take into account pressure losses in the evaporator, and the pressure balancing the diaphragm is the pressure behind the valve nozzle (at the inlet to the evaporator). The consequence of this solution is less accuracy in measuring superheat.
TEV with external pressure equalization
This type of valve, in addition to the inlet and outlet ports for the refrigerant, contains an additional port to which an equalization line (additional capillary tube) is attached. It is installed at the outlet of the evaporator, right next to the thermostatic element. As a result, the superheat value is influenced not only by the measured temperature, but also by the pressure, which increases the accuracy of the control. The components of a TEV are shown in the drawing. The bulb, which transmits the corresponding pressure of the superheated gas, consists of a hollow metal container filled with a refrigerant fluid. A capillary tube connects the bulb to the valve housing. The bulb is fitted in direct contact with the suction pipe, on the outlet of the evaporator.
If the superheating increases, the pressure inside the bulb will increase, because more refrigerant inside the bulb evaporates. The increased pressure is transmitted through the capillary tube and depresses the membrane inside the head of the TEV.
This moves the needle, opening the valve orifice and thus increasing the refrigerant mass flow. The balance across the membrane is adjusted with a spring that may be adjustable manually or set at the factory. The stiffer the spring, the higher the level of superheating required to open the valve.
Electronic expansion valve (EEV)
Electronic expansion valves can be mostly used on very large systems and systems with a high demand for precise regulation. We distinguish modulating electronic expansion valves (stepper), with a continuously adjusting orifice, and electronically controlled ON/OFF valves (impulse), with a solenoid valve that is opened and closed periodically.
Stepper expansion valve
Modulating electronic expansion stepper valves are controlled by temperature or pressure sensors. Transcritical iCOOLTM CO2 condensing units are equipped with this type of expansion valves. The controller can be programmed to correct differences in temperature and pressure at any point of the system. Because the electric actuator reacts only to signals from the regulator, there are good possibilities for achieving a lower level of superheating than with a thermostatic expansion valve. Pressure differences between the valve and the sensor caused by refrigerant distribution systems are also corrected. Furthermore, the same valve can be used for different refrigerants after reprogramming. The electric actuator controls the shutter to adjust the orifice area continuously to allow a higher or lower mass flow of refrigerant to pass, depending on the signals from the regulator.
This type of valve can handle large variations in operating conditions, for example changes in pressure difference and cooling capacity. The biggest disadvantage of electronic valves is the relatively high cost and complexity of components. Programming the regulator box is not trivial, and system performance with a poorly adjusted electronic expansion valve may be lower than with a thermostatic expansion valve.
Pulse expansion valve
The electronic ON/OFF valve is an electronically controlled solenoid valve that functions both as an expansion valve and as a solenoid valve. When functioning as an expansion valve, ON/OFF control is used. During one cycle period, typically 6 seconds, the valve is opened and closed once.
The capacity of the expansion valve, so the amount of refrigerant flowing through it, is determined by the relationship between the opening and closing times. A regulator controls the opening and closing of the valve in order to reach the correct level of superheating. The inputs to the regulator are the temperature and pressure at the evaporator outlet. The inputs could also be the inlet and outlet evaporator temperatures, as for an electronic valve with continuous control.
When the demand for refrigerant is high (high cooling capacities), the valve remains open for almost the entire 6 seconds. When the demand is very low (low cooling capacities), the valve opens only for a fraction of the 6 seconds. When the compressor is shut off, the valve closes and functions as a solenoid valve.
The electronic ON/OFF valve can operate satisfactorily even with large variations in operating conditions, such as changes in pressure difference or cooling capacity. The valve capacity is adjusted simply by changing the relation between the opening and closing time. This allows the minimum stable superheating to be found for a wide range of operating conditions.
Capillary tube
A capillary tube is a long, wound-up copper tube with a tiny opening that receives hot, high-pressure liquid refrigerant from the condenser.
This small opening holds high pressure on one side of the tube and low pressure on the opposite side. The friction from the walls of the tube significantly reduces the pressure of the refrigerant flowing through it.
Since it is a fixed hole, it is very important that the system has the proper refrigerant charge — the device could break if there is excessive pressure due to additional refrigerant in the system.
Advantages
- Low manufacturing cost.
- It has no mobile part.
- Maintenance free.
- During the stop time of the unit, since the tube is a hole that is always open, it allows the balance of the pressures between the high- and low-pressure sides. This reduces the necessary starting torque of the motor, since it must start with the same pressure on both the low- and high-pressure sides.
- It is ideal for use with systems equipped with fractional hermetic compressors.
Disadvantages
- During the stop cycle of a refrigeration unit, the liquid refrigerant is directed to the evaporator due to the pressure difference between the evaporator and the condenser. The evaporator could flood, and the liquid refrigerant could go to the compressor and damage it when it starts. Therefore, the refrigerant charge is very critical in capillary tube systems.
- It cannot control or regulate the amount of refrigerant that enters the evaporator, when there are variations in the temperature during the day or caused by seasonal changes or variations in the load.
- It is very susceptible to being obstructed due to its small internal diameter.
