For chillers and modern air conditioning systems, the most common types of expansion device are thermostatic or electronic expansion valves.
In this article we consider the differences between them, and the features and benefits of both types of device.
About Expansion Valves
Primary and secondary regulators control the flow of refrigerant in a refrigeration circuit. Here, the refrigerant mass flow must correspond to the capacity requirement. If more refrigerating capacity is required more refrigerant must be evaporated.
In addition, controllers are required to ensure that all components of the refrigerant circuit, such as evaporator, condenser and compressor, are operated in their optimal pressure and temperature range. This is the only way to ensure that a refrigeration system is operated safely and economically.
Primary regulators are also referred to as chokes or expansion elements. They directly control the capacity of the evaporator via the amount of refrigerant injected. We distinguish between these types:
- capilliary tubes
- pressure-regulated expansion valve
- thermostatic expansion valve
- electronic expansion valve
In More Detail…
In small systems such as refrigerators, capillary tubes are often used as expansion elements. The capillary tube is a copper tube with a very small internal diameter. The throttling effect is adjusted experimentally by the length of the capillary tube. Capillary tube systems do not contain a collector and the refrigerant quantity is precisely matched to the system.
A heat-conductive metal, such as copper, is commonly used to transport the vapour. The efficiency of a condenser is often enhanced by attaching fins, which are flat sheets of conductive metal, to the tubing so as to accelerate the removal of heat. Typically, such condensers employ fans to force air through the fins and carry the heat away. In many cases, large condensers for industrial applications use water or some other liquid in place of air to achieve heat removal.
Thermostatic Expansion Valves
The thermostatic expansion valve (TEV) is the most commonly used expansion device. The valve regulates the amount of refrigerant injected into the evaporator in the refrigeration circuit depending on the heat supply from outside.
A thermostatic expansion valve consists of a thermally controlled part and a pressure controlled part. The thermostatic element is separated from the valve housing by a diaphragm. The element is connected by a capillary tube to a sensor, a valve housing with valve seat and a spring. The spring is used to adjust the static superheat. We will look at this element in more detail later in this paper.
1. Below: typical thermostatic expansion valve
2. Internally, the expansion valve looks something like this (below):
How does this work?
The function of a thermostatic expansion valve is determined by three basic pressures:
P1: Sensor pressure acting on the top of the diaphragm and opening the valve.
P2: Evaporator pressure acting on the bottom of the diaphragm and closing the valve.
P3: Spring pressure, which also acts on the underside of the diaphragm and closes the valve.
When the expansion valve regulates, there is a balance between the sensor pressure on the top of the diaphragm and the evaporator pressure plus spring pressure on the bottom of the diaphragm i.e.
P1 = P2 + P3
Some more terms that we will use in due course:-
Superheat [K] is the difference between the temperature measured at the sensor of the thermostatic expansion valve and the evaporating temperature. The evaporating temperature is determined via manometer on the suction side.
Subcooling [K] is defined as the difference between liquid temperature and condenser temperature at the inlet of the expansion valve. Subcooling of the refrigerant liquid is necessary to avoid vapour bubbles before the expansion valve which would reduce the liquid supply to the evaporator.
Subcooling cools the liquefied refrigerant down a little more which extracts energy from the liquid refrigerant which is then reabsorbed during evaporation.
The sensor with a suitable gas or liquid filling (see later) is attached to the suction line behind the evaporator. A capillary tube transmits temperature changes to the diaphragm system that serves as the actuator of the valve cone. The pressure at the beginning of the evaporator acts on the underside of the diaphragm (internal pressure compensation). Rising superheat in the suction line increases the temperature sensor pressure on the diaphragm and opens the valve: the controlled variable is therefore the superheat temperature.
With decreasing evaporator capacity, there would still be unevaporated refrigerant at the refrigerant-side outlet; consequently, no superheat if the thermal valve did not immediately reduce the refrigerant supply.
If there is a greater pressure loss in the evaporator, valves with external pressure compensation are used because otherwise the superheat would be too high. In this case, the space under the diaphragm is connected to the evaporator end by the compensating line, so that perfect correlation of t0 and t0h, i.e. constant superheat and thus dry suction of the compressor, is ensured, even if the evaporator produces a pressure drop.
The evaporator and expansion valve form a control circuit whose stable behaviour must be guaranteed in every operating condition.
When the compressor is at a standstill, a so-called “re-injection” cannot be ruled out as soon as the pressure has equalised. Therefore, it is common to place a solenoid valve in the liquid line upstream of the expansion valve.
In a nutshell: The TEV compares the temperature of the refrigerant at the outlet of the evaporator with the inlet temperature. The TEV ensures that the refrigerant does not overheat at the outlet of the evaporator. In the optimum case, the TEV feeds the maximum possible amount of refrigerant, which can just be completely evaporated, into the evaporator. It is important that no liquid refrigerant leaves the evaporator, as this can cause severe damage to the compressor. The degree of superheat can be adjusted by preloading the diaphragm spring.
Liquid fillings of the sensor mentioned before may be of one of these different ones:
- universal filling: For most of the chillers, no MOP (maximum operating pressure) required, for chillers with high evaporation temperatures
- MOP filling: Expansion valves with MOP filling are used in systems where it is necessary to limit the suction pressure during start-up. Expansion valves with MOP have a very small filling in the sensor. This means that the thermostatic element must be warmer than the sensor – otherwise, a shift of filling from the sensor to the element can take place, which prevents the expansion valve from functioning.
- MOP filling with ballast (standard): Expansion valves with MOP ballast charges are preferably used in chillers in air conditioners and plate heat exchangers, which have a large transfer capacity with small internal volumes. With MOP ballast charge, 2 to 4 K less superheat can be achieved than with other charge types.
To eventually select a proper thermostatic expansion valve we need to know the following features:
- Evaporator capacity
- Evaporating temperature
- Condensing temperature
- Pressure drop across valve
- Internal or external pressure compensation
Worth knowing about assembly..
- The expansion valve must be mounted in the liquid line ahead of the evaporator and its sensor must be mounted on the suction line as close as possible behind the evaporator
- If the valves are external pressure compensating valves, the compensating line must be mounted immediately near the sensor on the suction line
- The sensor is mounted on a horizontal pipe on the suction line, in a position that corresponds to the time between 1 and 4 o’clock when compared with the dial of a clock. The mounting depends on the outer diameter of the pipe
- The sensor must never be attached to the bottom of the suction pipe, as it will pick up false signals there if there is oil at the bottom of the pipe
- The sensor is to determine the temperature of the superheated suction steam and must therefore not be mounted in such a way that it can be influenced by external heat/cold
- The expansion valve is delivered with a factory setting that does not need to be corrected in most cases
- If readjustment is necessary, this is done by means of the adjustment spindle on the expansion valve
- Turning to the right (clockwise) increases the superheat of the expansion valve, turning to the left (counter clockwise) decreases it
Electronic Expansion Valves
The electronic expansion valve (EEV) is a more advanced and subtle device. Optimum evaporator filling even with strong load fluctuations, flexible MOP points and highest possible evaporating temperatures to increase energy efficiency are keywords for this type of expansion valve.
These requirements can often not be adequately met with the usual thermostatic expansion valves. Electronic expansion valves are excellently suited for this purpose.
The advantages of an electronic superheat control are that the evaporator is always optimally filled with refrigerant. Even in the case of strong power fluctuations, i.e. the most diverse partial load cases, the refrigerant quantity to be injected can be precisely dosed. This is done by promptly transmitting the current superheat in the evaporator to a controller via a pressure transducer and a very sensitive temperature sensor.
This adaptive control of the refrigerant injection leads to an optimal use of the evaporator and thus to the highest possible evaporation pressures that can be realised in this chiller. This not only leads to higher COP values, but also to energy savings, because the COP is the quotient of achieved cooling power and power consumption of the chiller.
The superheat strives to adapt the minimum stable signal (MSS line) of the evaporator so that the signal cannot drift into the unstable range. The controller first picks an arbitrary superheat setpoint and use this value as the initial minimum stable signal. Then it tries to realise this value as a setpoint in the chiller. Since all the required information, i.e. the superheat temperature and the current evaporation pressure, are available by means of proper sensors and, in addition, an accumulated history is stored for these two values in order to optimise the control function, the controller decides whether the currently targeted value is feasible under the prevailing load conditions.
The permanent checking of the optimum superheat is a decisive advantage of an electronic expansion valve compared to thermostatic valves. These would have to be set in advance to the maximum superheat setpoint described by the individual MSS characteristic curve of the chiller, see before.
However, this value is not so easy to determine, so that in the case of a mechanical thermostatic valve, this already poor starting position is usually worsened by the installer adding a “safety margin” to the necessary minimum superheat value during commissioning.
With regard to the functional safety of a system, this is not wrong, however, this measure has a negative effect on the energy situation in the chiller. With electronically controlled expansion valves this “safety margin” does not apply, as the system regulates itself with regard to overheating, as described.