AIR CONDITIONING TECHNOLOGY PART 10 Expansion Devices ___________________________________________________________________ IN PART 8 and 9 over the last three months, the methods of heat rejection utilising air cooled and water cooled condensers were studied including the fundamentals of cooling towers. Following the flow of refrigerant around the vapour compression cycle circuit now takes us to the Expansion Device. PURPOSE The Expansion device performs the following functions within the Vapour Compression Cycle: Causes a resistance to refrigerant flow created by the compressor resulting

in the creation of a High Pressure and a Low Pressure within the appropriate regions of the circuit. These pressures are termed the Evaporating Pressure and the Condensing Pressure, which in turn determine the Evaporating Temperature and the Condensing Temperature.

Regulates the flow and thus the mass of refrigerant in the Evaporator Coil to

match the instantaneous cooling load. TYPES OF EXPANSION DEVICE The following types of expansion device are generally available: Capillary Tube Restrictor Expansion Device Thermostatic Expansion Valve Externally Equalised Expansion Valve Electronic Expansion Valve Hand Operated Expansion Valve Automatic Expansion Valve Flooded Evaporator Float Control Low Pressure Float Valve Control

CAPILLARY TUBE The Capillary Tube is the simplest and cheapest form of expansion device consisting simply of a tube with a very small internal bore. The length of the tube coupled with the small internal diameter determines the resistance to refrigerant flow and is sized carefully to suit the application, operating pressure range (high pressure and low pressure across the capillary) and refrigerant type. The length of the tube of a given internal diameter is initially calculated to suit the application and is normally varied in accordance with test results by the equipment manufacturer until the desired operating conditions are achieved. In vapour compression systems, all expansion devices are located between the outlet of the condenser and the inlet of the evaporator. In small systems where the evaporator and condenser are positioned closely together, a long capillary tube is used to substitute the liquid line. Careful sizing of the capillary tube is essential in order to match the refrigerant flow rate through the compressor whilst maintaining the desired evaporating and condensing temperatures. If the capillary tube is correctly sized, the resistance against the compressor to refrigerant flow will result in a condensing pressure and temperature suitably positioned above the maximum ambient temperature to achieve heat rejection at reasonable efficiency and power consumption. If the restriction is too great, liquid refrigerant will not flow at a sufficiently high rate and will be retarded within the condenser. The increased liquid level within the condenser will result in reduced latent heat of condensation thus resulting in a rise in condensing pressure and condensing temperature until sufficient heat energy flows from the condenser to ambient. Power consumption at the compressor will increase and system efficiency therefore declines. Refrigerant flow to the evaporator will also be reduced and the suction effect of the compressor will cause the evaporating pressure and evaporating temperature to fall. This will result in excessive latent cooling, systems inability to hold design temperature and icing of the coil under certain circumstances when the evaporating temperature falls below freezing. If the restriction provided by the capillary tube is too low, condensing pressure and temperature will be too low. Liquid refrigerant within the condenser and liquid line will tend to flash to vapour form thus starving the evaporator of liquid refrigerant for essential refrigeration effect. The compressor will not be able to reduce the evaporator pressure and thus evaporating temperature to meet the desired rate of heat extraction due to inadequate temperature difference.

Component Balancing To summarise the perfectly sized capillary, the refrigerant flow rate through the capillary should match the refrigerant flow rate through the compressor whilst maintaining the desired evaporating pressure/saturation temperature and condensing pressure/saturation temperature. The capillary tube will only be perfect however at one set of operating conditions. Liquid Flooding & Compressor Damage The capillary tube has no means of shutting off refrigerant flow and therefore cannot protect the compressor from damage, which will occur if liquid refrigerant enters the compressor. When the system is switched off, refrigerant liquid continues to flow through the capillary tube into the evaporator as the system high and low side pressures equalise. The evaporator fan transporting heat energy to the evaporator coil may also have stopped and the liquid refrigerant is therefore not vaporised. If the system is accurately charged with the precise amount of refrigerant, the amount of liquid refrigerant in the evaporator coil will not be sufficient to cause compressor damage. If the system contains too much refrigerant, the excess liquid refrigerant will be drawn into the compressor at start up resulting in compressor damage and subsequent failure. If the capillary tube is too large in internal diameter or too short, the same situation can result. Small Heat Pump Systems Many small heat pump packaged and split systems use a capillary tube. Precise charging of such systems is absolutely essential if the desired heating output at reasonable power consumption is to be achieved, particularly as the smaller indoor coil acts as the condenser in heating mode and excess refrigerant will cause reduced area for heat rejection and excessive high side condensing pressure. Suction-Liquid Interchange A well-designed refrigeration or air conditioning system should employ suction-liquid interchange as this provides an increase in system efficiency and reduced power consumption. There is however a capital cost attached to this and many systems therefore exclude the facility on cost and competitive grounds. This technique will be described later in the series.

However, the general principle can be applied to small air conditioning and refrigeration systems very cheaply and with good effect. As the liquid refrigerant flows through the capillary from high side to low side pressure regions of the system, a proportion of the liquid refrigerant entering the capillary tube flashes to vapour, partly due to heat energy entering the walls of the tube. This represents a loss of cooling capacity and efficiency. Since the suction line leaving the evaporator is always colder than the liquid entering the capillary, the suction line can continually cool the capillary if they are brought into close contact. This will eliminate heat energy entering the tube through the walls. In fact, heat energy will flow from the liquid refrigerant passing through the tube causing sub-cooling, which will minimise the amount of refrigerant liquid flashing to vapour whilst travelling through the capillary. This results in an increase of cooling capacity, lower power consumption and increased efficiency. This technique is very often employed on domestic refrigerators. Variations in Cooling Load The capillary tube is designed to perform correctly across a defined range of operating conditions (ambient and room temperatures). However, the refrigerant flow must be correctly controlled over a considerable range of cooling load variations. The capillary tube is self-regulating within certain parameters since increasing ambient temperature results in increasing load on the conditioned space and the need for increased capacity. The condensing pressure will rise under these conditions forcing more refrigerant to flow through the capillary to meet the increased cooling load at the evaporator. RESTRICTOR EXPANSION DEVICE The Restrictor Expansion Device has been developed to overcome several limitations imposed by the capillary tube. The construction is illustrated in Figure 1. FIGURE 1 In Figure 1, the device is shown in the normal cooling mode where the bullet, a small cylindrical pellet with a fine precision machined orifice drilled through the centre, is forced by refrigerant liquid flow on to the forward seat, forcing the refrigerant to flow thought the hole. This fine orifice acts as the restrictor between the condenser outlet and evaporator inlet.

Capillary tubes suffer from variations on internal tube diameter over their length which result changes to predicted performance. The restrictor expansion device can be produced more accurately resulting in greater performance consistency. It is important to note that this device is located directly at the outlet of the condenser rather than at the evaporator inlet as is more conventional. This results in what is normally the liquid line becoming an extension of the evaporator coil. Clearly, the liquid line must be insulated to prevent heat energy evaporating the low-pressure liquid refrigerant before it reaches the evaporator. The flash gas refrigerant vapour produced during the expansion process is now transported along this line rather than being produced near the evaporator. Whilst this arrangement at first appears to be detrimental, there are the following advantages: 1 Greater line lengths are possible because the mixture of flash gas

vapour and liquid refrigerant presents a less arduous task for the compressor to transport.

2 The orifice is removable from the housing and therefore allows changes

in orifice size to be made to attain the optimum performance from the system.

3 With a reverse cycle heat pump system and two expansion devices, two

check valves have to be used to allow the refrigerant to flow through the appropriate expansion device. This type of restrictor allows the orifice to float free from the seat, allowing refrigerant to pass around it as well as flowing through it and thereby maintains a higher pressure in the line after the first device thus limiting further flash gas and system efficiency losses. This mode is shown in Figure 2.

FIGURE 2 The restrictor expansion device is shown in more detail in Figure 3. FIGURE 3 THERMOSTATIC EXPANSION VALVE (TEV) The Capillary Tube and Restrictor Expansion Devises are both relatively low in cost. But both require that the refrigerant charge be exactly correct and that the range of operating conditions be limited to within a certain band for reasonably efficient operation. Neither have the ability to continuously regulate the refrigerant flow into the evaporator at a rate, which matches that

precisely required for the instantaneous load. The Thermostatic Expansion Valve, popularly known as the TEV, achieves this objective. The Thermostatic Expansion Valve is designed to maintain a pre-set superheat value at the outlet of the evaporator, usually 5 - 7 K, this being the difference between the saturation temperature and the superheated vapour temperature leaving the coil. The TEV is usually designed to prevent variations in high side system condensing pressure having any effect on the operation of the valve and flow rate. This is ideal (other than for special systems), as this ensures close control of superheat without other forces acting on the valve. This makes the TEV the most commonly used form of refrigerant flow control. By maintaining constant superheat, the TEV not only ensures the evaporator is correctly filled with the required refrigerant but also prevents carry-over of liquid refrigerant to the compressor which might otherwise occur with a sudden reduction in cooling load. The valve is therefore ideally suited to extremely wide variations ion cooling load, which indeed most systems require in any event! The outline structure of the TEV is shown in Figure 4. FIGURE 4 The TEV consists of the following major components: 1) Needle Valve 2) Orifice 3) Diaphragm 4) Spring 5) Adjusting Screw 6) Remote Sensing Bulb 7) Filter/Strainer The operating principle below is described using the pressures and temperatures associated with refrigerant HCFC22: It is firstly important to note that the evaporating pressure (and saturation temperature) rises as the cooling load increases and vice-versa. The diaphragm is exposed to the evaporating pressure and the spring pressure on the underside. The upper surface is exposed to the pressure

created in the remote sensor bulb and the capillary connecting these due to the presence of a small quantity of the same refrigerant as is used in the system. When the load is constant, the diaphragm will be in a position of equilibrium. The superheat value of 7 K causes the remote sensor bulb to be held at 12 Deg C and the refrigerant contained therein will create a pressure of 6.2 bar. The evaporating pressure of 4.8 bar is generated by a balance of cooling load and compressor pumping capacity resulting in 5 Deg C saturation (evaporating) temperature. If the load increases, the rate of refrigerant liquid supplied by the TEV is required to increase. However, this does not happen immediately. The additional heat energy received by the evaporator coil evaporates all the liquid refrigerant and causes a greater temperature rise in the remaining vapour (increased superheat). This causes the remote sensor bulb to increase in temperature and the liquid/vapour refrigerant mix contained in the bulb and capillary line will also increase in temperature with a corresponding rise in pressure. The increased pressure over the diaphragm cause the needle to move downward, increasing the refrigerant flow through the orifice. The increased flow provides increased cooling (latent heat of vaporisation) with a corresponding reduction in superheat which returns to the original setting governed by the spring which has been adjusted to give 7K superheat. A cross-sectional view of a simple Thermostatic Expansion Valve is shown in Figure 5. The key components are as follows: 1 Thermostatic Element incorporating Diaphragm 2 Spring 3 & 4 Orifice and Needle Valve Assembly 5 Filter Screen 6 Valve Body 7 Superheat Adjustment FIGURE 5 NEXT MONTH: Part 11- Expansion devices continued.