To control the amount of refrigerant volume flowing through the evaporator a metering device must be used. The function of the metering device is as follows:
● To separate the high pressure and low pressure side of the system.
● To meter the volume of refrigerant and hence the cooling capacity of the evaporator.
● To ensure that there is superheated refrigerant exiting the evaporator.
Currently there are two main categories of metering device used, a Thermostatic Expansion Valve (TXV or TEV) and a Fixed Orifice Valve (FOV). The pressure drop across the evaporator is used to determine which type of valve is the most appropriate. Simple air-conditioning systems will generally use only one of these metering devices. Dual air-conditioning systems may use both a TXV and an FOV within the system.
Superheat
Superheat within an expansion system is very important. It ensures that the entire liquid refrigerant inside the evaporator has been vaporised from a liquid. It is generally measured by the difference in the boiling point of the refrigerant between the inlet and the outlet of the evaporator which can be as high as a 10°C difference. Evaporators, depending on their design characteristics, operate within different amounts of superheat and expansion valves are matched against this by adjusting the superheat valve spring tension.
Thermostatic expansion valve
There are also some variations in the design of a TXV.
● Internally equalised thermostatic expansion valve.
● Externally equalised expansion valve.
● Box or H-valve type.
Operation of externally equalised expansion valve
The externally equalised expansion valve (Fig. 2.36) has the benefit of having refrigerant pressure from the outlet of the evaporator acting directly on the underside of the diaphragm of the TXV. This arrangement overcomes the problem of sensing the pressure drop across the evaporator. The TXV has a pintle valve which is controlled by a diaphragm.
There are three pressures acting on the diaphragm of a TXV valve:
- Refrigerant within or exiting the evaporator applies pressure under the diaphragm (Pe).
- Spring pressure (called superheat spring) applying pressure against a pintle valve (ball and seat). This pressure is applied under the diaphragm (Ps).
- Pressure from the expansion of the liquid within the heat sensing bulb via the capillary tube (Pf) which is above the diaphragm.
The amount of refrigerant that is allowed to flow into the evaporator is determined by the vertical movement of the diaphragm and valve. This is controlled by the difference in pressure above the diaphragm Pf and below the diaphragm which is the sum of Pe and Ps.
When the load on the system is high and additional cooling is required the temperature exiting the evaporator will be high.
Figure 2.36
This high temperature will be transferred to the heat sensing tube (remote bulb) which contains an inert liquid (different to the refrigerant) which expands within the bulb increasing in pressure due to its fixed volume. The increase in pressure is transferred via the capillary tube to the top of the diaphragm enabling the diaphragm to overcome the combined pressure of the superheat spring (Ps) and the refrigerant exiting (Pe). The valve opens further and allows an increase in volume of refrigerant to flow through the evaporator to cope with the increase in load. This will lower the temperature of the refrigerant exiting the evaporator. This reduction in temperature will be transferred to the heat sensing tube and cause the liquid inside to contract. This will reduce the pressure inside the capillary tube and diaphragm. This reduction in force Pf above the diaphragm will allow the superheat sensing spring pressure Ps and refrigerant exiting the evaporator pressure Pe to force the diaphragm upwards reducing the orifice size and thus reducing the volume of refrigerant.
This continual adjustment and balancing of forces controls the volume of refrigerant to ensure that a superheated refrigerant exists within and exiting the evaporator.The manufacturers adjust expansion valves to ensure they operate under superheat conditions. Factory settings must not be tampered with and the correct TXV must always be used as a replacement for a faulty one.
Some expansion valves have a small V cut into the valve seat to ensure that if the valve is closed a small quantity of refrigerant may still flow around the system if a fault occurs or they are seized closed.
Operation of internally equalised expansion valve The operation of the internally equalised expansion valve is almost the same as the externally equalised expansion valve. The difference is the loss of the benefit of sensing the pressure of the refrigerant as it leaves the outlet of the evaporator. The pressure is sensed on the inlet of the evaporator or just before, using an internal drilling inside the housing of the TXV where refrigerant entering can apply pressure on the underside of the diaphragm.This means that the pressure drop across the evaporator is unknown with this type of valve.
Box or H-valve type
The box type expansion valve (Fig. 2.37) includes the pressure sensing and temperature sensing functions of the externally equalised TXV but has no external tubes (capillary or pressure
Figure 2.37
sensing).This is achieved by using two passages – one entering the evaporator and one exiting
the evaporator.
Liquid refrigerant enters the block valve housing (1). The orifice is very small and there is a large pressure drop on the other side of the ball valve (8).The liquid and small amount of vapour refrigerant (flash gas) enters the evaporator. The liquid/vapour will boil due to the drop in pressure and the vapour will become saturated. The saturated vapour will continue to flow through the evaporator becoming superheated.The superheated refrigerant will enter the box type expansion valve at point (7). The valve position is controlled by the temperature and pressure of the superheated vapour entering it from the evaporator at point (7). If the temperature of the refrigerant is high due to a high cooling demand then this additional heat will be transferred to the sensing element and diaphragm head. The liquid will expand and apply pressure downwards on the ball valve and superheat spring enlarging the orifice and allowing an increased volume of refrigerant to flow through the evaporator. The increased volume of refrigerant will provide additional cooling capacity and the refrigerant temperature entering the valve at point (7) should reduce. When this occurs the sensing element will transfer the reduced temperature to the diaphragm head which will then contract and reduce the pressure applied to the ball valve and superheat spring. The pressure of the superheated vapour is sensed directly under the diaphragm head via internal drillings. If the pressure is high then the diaphragm head flexes upward reducing the pressure applied to the ball valve and superheat spring. This causes the orifice to reduce in size thus reducing the volume and pressure of the refrigerant flowing into the evaporator. If the pressure applied to the diaphragm is low then the diaphragm will flex downward and apply additional pressure to the ball valve and superheat spring. This will increase the size of the orifice and allow a larger volume of refrigerant at a higher pressure to flow through the evaporator.
2.38
Fixed orifice valve (FOV)
The fixed orifice valve (Fig. 2.38) is positioned inside the high pressure line between the condenser and the evaporator. This can often be seen when looking at a system with an FOV fitted due to the increased diameter of the aluminium tube where the valve is situated. This is
also where you can sense the change in temperature from hot to cold.
The volume of refrigerant flowing through the orifice is determined by the pressure of the refrigerant and the size of the orifice.The orifice is fixed so the only way to control the volume is to vary the pressure of the refrigerant (1.5–2.9 bar low pressure side). This type of system tends to cycle (switch on/off) the compressor on a regular basis to vary the volume/pressure of refrigerant matched against the load on the system. The compressor is cycled by a cycling switch located in the low side of the system. A way to improve this system is to fit a variable orifice valve or smart valve or a variable displacement compressor.
As previously stated the function of the FOV is to divide the system creating a high pressure and low pressure side and to meter refrigerant into the evaporator.
Liquid refrigerant flows from the condenser at high pressure to the inlet of the FOV. The refrigerant travels through filter screens to remove any foreign particles and then to the calibrated fixed diameter tube inside the plastic valve body. Dependent on the pressure of the refrigerant a small volume flows through the orifice into the evaporator inlet. This changes the refrigerant into a low pressure liquid ready to boil off inside the evaporator. The orifice size of the FOV is matched to deliver the correct volume of refrigerant under maximum cooling loads. This causes problems under light loads when the evaporator is in danger of becoming flooded due to not enough heat available to vaporise the entire refrigerant inside the evaporator.This is the reason why an accumulator must be fitted between the outlet of the evaporator and the compressor ensuring no liquid refrigerant flows to the evaporator, except through the oil bleed inside the accumulator which is used to deliver oil to the compressor (5%).
The FOV comes in a range of orifice sizes (1.19 mm–1.70 mm) which are colour coded to
enable them to be matched to the manufactured vehicle (see Table 2.2). Special tools are required
to remove the valve (Chapter 5).
Variable orifice valve (Smart VOV)
The fixed orifice valve (Fig. 2.39) is limited due to its design which often leads to poor performance and excessive cycling of the compressor (if fixed displacement).To improve the performance a VOV can be fitted which can respond to the change in pressure of the refrigerant
and thus vary its orifice size to compensate. It is important for the correct operation of the
Table 2.2
Figure 2.39
A/C system to create the correct pressure distribution throughout the system. This must be matched to the load on the system. This will promote the correct condenser and evaporator performance.
The valve assembly contains two ports, a fixed and a variable port. The fixed port is matched against the required refrigerant flow for high vehicle speed.The variable port reacts to the temperature applied to it from the refrigerant exiting the condenser.The temperature is sensed by a bimetallic spring which expands and contracts with changes in refrigerant temperature. At idle and low compressor speeds the orifice size can be reduced. This creates a larger pressure differentiation across the valve and reduces the volume of refrigerant and evaporator flooding.When either the output of the compressor due to vehicle speed or the external load changes the orifice size can respond to this change. The main benefits which are published by aftermarket companies that sell VOV state that the valve is suited to drivers who spend a great deal of time idling in traffic or moving slowly. Performance is improved through reduced compressor loading and greater cooling. There are also published data on improved fuel economy due to reduced compressor loading. Currently VOV are not in widespread use by Original Equipment Manufacturers (OEM). Whenever working on an A/C system the OEM must always be contacted for advice when deviating/changing the original set-up of a system (replacing an FOV with a VOV).
The pressure in the high pressure line pushes the metering pin against a spring with a force which depends on the type of driving (idle, town or motorway/highway). The aperture of the valve is increased or decreased by the different diameters of the metering pin and thus matched to the driving situation.
Figure 2.40
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