GB2033066A - Refrigeration - Google Patents

Abstract

An air conditioning system comprises a compressor 1, a condenser 2, first and second evaporators 3, 4 and valve means e.g. valves 5, 6 for controlling the flow of refrigerant into the first and second evaporators. The first evaporator is placed within the air flow to the space to be air conditioned while the second evaporator absorbs heat from outside that space. Thus the degree of cooling produced in the air conditioned space may be controlled by means of the valves instead of by switching the compressor on and off or by-passing the condenser. Suitable for use in Outside Broadcast Vehicles, in confined spaces or where a precise control of temperature is required. <IMAGE>

Description

SPECIFICATION Air conditioning system The invention relates to air conditioning systems comprising a compressor, a condenser, an evaporator and means for passing air over the evaporator and into the area to be conditioned.

Air conditioning systems according to the invention may be used where comparatively small spaces are to be conditioned, for example, Outside Broadcast Vehicles or where a precise temperature has to be maintained, for example, in laboratory cabinets.

As is weli known an air conditioning system comprises three main components-a compressor, a condenser and an evaporator which are generally connected together in a sealed system to which a quantity of refrigerant gas is added. In operation, the function of the compressor is to compress the gas which is then fed to the condenser unit where heat is removed to the outside air and the gas liquifies. The cooled liquid refrigerant is then allowed to expand into the evaporator where, due to the latent heat of evaporation, heat is absorbed. Having passed through the evaporator the gas at normal temperature and pressure is fed to the inlet of the compressor where the process is repeated.The process of compression and evaporation which provides heat absorption at the evaporator and heat transfer to the outside air via the condenser operates at a rate dependent on the work done by the compressor. Conventionally control of the cooling effect is achieved by switching the compressor on and off by means of a thermostat located in the room to be air conditioned. While this method is satisfactory when applied to commercial and domestic premises, it has certain disadvantages when used in special applications such as Outside Broadcast Vehicles.

The first problem is that switching the compressor on and off leads to cyclic variations in temperature. These variations may only be a few degrees and in a normal sized room tend to be averaged out by the large volume of air.

However, within the confines of an Outside Broadcast Vehicle or studio control room the smallest change in temperature can be apparent and uncomfortable.

The second problem which occurs principally in Outside Broadcast Vehicles is the high starting current taken by the compressor.

Since the compressor has to start at peak loads the motor is designed to produce high torque until its running speed is reached. A variation of the capacitor start motor is used which can take six times the normal running current for up to five seconds after starting, depending on conditions. This high current leads to a momentary drop in the voltage of the power supply which can adversly affect the vision and sound output of an Outside Broadcast Vehicle. Because of this problem it is usual for Outside Broadcast Vehicles to specify that the compressor should run continuously and that control of cooling be achieved by other means.

The various known means of controlling air conditioning and refrigeration systems without stopping the compressor fall into two categories. Those which involve controlling the output of the compressor and those which operate by causing a proportion of the hot refrigerant to by-pass the condenser and return to the input of the compressor.

Several methods have been used or suggested for varying the output of the compressor for example, integral cylinder unloading, variable speed and multiple compressors.

None of these methods are readily applicable to Outside Broadcast Vehicle designs because of the increased cost, size and weight they involve. By-passing the condenser by a proportion of the hot refrigerant has been the accepted practice for many years in Outside Broadcast Vehicles. When the internal thermostat calls for a reduction in cooling capacity, that is the temperature is below the set value, an electrically controlled valve opens and causes the hot refrigerant to by-pass the condenser to the low pressure side. The bypassed refrigerant can enter the low pressure side either at the evaporator inlet or at the compressor input. Injection to the compressor input is the simpler of the two methods because the valve and pipework is all contained in the compressor/condenser unit. It is therefore a fairly simple modification of a standard system.Unfortunately, the method suffers from the disadvantage of inefficient oil return to the compressor (lubricating oil for the compressor in a sealed system is normally carried round the circuit with the refrigerant). In addition, due to the heat from the refrigerant being returned to the compressor without cooling, the compressor is likely to overheat and fail in high ambient temperatures.

Thus the proportion of the total refrigerant and hence the measure of cooling control is limited by the amount of overheating the compressor can withstand. Injection of hot refrigerant into the evaporator inlet, in spite of the additional pipework required, is a more satisfactory method. Temperature rise of the compressor is reduced due to the cooling effect of the evaporator and oil return is improved due to the high refrigerant velocity in the evaporator where oil retention can otherwise occur. The main problem with this method is that the cooling capacity can only be reduced to about 40% of the maximum rate. This means that when conditions call for only a small amount of cooling, the minimum of 40% can soon create a runaway condition.

The only solution then is to switch off the compressor or add heating. Both undesirable measures in Outside Broadcast Vehicles Thus, as a means of maintaining an exact temperature in Outside Broadcast Vehicles the hot refrigerant by-pass system leaves a lot to be desired but has been the best available solution within the restrictions of this application.

It is an object of the invention to provide an air conditioning systems in which the temperature of the air flowing into the conditioned space may be controlled without switching the compressor on and off and without by-passing the condenser with a proportion of the refrigerant.

The invention provides an air conditioning system comprising a compressor, a condenser, an evaporator and means for passing air over the evaporator into a space to be conditioned characterised in that there is provided a second evaporator thermally isolated from the space, valve means for controlling the flow of refrigerant through the first mentioned and second evaporators, and control means responsive to the temperature within the controlled space for controlling the operation of the valve means.

Since the flow of refrigerant through the first evaporator is controlled by valve means small amounts of cooling can be produced by causing small amounts of refrigerant to be injected into the first evaporator. The switching of the valve means can be at a much faster rate than the compressor can be switched and hence the temperature of the first evaporator can be held to a more precise value with only a small cyclic variation in temperature.

The second evaporator may be positioned adjacent to the condenser and means may be provided for passing air over the second evaporator, the air being discharged over the condenser. In order to reduce icing of the second evaporator in cold climates means may be provided for reversing the direction of the air flow such that in the first direction the air flows over the second evaporator and is discharged over the condensor and in the reverse direction the air flows over the condensor and is discharged over the second evaporator. Thus when ice is detected on the second evaporator the air flow may be reversed so that heat extracted from the condenser is available to melt the ice an the second evaporator.

The invention further provides an Outside Broadcast Vehicle including such an air conditioning system.

Embodiments of the invention will now be desribed, by way of example, with reference to the accompanying drawings, in which: Figure 1 shows in block schematic form an air conditioning system according to the invention.

Figure 2 is a plan view of a mechanical arrangement of the compressor, condenser, outside evaporator and control valves of Fig.

1.

Figure 3 is a perspective view of an Outside Broadcast Vehicle having an air conditioning system as shown in Fig. 1.

Figure 4 is a plan view of an alternative mechanical arrangement to that shown in Fig.

2.

Figure 5 is an elevation of the arrangement shown in Fig. 4.

Figure 6 is a plan view of a modification of the mechanical arrangement shown in Fig. 5, and Figure 7 is a circuit diagram of the control circuit of Fig. 1.

The air conditioning system shown in Fig. 1 comprises a compressor 1, a condenser 2, first and second evaporators 3 and 4, first and second control valves 5 and 6, a control circuit 7 and a temperature sensor 8.

In operation the compressor 1 compresses the refrigerant gas to raise its pressure. The refrigerant passes into the condenser 2 where heat is extracted from the refrigerant by passing air over the surface of the condenser 2 which is of thermally conductive material. The refrigerant which has now condensed to form a liquid is then allowed to expand into either evaporator 3 or evaporator 4, depending on the status of control valve 5 and 6, where the refrigerant returns to vapour form thus absorbing latent heat of evaporation. This causes the evaporator 3 or 4 to cool and thus to absorb heat from air which is passed over it. Air passed over evaporator 3 is arranged to pass into the space to be air conditioned while air passed over evaporator 4 is arranged to be discharged outside the space to be air conditioned.Thus by suitable control of valves 5 and 6 the cooling effect can be transferred from the space to be air conditioned to outside that space. Hence with constant compressor speed the cooling effect may be varied between 0 and 100% by use of the control valves 5 and 6. The valves 5 and 6 are controlled by the control circuit 7 which produces an output to open valve 5 and close valve 6 when the temperature in the space to be air conditioned is higher than a pre-selected value and to close valve 5 and open valve 6 when the temperature drops below the pre-selected value. The control circuit has as its input a temperature sensor 8, which may be a thermistor. The temperature sensor 8 may be placed in the air stream from evaporator 3. Since the valves 5 and 6 may be switched quickly, several times a second if required, it is possible to maintain a precise control of the temperature of the air passing into the controlled space since small quantities of refrigerant can be passed into the evaporator 3 and hence small quantities of heat extracted if desired.

In the arrangement shown in Fig. 2 the condenser 2 and evaporator 4 are arranged with a fan motor 9 and fan blades 10 and 11 to produce an air flow which normally passes over the evaporator 4 and subsequently over the condenser 2. This has the advantage of reducing the temperature at which the condenser operates giving more effective cooling of the refrigerant in the condenser 2. The fan motor 9 may be reversible so that, when used in cold climates, icing of the evaporator 4 may be prevented by reversing the air flow so that air heated by the condenser passes over evaporator 4. The solid arrows show the normal direction of air flow while the dotted arrows show the reverse, or de-icing, flow.

The reversal of the air flow may be controlled by an icing detector so that the reverse air flow is only produced when icing has taken place. The arrangement of Fig. 2 can be placed at a distance from the space to be air conditioned. Two pipes 12 and 13 then carry the refrigerant to and from the evaporator 3 which is placed adjacent to the space to be air conditioned.

Fig. 3 shows such an arrangement in an Outside Broadcast Vehicle. The unit 20 is mounted within the body of an Outside Broadcast Vehicle 30 such that air may be drawn in under the body of the vehicle and exhausted through a grill in the side of the body. The unit 20 is isolated from a space 31 to be air conditioned. The evaporator 3 which is located in a space 32 above the cab of the vehicle and is fed via pipes 1 2 and 1 3 circulates cooled air within the space 31. The evaporator 3 is provided with its own fan 33 to circulate the air. A controlled vent may be provided in the vicinity of the evaporator 3 to enable a controlled quantity of fresh air to be introduced in to the circulating air so as to enable a continuous replacement of stale air.

The space 31 includes a volume 34 in which equipment is mounted together with the accommodation for operators of the equipment.

The conditioned air is arranged to pass through grills in a wall 35 dividing the spaces 31 and 32 to circulate round space 31 and be returned to the space 32 via further grills and associated ducting.

Figs. 4 and 5 show a unit which may conveniently be mounted in the space 32 (Fig. 3) above the cab of the vehicle. In this case the evaporator 3 is mounted in the same unit as the compressor 1 and condenser 2.

The fan motor 9 drives a radial fan blade 11 to draw air across the condenser 2 and a centrifugal fan within the evaporator assembly 3. A barrier 40 is provided between the evaporator assembly 3 and the rest of the unit to isolate the rest of the unit from the space to be air conditioned. An opening 41 for the delivery of conditioned air is provided in the evaporator unit 3 and appropriate ducting is then fitted in the vehicle to direct the air to the space to be air conditioned. A separate fan comprising a motor 1 5 and a blade 1 6 is provided for the evaporator 4 and the discharge of the air flowing over evaporator 4 may conveniently be via a grill in the roof of the vehicle.With this arrangement air from the outside of the vehicle is drawn over the condenser 2 and part of this air flow subsequently passes over evaporator 4 thus reducing the possibility of ice forming on evaporator 4 in cold conditions and eliminating the requirement for a reversible air flow. The arrows in Figs. 4 and 5 show the typical air flow over the condenser 2 and evaporator 4.

A modification of the arrangement shown in Figs. 4 and 5 suitable for mounting in the position shown for the evaporator/condenser unit 20 of Fig. 3 is shown in plan view in Fig.

6. In this arrangement the evaporator 4 has been moved alongside the condenser 2. Airflow for the condenser 2 and evaporator 4 is drawn and discharged from beneath the vehicle or from a grill in the side of the vehicle. It has been found that a proportion of the air from the condenser discharge is normally drawn over the second evaporator but if required deflector plates may be mounted so as the deflect more of the air discharged over the condenser towards the second evaporator.

Fig. 7 is a circuit diagram of the control circuit 7 shown in Fig. 1. The control circuit comprises two operational amplifiers Al and A2, a transistor T1 and a relay RL1. A thermistor, which is the temperature sensor 8 shown in Fig. 1, is connected between two input terminals 70 and 71. The terminal 70 is connected to the positive supply rail via a resistor R1 and via a resistor R2 to the negative input of the operational amplifier Al while terminal 71 is connected via a resistor R3 to the negative supply rail. The series arrangement of a resistor R4, potentiometer R5 and resistor R6 is connected between the positive and negative supply rails, the wiper of potentiometer R5 being connected to the positive input of amplifier Al. The output of amplifier Al is connected via a resistor R7 to the positive input of amplifier A2.The series arrangement of a resistor R8, potentiometer R9 and resistor R10 is connected between the positive and negative supply rails, the wiper of potentiometer R9 being connected via a resistor R11 to the negative input of amplifier A2. The output of amplifier A2 is connected via the series arrangement of a zener diode D1 and a resistor R12 to the base of an n.p.n. transistor T1, the emitter of which is connected to the negative supply rial. The parallel arrangement of a resistor R 1 3 and a capacitor C1 is connected between the base of transistor T1 and the negative supply rail.

The coil of a relay RL1 is connected between the collector of transistor T1 and the positive supply rail and has a diode D2 connected in parallel to surpress switching transients. The collector is further connected to the negative supply rail via the series arrangement of a resistor R14 and a capacitor C2. A changeover contact set of relay RL1 has the pole connected to the positive supply rail and contacts connected to output terminals 72 and 73.

In operation a thermistor which is preferably located in the air stream from evaporator 3 is connected across terminals 70 and 71 while terminal 72 is connected to valve 6 and terminal 73 to valve 5, valves 5 and 6 being solenoid operated valves. The circuit is adjusted by setting potentiometer R5 so that at the required temperature relay RL1 is unoperated and hence valve 6 is open. When the temperature rises the resistance of the thermistor decreases causing the output voltage of amplifier Al to increase thus increasing the output voltage of amplfier A2 which causes transistor T1 to conduct and operate relay RL1. Hence the changeover contact now causes valve 5 to open and valve 6 to close.

Refrigerant therefore enters evaporator 3 and cools the air flow into the conditioned space.

Various modifications may be made to the embodiments shown without departing from the scope of the invention. As described the refrigerant is switched between evaporators 3 and 4 by the action of valves 5 and 6, which could be replaced by a changeover valve. If desired a proportioning valve could alternatively be utilised so that an adjustable proportion of the refrigerant is diverted to each of the evaporators, for example, one third to evaporator 3 and two thirds to evaporator 4.

The control circuit 7 would then form a closed loop servo system in combination with the temperature sensor and evaporator 3.

Claims (8)

1. An air conditioning system comprising a compressor, a condenser, an evaporator and means for passing air over the evaporator into a space to be conditioned characterised in that there is provided a second evaporator thermally isolated from the space, valve means for controlling the flow of refrigerant through the first mentioned and second evaporators, and control means responsive to the temperature within the controlled space for controlling the operation of the valve means.

2. An air conditioning system as claimed in Claim 1 in which the second evaporator is positioned adjacent to the condenser.

3. An air conditioning system as claimed in Claim 2 comprising means for passing air over the second evaporator, the air being discharged over the condenser.

4. An air conditioning system as claimed in Claim 2 comprising means for passing air over the condenser, the air being discharged over the second evaporator.

5. An air conditioning system as claimed in Claim 2 comprising means for passing air over the second evaporator and means for reversing the direction of the air flow so that in the first direction the air flows over the second evaporator and is discharged over the condenser and in the reverse direction the air flows over the condenser and is discharged over the second evaporator.

6. An air conditioning system substantially as described herein with reference to Fig. 1, or to Figs. 1 and 2 or 4 and 5 or 6, or to Figs. 1 and 7 or to Figs. 1, 7 and 2 or 4 and 5 or 6 of the accompanying drawings.

7. An Outside Broadcast Vehicle including an air conditioning system as claimed in any preceding claim.

8. An Outside Broadcast Vehicle substantially as described herein with reference to Fig. 3 of the accompanying drawings.