GB2185094A - Liquid heating system - Google Patents

Abstract

A liquid, for example water, contained in a storage cylinder 1 is selectively heated by means of a heat pump 2 utilising waste or reject heat and an electric or fuel-fired boiler 3. The heat pump 2 and boiler 3 are operable to heat the liquid to a pre-set temperature, for example 55 DEG C, at which temperature, which may be adjustable, the boiler 3 cuts-out and the liquid is further heated up to a maximum temperature, for example 63 DEG C, by the heat pump 2 only. In this way efficient use is made of the heat recovered by the heat pump 2 towards the heating requirement of the liquid and the boiler 3 acts as a back-up to the heat pump 2 to assist in meeting a large demand for heated liquid sufficient to cause the temperature of the liquid in the storage cylinder 1 to fall below the cut-out temperature of the boiler 3. The heat pump 2 may collect the heat from air in an enclosure, for example a beer cellar, cold room etc., and include both a liquid cooled and an air cooled condenser so that space cooling of the enclosure can continue when the liquid heating requirement is satisfied. <IMAGE>

Description

SPECIFICATION Liquid heating system This invention relates to liquid heating systems in particular, though not exclusively, for heating water.

It is well known to heat and store water until required in a storage cylinder by means of an electric immersion heater or a fuel-fired boiler. However, with the recent rapid escalation in cost of electricity, coal, gas and the like fuels, there have been numerous proposals for replacing such known heating devices with devices utilising other forms of low cost energy.

In particular, solar heaters which collect solar radia- tion and heat pumps which collect waste or reject heat have been proposed.

A disadvantage of solar heaters and heat pumps is that they rely on a heat source which is not constant and undercertain conditions such devicesareun- able to heat the water to an acceptable temperature and/or meet the demand for hot water.

It is an object ofthe present invention to provide liquid heating system which overcomes or reduces the disadvantages aforementioned.

According to the present invention there is provided liquid heating system comprising a storage vessel for liquid to be heated, a heat pump for heating the liquid to a first pre-determined temperature, said heat pump including an evaporatorforcollecting waste or reject heat and a condenser for dissipat- ing the collected heat to the liquid and, an electric or fuel-fired heating devicefqr heating the liquid to a second pre-determinedtemperature, said second pre-determined temperature being lower than said first pre-determined temperature.

By the instant invention, when there is sufficient waste or reject heat available, the heat pump isoperable to heat the liquid to the first pre-determined temperature and the difference between the first and second pre-determined temperatures enables an average demand for heated liquid to be met without the temperature ofthe liquid in the storage vessel falling belowthesecond pre-determined tem perature at which the electric orfuel4ired heating de- vice would become operable.However, if there is insufficient waste or reject heat available for the heat pump to heat the liquid to the second pre determined temperature or ifthere is a large demand for heated liquid causing the temperature ofthe liquid in the storage vessel to fall below the second pre-determined temperature, the electric or fuelfired heating device is operable as a back-up to the heat pump to heat the liquid to the second pre determined temperature at which the electric or fuelfired heating device cuts out and further heating of the liquid is provided by the heat pump only up to the first pre-determined temperature. In this way, use of the electric of fuel-fired heating device is reduced to the minimum necessaryto accomodate fluctuations in the waste or reject heat available whilst ensuring thatthe liquid in thestoragevessel is heated to the second pre-determined temperature at least. As a resuit, there is available a supply of heated liquid atan acceptable temperature qs determined by the second pre-determined temperature.

Preferably the condenser forms one coil of a twin coil heat exchanger and the liquid in the storage vessel is heated either directly by circulation through the other coil to collectthe heat dissipated bythe condenser thereby heating the liquid or indirectly by circulation of a heat transfer medium through the other coil to collect the heat dissipated by the condenser and a calorifier coil positioned in the storage vessel thereby heating the liquid.

Conveniently the liquid or heat transfer medium is circulated through the other coil of the heat exchanger by means of a pump and the pump and electric or fuel-fired heating device are controlled by respective thermostats arranged to monitor the liquid temperature in the storage vessel.

The evaporator may be positioned to collect waste or reject heat from any suitable source. Advantageously, however, the liquid heating system is combined with a space cooling system for controlling the air temperature in an enclosure,forexample a beer cellar, a meat or vegetable cold room, ora building.

In the combined liquid heating/space cooling system, the evaporator is positioned in the enclosure to collect reject heat from the air within the enclosure thereby cooling the air which is returned to the enclosure at a reduced temperature.

Advantageously the heat pump includes a second condenser operable when the liquid is heated to the first pre-determined temperature to dissipate the heat collected by the evaporator by forced airconvection thereby enabling the cooling systemm to continue operation when the liquid heating requirement has been satisfied. Conveniently a fan is arranged to direct air over the second condenser and the thermostatfor controlling the pump also controls the fan to operate selectively the water cooled and air cooled condensers in response the the liquid tem- perature sensed. Preferably a furtherthermostat positioned in the enclosure is operable to switch off the heat pumpwhen the airtemperature in theenciosure is reduced to a pre-set minimum level.

Conveniently the air cooled condenser isconnec- ted in series with the water cooled condenser so that the hot refrigerant from the evaporator passes through both condensers. In this way, when the liquid heating requirement is satisfied and the pump is switched off, the liquid in the storage vessel continuesto be heated bythermo-syphoning of the liquid or the heattransfer medium through the other coil ofthe heat exchanger. As a resultthetemperature of the liquid in the storage vessel is maintained at or slightly above the first pre-determined temperature and heat loss from the storage vessel does not cause intermittent operation of the pump.

Where the airtemperature in the enclosure is dep endent on the external ambient temperature it is preferred to provide for operation of the electric orfuelfired heating device to heat the liquid in the storage vessel to the second pre-determined temperature or a third pre-determined temperature intermediate the first and second pre-determined temperatures depending on the external ambienttemperaturn.

The electric or fuel-fired heating device is operable to heat the liquid to the third pre-determined tem perature when the external ambienttemperature is below a pre-set level at which the cooling require ment in the enclosure is low and therefore there will be insufficient reject heat availableforthe heat pump to heatthe liquid in the storage vessel. When the external ambienttemperature is above the pre-set level indicating that the cooling requirement in the enclosure is high and consequently there will be sufficient reject heat availabie forthe heat pump to heat the liquid in the storage vessel, the electric orfuel- fired heating device is setto heatthe liquid to the second pre-determined temperature.

By arranging forthe third pre-determined temperature to be slightly less than the first predetermined temperature, the liquid in the storage vessel is heated to a substantially constant temperature by either the heat pump or the electric or fuel-fired heating device for all operating conditions whilst ensuring that minimum use is made of the el ectric or fuei-fired heating device by arranging for the first pre-determined temperature to be the highest. In this way the heat pump provides the main heating requirement whenever there is sufficient reject heat available for collection by the evaporator.

As a result, the most economical use is made of the heat pump and the electricorfuel-fired heating device.

An exemplary embodiment of a liquid heating system according to the present invention in combination with a space cooling system will now be described with reference to the accompanying drawings, wherein Figure lisa schematic lay-out of a liquid heating system according to the present invention in combination with a space cooling system; Figure2 is section through the heat recovery unit ofthe heat pump of the combined system shown in Figure 1; and Figure 3 is a diagrammaticview of the electrical supply circuitfor controlling the boiler of the com- bined system shown in Figure 1.

Referring first to Figures 1 and 2 of the accompanying drawings, there is shown a liquid heating system for heating water in a storage cylinder 1 by means of a heat pump 2 and/or a fuel fired boiler 3 in combination with a space cooling system for controlling the airtemperature in an enclosure 4 by means ofthe heat pump 2.

In known mannerthe storage cylinder 1 has a cold water feedS connected to the lower end and a ventline 6 connected to the upper end from which branches a supply line 7 for delivering hot waterto various outlets, for example wash basins, baths, bidets, showers, sinks, washing machines, dishwashers etc.

The heat pump 2 has an evaporator8 positioned within the enclosure 4 and a pair of condensers 9,10 connected in series and positioned externally ofthe enclosure4 in a generally rectangular housing 11.

The evaporator 8 and condensers 9, form part of a closed refrigerant circuitthroug h which a refrigerant is circulated and which, in known manner, includes a compressor 12, refrigerant drier (not shown), sight glass (not shown), receiver (not shown) and expansion device (not shown).

The evaporator 8 is of looped and finned construction and a fan 13 positioned in front of the evaporator 8 is operable to direct air from within the enclosure4 over the evaporator8 backto the enclosure.

The housing 11 is divided into upper and lower compartments 14 and 15 respectively by a partition wall 16. One of the condensers 9 is water cooled and forms the outer coil of a twin coil heat exchanger 17 located in the lower com pa rtment Sand surroun- ded by thermal insulation material 18. The inner coil of the heat exchanger 17 is connected by supply and return lines 19and 20 respectivelytothe storage cylinder 1 and a pump 21 positioned in the supply line 19 is operable to pump water from the storage cylinderl throughtheinnercoil andbacktothestor- age cylinder 1. An automatic air vent 22 is positioned in the return line 20 to release expansion gases generated by heating of the water.

The upper compartment 14 is sub-divided by an uprightwall 23 on opposed sides ofwhich are located the other condenser 10 and compressor 12. The condenser 10 is air cooled by a fan 24 positioned in front of the condenser lOin an opening in the wall 23.

Thefan 24 is operableto draw airfrom the sur roundings through an inlet (not shown) into the housing 11 and to direct air over the condenser 10 and backthrough an outlet (not shown) to the surroundings.

The condensers 9, 10 and compressor 12 contained in the housing 11 form a separate heat recovery unit 25 having appropriate fittings for connection to refrigerant supply and return lines 26 and 27 from and to the evaporator 8 and vvater supply and return lines 19 and 20 from and to the storage cylinder 1. In this waythe heat recovery unit25 may be positioned in anysuitablelocation externallyofthe enclosure 4 and connected to the other components of the system.

The boiler 3, which may be gas, coal or oil fired, is operable to heat a heattransferfluid which iscirculated by means of a pump 28 th rough a closed circuit 29 including a calorifier coil (not shown) positioned in the upper half of the storage cylinder 1 for heating the water in the storage cylinder 1. In known manner the fluid circuit 29 may also serve one or more radiators of a central heating system (not shown).

For controlling operation of the combined liquid heating and space cooling system there are a number of thermostats T1 ,T2,T3,T4 and T5. T1 ,T2,T3 are arranged to monitor the temperature of the water in the cylinder 1. T4 is arranged to monitorthe external ambient temperature and T5 is arranged to monitor the airtemperature in the enclosure 4which in this particular embodiment is a beer cellar.

T1 and T2 control operation of the boiler 3 and are set to 55and 60"C respectively. T3 controls operation of the heat pump condensers 9, 10 and is setto 63"C. T4 controls operation ofthe boilerthermostats T1 and T2 and is set to approximately 13/14"C. T5 controls operation of the heat pump 2 and is setto 10/1200.

Referring now to Figure 3, the electrical supply cir cuitforthe boiler 3 is shown in which the supplyfirst passes through the conventional time clock 30 for setting the time periods in which the boiler 3 is oper able. From the time clock, the supply passes to T4 and from T4to either T1 orT2 depending on the temperature sensed byT4. From T1 or T2 the supply passes to the boiler3.

The electrical supply to the heat pump 2 is constant and passes to either the pump 21 of the water cooled condenser 9 or the fan 24 ofthe air cooled condenser 10 depending on the temperature sensed byT3.

In operation ofthe combined system abovedescribed, when the air temperature in the enclosure 4 is above 10-12"C, there is a cooling requirement in the enclosure 4 and hence waste or reject heat available for collection by the heat pump 2 as will now be described.

Air within the enclosure 4 is directed over the evaporator 8 by the fan 13 where it is cooled by heat exchange with the refrigerant in the evaporator 8 therebyvaporising the refrigerant and cooling the air which is returned to the enclosure 4 at a reduced temperature.

The vaporised refrigerant is then passed in supply line 26 to the compressor 12 of the heat recovery unit 25 where the refrigerant is compressed thereby increasing its temperature. From the compressor 12, the hot refrigerant is passed first th rough the water cooled condenser 9 and then through the air cooled condenser 10.The heat collected is dissipated thereby cooling and condensing the refrigerant in primarily either the water cooled condenser 9 by heat exchange with the waterfrom the storage cylinder 1 which is pumped through the inner coil ofthe heat exchanger 12 if the water temperature sensed by T3 is less than 63"C or the air cooled condenser 10 by heat exchange with atmospheric air which is forced over the condenser 10 by fan 24 ifthewatertem- perature sensed by T3 is greater than 63"C. The condensed refrigerant from the air cooled condenser 10 is passed backto the evaporator8 in return line 27.

To maximise heat exchange between the water and hot refrigerant in the heat exchanger 17, the flow of waterthrough the inner coil is arranged to be in the opposite direction to the flow of hot refrigerant through the outer coil.

When the airtemperature in the enclosure 4 is reduced to 10/12"C thereby satisfying the cooling requirement, thermostat T5 operates a valve 31 in the refrigerant circuit preventing flow of the refrigerant which causes the heat pump 2 to shut-off until the air temperature in the enclosure 4 again rises above 10/ 12"C.

The air temperature in the enclosure 4 is subjectto the external ambient tem peratu re so that the cooling requirement and hencethe amount of waste or reject heat available for collection by the heat pump 2to heat the water in the storage cylinder 1 as above- described varies with the external ambienttemperature. As a result, under certain conditions, operation of the heat pump 2 alone may be insufficient to satisfy the water heating requirement in the storage cylinder 1. For this reason the boiler 3 is provided as a back-uptothe heat pump 2 and is operable to compensate automaticallyforvariations in the external ambienttemperature as will now be described.

For conditions where the ambient temperature is greaterthan 13/14"C, for example in su mmer, and consequentlythe cooling requirement in the enclosure 4 is high, T4 actuates T1 so that in any timed period in which the boiler3 is operable, the heat pump 2 and boiler 3 will heat the water in the storage cylinder 1 to 55"C whereupon the boiler 3 will cut-out and further heating of the water up to 63"C will be provided by the heat pump 2 alone.In this way when there is a significant contribution to water heating from the heat pump 2 enabling normal demand for hot water to be met by operation of the heat pump 2, the boiler 3 is set to operate up to a watertemperature of 55"C so that a large demand for hot water can be met.

For conditions where the ambienttemperature is lessthan 13/14"C, for example in wi nter, and con- sequentlythe cooling requirement in the enclosure is low,T4 actuates T2 so that in anytimed period in which the boiler is operable, the heat pump and boilerwill heat the water in the storage cylinderto 60"C whereupon the boiler will cut-out and further heating of the water up to 6300 will be achieved by the heat pump alone. In this way when there is only a small contribution to water heating from the heat pump, the boiler is setto operate upto a watertemperature of 60"C to compensate for such reduced contribution so that all demand for hot water can be met.

As will be understood from theforegoing descrip- tion of combined liquid heating/space cooling system, by arranging forthe heat pumpto always heat the water to a highertemperature than thatto which the boiler is operable to heat the water, the boiler is only used when there is insufficient waste or reject heat available to meet the demand for hot water.

Furthermore, by arranging for the boilerto com pensateforvariations in the contribution to water heating from the heat pump, maximum use is always made of the available waste or reject heatforwater heating whilst the boiler provides adequate back-up to ensure that all demand for hot water can be met.

Additionally, because the heat pump is operable whenever there is a cooling requirement in the enclosure, heating of the water in the storage cylinder by the heat pump during a time period in which the boiler is inoperable will further reduce or preclude the contribution to water heating from the boiler during a time period in which the boiler is operable.

The invention is not limited to the embodiment above-described, for example the thermostats Tl ,T2,T3 may be setto any desired temperatures provided that T3 is always greater than T1 or T2. Like wiseT4 and T5 can be set to any desired temperatures. For example T5 may be setto provide an air temperature in the range 300C to +25 C. To permit operation at low settings of T5, for example in the range 0 C to -30 C, the evaporator will include an electricdefrost.

The combined liquid heating/space cooling system above-described in which the airtemperature in the enclosure is dependent on the external ambient temperature has application to heat recovery otherthan from cellar cooling as described, forexampleairconditioning in bpildings(offices, factories), and cold rooms for meat, vegetables, fruit and the like, to heat water for use on the premises.

It will be understood that the invention is not limited to combined liquid heating/space cooling systems and in its simplestform consists in a liquid heating system having a heat pump operable to col lect heat from any source of waste or reject heat irrespective of cooling requirementfor heating liquid in a storage vessel to a first temperature with back-u p from an electric orfuel-fired heating device for heating the liquid to a second lowertemperature in the event the waste or reject heat available for collection by the heat pump is insufficient to meet the demand for heated liquid. In such simple form oftheinvention the heat pump and boiler may each be controlled by a respective thermostat and the air cooled condenser ofthe heat recovery unit may be dispensed with.

Claims (20)

1. A liquid heating system comprising a storage vessel for liquid to be heated, a heat pump for heating the liquid to a first pre-determined temperature, said heat pump including an evaporatorfor collect- ing waste or reject heat and a condenserfordissipat ing the collected heat to the liquid and, an electric or fuel-fired heating device for heating the liquid to a second pre-determined temperature, said second pre-determined temperature being lower than said first pre-determined temperature.

2. Asystem according to claim 1 wherein the condenser forms one coil of a twin coil heat exchanger ofwhich the other coil is connected to the storagevesselforcirculation ofthe liquid.

3. A system according to claim 1 wherein the condenser forms one coil of a twin coil heat exchan gerofwhichthe othercoil is, connectedto a calorifier coil positioned within the storage vessel for circulation of a heattransfermedium.

4. Asystem according to claim 2 or claim 3 wherein the circulation of the liquid or heat transfer medium is arranged to be in the opposite direction to the flow of refrigerant through the condenser coil.

5. A system according to any one of claims 2 to 4 wherein the circulation ofthe liquid or heattransfer medium is controlled by a pump operable in re sponse to thetemperature ofthe liquid in the storage vessel.

6. Asystem accordingto anyoneofciaims2to 5 wherein the heat exchanger is thermally insulated.

7. A liquid heating system according to claim 1 substantially as hereinbefore described with reference to the accompanying drawings.

8. A liquid heating system according to any one of the preceding claims in combination with a space cooling system for controlling the air temperature in an enclosure wherein the evaporator is positioned in the enclosure for collecting heatfrom the air in the enclosurethereby cooling the air which is returned to the enclosure at a reduced temperature.

9. The combination according to claim 8 wherein the air in the enclosure is circulated overthe evap oratorbyafan.

10. The combination according to claim 8 or claim 9 wherein the heat pump is operable to reduce the airtemperature in the enclosure to a pre-set minimum level.

11. Thecombination accordingtoclaim 10 wherein the heat pump includes a second condenser for dissipating the heat collected by the evaporator to air externally of the enclosure.

12. The combination according to claim 11 wherein the air is circulated over the second condenser by a fan operable in response to thetemperature of the liquid in the storage vessel.

13. The combination according to claim 11 or claim 12 wherein the condensers are selectively operable in response to the liquid heating and space cooling requirements.

14. The combination according to claim 13 wherein the liquid heating condenser is operable when there is both a liquid heating and a space cooling requirement, and the air cooled condenser is operablewhen the liquid heating requirement is satisfied and there is still a space cooling requirement.

15. The combination according to any one of claims 11 to 14 wherein the condensers are connected in series.

16. The combination according to claim 15 wherein the air cooled condenser is arranged downstream ofthe liquid heating condenser.

17. Thecombination accordingto anyoneof claims 11 to 16 wherein the condensers are mounted in a housing forming a separate unit for location externally ofthe enclosure and having the appropriate means for connection to the storage vessel and evaporator.

18. The combination according to any one of claims 8 to 17 wherein the electric or fuel-fired heating device is selectively operable to heat the liquid to either the second pre-determinedSemperature or a third pre-determined temperature intermediate the second and first pre-determined temperatures in re sponse to the air temperature outside the enclosure.

19. The combination according to any one of claims 8 to 18 wherein the electric or fuel-fired heating device is controlled by timer means for setting the time period in which the device is operable.

20. The combination according to claim 8 substantially as hereinbefore described with reference to the accompanying drawings.