GB2062195A - Improvements in water heating installations employing boilers and heat pumps - Google Patents

Abstract

A heating installation comprises a burner 11 with the usual boiler or heat exchanger 12 and a heat pump comprising a compressor 19, an evaporator 22 and a condenser 20. The condenser is connected in the water circuit of the boiler so as to pre- heat the water entering the boiler; when the burner is not lit the condenser provides all the heat for the water. In the flue 13 there is an economiser 16 in the form of a second evaporator. This evaporator economiser is brought into operation by a switching arrangement 24 under the control of an external temperature senser when the external temperature senser senses the temperature is below a certain value and calls in the burner; otherwise the heat pump alone is called in. <IMAGE>

Description

SPECIFICATION Improvements in heating installations The present invention concerns heating installations for varying loads, for example, for central heating and hot water supply.

Domestic heat demands vary from a base load for a hot water supply in Summer to a peak demand in the winter months. Thus domestic heating installations are usually operating on inefficient part loading and are mainly operated intermittently especially since the installations have to have a reserve to cope with freakish cold spells. Various ways have been proposed to provide heating such as heat pumps extracting low grade heat from a suitable source and boosting its temperature to a usable level. Heat pumps are superficially attractive since they provide more energy than they need to run.

However they are least efficient in winter when the source temperature is low and are expensive in prime cost especially since the enhanced size of the heat pump to cope with the peak demand when working inefficiently usually means that rewiring the building to provide a three-phase supply. Thus conventional burner-type installations remain the most cost-effective despite wasting a great deal of the paid-for heat up the chimney or flue.

The present invention aims at combining heat pumps and burner installations and in accordance with the present invention a heating installation for houses and other buildings comprises a burner having a flue for the combustion gases, at least one hot water circuit for connection with space heating radiators and hot water taps, a heat exchanger in the circuit and disposed in the flue for heating the water from the combustion gases, the heat exchanger having inlet and outlet connections, an economiser for extracting further heat from the flue to pre-heat the water entering the heat exchanger, and a heat pump comprising an evaporator, compressor and a condenser with the condenser being in heat exchange relationship with the water.

The capacities of the burner and the heat pump would in any specific installation be such that the heat pump alone would supply the entire heat demand when the source temperature was above a certain value and at this time the burner could be switched off, with the burner capable of keeping the building comfortable in winter. In any specific installation, the certain value would be preestimated to give the most economical balance between running and prime cost. It would be preferably above the atmospheric dew point as determined from meteorological data for the locality (a typical value is 80C) so as to avoid problems with the heat pump evaporator icing up but could be above this so that the heat pump size could be low. A large heat pump could entail rewiring the building to give a three phase supply.

For example a single phase supply can drive a one or two kilowatt compressor of a heat pump and deliver sufficient heat for a normal building in the summer months.

A normal economiser is a water tube arrangement which pre-heats the water entering the heat exchanger from the waste heat in the flue. The invention can use a normal economiser but it is preferred to make use of the heat pump in combination with the economiser. In winter the heat pump if trying to extract heat from the cold surroundings is inefficient because of its lower source temperature. The flue gases would provide a higher temperature source for the heat pump.

Thus it is advantageous to switch the heat pump to take heat from the ambient or from the flue gases. This can be achieved by having the economiser in the form of an evaporator. This evaporator economiser can be completely separate from the heat pump evaporator or be the same evaporator. If separate, it can be connected in series or parallel with switching taking place automatically in that if the heat pump medium is passed through the heat pump evaporator in cold conditions there will be little pick-up of heat until the medium passes through the evaporator economiser or with a definite switching arrangement. With a parallel non-switched arrangement, one evaporator will pick up heat and evaporate the medium and the other would be inoperative. Using the same evaporator involves a switching arrangement for switching ambient air or the flue gases through the evaporator.

The burner could consume any type of fuel such as wood, coal, oil, fuel gas, or hydrogen but the initial moisture content and chemical make-up of the fuel has an influence on the installation in that moisture initially there in the fuel or combustion air or formed by combustion of any hydrogen content combines with any sulphur oxides or carbondioxide to form corrosive products which attack the usual mild steel flue if the temperature falls below a flue dew point. It is known to depress this flue dew point by using considerable excess air which also cools the burner; this excess air can be ten times the stoichometric combustion air.

Alternatively the greater proportion of the corrosive products can be condensed out. With burners which are switched off and on, the corrosion problem is most severe during start-up since then the flue gases contact a cold flue but it can be arranged that the heat pump is switched on only after the flue has warmed up so that the heat pump does not over-cool the flue. With the flue gas temperature reduced to a low level, it would be possible in new installations to use a corrosion-resistant plastics material flue but to avoid the risk of over-heating if the economiser failed, it would be advisable to have provision for inducing a considerable flow of air if required. It would of course be advantageous to by-pass any excess air needed in normal operation merely to depress the flue dew point around the economiser.

The economiser especially if it contains heat pump medium can be protected against corrosion by a thermo-syphon-or other buffer which transfers heat from within the flue to the economiser which would be disposed in a non-corrosive atmosphere outside the flue.

It is thought that the present invention skilfully employed could enable the excess air to be reduced leading to more ideal combustion.

A heat pump when extracting heat from a source and delivering it to a sink delivers a greater output for a given driving energy input when the temperature difference between source and sink is smaller. Thus the heat pump gain is higher when working from a larger source temperature in Summer. If the heat pump fails, then the burner should keep the building comfortable.

A simple control of an installation would consist of an external temperature senser sensing when the external temperature was above or below the certain value and summoning either the burner or the heat pump as required. The control could be adjustable to allow the said certain value to be adjusted. Such a control would be combined with a normal demand control. The control could also be arranged to stagger the start-up of any fan associated with the heat pump economiser so as to spread the starting surges of the heat pump and the fan.

Embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings in which: - Figure 1 is a schematic circuit diagram of the simplest form of the invention, Figure 2 is a block circuit diagram of a more advanced form, Figure 3 is a block diagram of a third form, Figure 4 is a block diagram of a fourth form, and Figure 5 is a block diagram of a control circuit.

In Figure 1 a burner 11 heats up water in a normal boiler or heat exchanger 12 and then delivers the combustion gases to a flue 13. The heat exchanger 12 has at least one inlet 14 and one outlet 15 whereby it is connected to a hot water circuit (not shown); in practice there will be one circuit for hot water supplying taps and one for hot water to central heating radiators with each circuit having its own inlet and own outlet.

An economiser 1 6 is connected iri the flue. This economiser is of the water tube variety and is connected to the inlets of the heat exchanger. A heat pump comprises a compressor 19, a condenser 20 and an evaporator 22. The condenser is located in a connection 23 between the outlets of the economiser and the inputs 14 of the heat exchanger. The heat pump medium circuit contains the usual restrictor 21.

The arrangement of Figure 2 is very similar except that the economiser is in the form of an evaporator connected in series with the evaporator 22 with the evaporator 22 on the upstream side of the economiser. The water does not pass through the economiser but is pre-heated by the condenser before entry to the heat exchanger.

The arrangement of Figure 3 is again similar to Figure 2 except the economiser is arranged in parallel with the evaporator 22 with a switching arrangement 24 controlling which is operative. In each embodiment there will normally be a fan 25 for passing air over the evaporator 22.

In the embodiment of Figure 4, the evaporator 22 is contained in a duct 26 through which ambient air can be blown by the fan 25 or which can be connected to the flue 13 under the control of a switching arrangement 24 which can be of any suitable type such as a rotary valve or a louvre system. Instead of the fan being arranged to blow air through the duct it could be arranged to suck the air through; this could be used to compensate for any loss of draught due to cooling the flue gases.

Figure 5 shows a simplified control circuit. An external temperature senser 29 with possible corrections for other factors influencing heat demand such as humidity, precipitation, wind and wind direction would feed an input to either gate 30 or gate 31 depending on the temperature sensed and thus on whether the demand should be met from the heat pump alone or whether the demand entails the burner. These inputs would condition the respective gate control by a normal demand sensers 32 and 33 such as sensers detecting the water temperature and building air temperature. The gate 30 if responding to a demand signal would summon in the heat pump compressor and the external evaporator 22 followed shortly by the fan 25.Associated with the gate 30 there could be a timer 34 to detect whether the heat pump was being summoned continuously in which case an alarm would be given and the demand transferred to the burner, or an adjustment would be made to the said certain value to see whether this was all that was needed.

The other gate 31 would call in the burner and shortly thereafter by means of a delay 35 the heat pump compressor as well as operating the switching arrangement 24 (if used). When the gate 31 is de-energised by the internal demand sensers, the heat pump and the burner are switched off but it would be possible to provide a latch 36 to drive off any liquid refrigerant in the evaporator economiser or to operate any other shut-down procedure. The control circuit is a simple form and could be combined with timing devices so as to vary the demand as required throughout the day.

The evaporator 22 could be disposed in a loft of the building to recover some building waste heat or in some other location possibly outside the building and the fan could be ducted so as to discharge the cold air outside the building. The size of the evaporator 22 would be related to the size of the compressor and the size of the condenser and would be capable of extracting sufficient heat from the air circulated through it by the fan to supply the building's heat demand when the external temperature was above a certain value which could be pre-set according to meteorological data for the district and would normally be above the dew point for that locality.

The certain value can be adjustable so that it can be varied to cope with varying heat demand patterns. No provision need be made for de-icing since the ambient temperature above which the heat pump can supply the building demand will not be less than the dew point below which icing can occur but if thought desirable the fan can circulate the air so that no condensation occurs (i.e. at a fast rate). The fan can be switched on before or after the compressor to avoid the switching surges coinciding. A temperature above the dew point (above 80C, say) is a relatively high minimum source temperature for a heat pump so that the heat pump operates with high gain. The heat pump medium or refrigerant should have a high energy transfer coefficient and low vapour pressures at the temperatures required for evaporation.

The burner can be a conventional burner but if an existing burner suitable for the demand exists, it is preferably downrated by about 30% to match in with the improved installation.

The cconomiser is rather special in that if used with heat pump medium or refrigerant it must resist corrosion by the flue gases, must be cleanable and must be capable of withstanding flue temperatures. This latter requirement arises in case of failure of the heat pump or if the start-up of the heat pump is delayed to allow the flue temperature to exceed the flue dew point.

Normally during start-up, the refrigerant will be safely stored outside the economiser. When the refrigerant is introduced into the high temperature economiser, the pressure generated would be the design pressure for the heat pump system.

If a solid-fuel burner is used, it cannot be switched on and off and thus the invention would be used to control dampers and it would be necessary to have an over-ride to prevent the controls calling in an unlit burner. The installation described matches well in with the normal demand pattern as derived from degree-day tables. A heat pump system can supply the building heat demand for six months of a year without being excessively large. For example a two kilowatt compressor can drive a heat pump giving six kilowatts of heat. There will be an increase of capacity in the hotter periods which capacity will not be needed but this slight mismatch is unavoidable, and merely requires shorter periods of operation of the heat pump. The burner supplements the heat pump in the colder months and a heat pump with a two kilowatt compressor can extract say ten kilowatts from the flue gases. So with a burner capable of providing twenty kilowatts in the normal way using a boiler or heat exchanger, the total capability is thirty kilowatts. A twenty kilowatt burner will normally waste ten kilowatts in the flue gases so when the heat pump recovers that ten kilowatts, no heat is being wasted up the chimney or flue.

Claims (11)

1. A heating installation for houses and other buildings comprises a burner having a flue for the combustion gases, at least one hot water circuit for connection with space heating radiators and hot water taps, a heat exchanger in the circuit and disposed in the flue for heating the water from the combustion gases, the heat exchanger having inlet and outlet connections, an economiser for extracting further heat from the flue to pre-heat the water entering the heat exchanger, and a heat pump comprising an evaporator, compressor and a condenser with the condenser being in heat exchange relationship with the water.

2. An installation according to claim 1 wherein the condenser is disposed at the inlet connection to the heat exchanger.

3. An installation as claimed in claim 1 or claim 2 wherein the economiser comprises a switching arrangement for directing either ambient air or the flue gases through the evaporator.

4. An installation as claimed in claim 1 or claim 2 wherein the economiser comprises an economiser evaporator connected in the heat pump circuit and switched into operation in preference to the first mentioned evaporator in dependence on the ambient temperature.

5. An installation as claimed in claim 4 wherein the two evaporators are connected in parallel with the switching being done by a switching arrangement.

6. An installation according to any one preceding claim in combination with a control circuit including an external temperature senser for summoning the burner and the economiser or the heat pump with the first mentioned evaporator in dependence on whether the external temperature is below or above a certain value, and on a demand for heat being made.

7. An installation according to claim 6 wherein the control circuit is adjustable to vary the said certain value.

8. An installation according to claim 6 or 7 as dependent on claim 3, claim 4 or claim 5 wherein the control circuit when calling the burner summons the heat pump with the economiser after a short delay.

9. An installation substantially as herein described with reference to Figure 1, Figure 2, Figure 3 or Figure 4 of the accompanying drawings.

10. An installation as claimed in claim 9 in combination with a control circuit substantially as herein described with reference to Figure 5 of the accompanying drawings.

11. A control circuit substantially as herein described and for the purpose herein described with reference to Figure 5 of the accompanying drawings.