Heat pump apparatus

This invention generally relates to heat pump apparatus and more particularly is directed to a split heat pump apparatus comprising - an outdoor unit comprising an expansion valve, a radiator adapted to be heated by outdoor air, and a refrigerant compressor, an indoor unit comprising a radiator adaptedto be cooled by air, a refrigerant circulation loop that is provided with the expansion valve and arranged to convey a refrigerant pressurised by the com- pressor from the compressor to the radiator of the indoor unit and back to the compressor, and a control device which comprises a first temperature sensor for sensing the refrigerant temperature at the exit of the compressor, a second temperature sensor for sensing the temperature of the refrigerant flowing from the radiator of the indoor unit to the expansion valve, and a third temperature sensor for sensing the temperature of an air flow conveyed to the radiator of the indoor unit.

Air-to-air heat pump apparatus of the kind defined above are manufactured as air-conditioners in extremely large numbers by numerous manufacturers fiercely competing with one another. The large volumes of production and the intense competition has furthered a highly cost-effective production and a price that is low in comparison with the cost of heat pump apparatus designed for the production of water-borne heat.

The cost-effective air-to-air heat pump apparatus are primarily meant for heating a single room. It is true that air-to-air heat pump apparatus comprising a plurality of indoor units and designed for heating of a plurality of rooms are marketed, but because of the cost these are normally not attractive.

From the point of view of heat comfort, low- temperature surface heating, such as underfloor heating, is preferred. With underfloor heating, and in comparison with room heating by introduction of warm air, a lower air temperature at the level of the head of persons occupying the room is adequate to provide the desired heat comfort, so that the energy required for the heating can be reduced. If a heat pump is used for underfloor heating, a plurality of rooms can be heated.

Air-to-water heat pumps exist, but their prices are several times the prices of air-to-air heat pumps with a single indoor unit and they only heat water.

It is known (Mitsubishi) to use an add-on unit to produce waterborne heat energy together with air-borne heat energy from the indoor unit by allowing the hot gaseous refrigerant pass through a heat exchanger for waterborne heat for floor heating before the refrigerant reaches the indoor unit of the heat pump apparatus.

A disadvantage of an arrangement of this kind is that the condensed and thus liquid refrigerant formed in the floor-heating heat exchanger has to be transported against the gravity forces from the floor-heating heat exchanger, which is positioned close to the floor level, to the indoor unit, which is usually placed close to the ceiling, by means of bubbles of gaseous refrigerant. This method of transport is unreliable.

Moreover, in a properly dimensioned heat exchanger for floor heating the refrigerant condenses at a temperature close to the supply temperature of the water in the floor heating system, e.g. slightly above 30 ⁰ C. This refrigerant temperature is not sufficient to provide a material heating of the room air that passes through the indoor unit, particularly so when the fan of the indoor unit is set at a low speed to operate with-

out causing disturbing noise as is desirable, especially in the nighttime.

An object of the invention is to remedy the above-described disadvan- tages of the prior art and provide an improved heat pump apparatus that is capable of economically producing not only airborne heat energy as in the conventional split-type heat pump apparatus, but also water- borne heat energy.

This object is achieved through the features which are set forth in the independent claim and described in more detail hereinafter with reference to the accompanying diagrammatic drawings.

The aforesaid object can advantageously be achieved with an add-on unit by means of which commercial air-to-air heat apparatus of the kind indicated initially can be modified for production of both airborne and waterborne heat energy. This modification can be carried out substantially without any intervention in the outdoor and indoor units. Suppliers of outdoor and indoor units can therefore be expected to apply the terms of their customary product warranties to apparatus modified according to the invention.

Brief description of the drawings:

Figure 1 is an illustration of a prior art heat pump apparatus, namely a split-type air conditioner;

Figure 2 is an illustration of the heat pump apparatus shown in Figure 1 as modified according to one embodiment of the invention;

Figure 3 is an illustration of a different modification of the heat pump apparatus shown in Figure 1.

In the following description of the heat pump apparatus according to the invention the focus is chiefly on the operation of the apparatus for production of warm air for room heating and of waterborne heat for heating of water, such as water for underfloor or other surface heating, or tap water. As is readily appreciated, the heat pump apparatus according to the invention may also be designed to be capable of use in the well-known manner for cooling, the direction of refrigerant flow being then reversed by means of a four- way valve. The direction of refrigerant flow is assumed to be the direction applicable to the heating mode (as opposed to the cooling mode), and designations such as, for example, inlet/ outlet and upstream/ downstream apply to that assumed direction of flow.

The prior art heat pump apparatus shown in Figure 1 comprises an outdoor unit 1 and an indoor unit 9 which is connected to the outdoor unit 1 through a refrigerant circulation loop generally designated by C. In operation of the heat pump apparatus, a refrigerant circulates through an expansion valve 2, a radiator 3 associated with a fan, and through a compressor 4, all included in the outdoor unit, and through a radiator 10 included in the indoor unit 9 and associated with a fan. Included in the outdoor unit 1 is also a control unit 5 which is connected to a temperature sensor 6 for the outdoor air and a temperature sensor 7 for the hot gaseous refrigerant exiting from the compressor. The indoor unit 9 also includes a control unit 11 which is connected to a temperature sensor 12 for the indoor air and a temperature sensor 13 for the partially condensed refrigerant exiting from the radiator 10. The control units 5 and 11 in the outdoor unit 1 and the indoor unit 9, respectively, communicate electrically with one another through a signal line 8 and form a control device for the heat pump apparatus.

Assuming heat pump operation, the cold, partially gaseous refrigerant exiting from the expansion valve 2 is preheated as it flows through the radiator 3, whereupon the refrigerant is compressed by the compressor 4. The heated refrigerant condenses in the radiator 10 of the indoor unit 9, in which it releases its latent heat before it returns to the outdoor unit 1.

By means of the temperature sensor 12 in the indoor unit 9 the control unit 1 1 operates to keep the indoor air at a desired set value. The temperature sensor 13 senses the refrigerant condensation temperature, which is an important intermediate quantity for the control of the temperature of the indoor air.

Figure 2, in which the reference characters 1 to 13 of Figure 1 are used to designate the same or corresponding elements as in Figure 1 , shows one embodiment of the heat pump apparatus according to the invention. This heat pump apparatus comprises a number of modifications of the prior art apparatus shown in Figure 1. Thus, a first heat exchanger 14 is provided which has its primary (heat releasing) side connected into the refrigerant circulation loop C and passes the refrigerant exiting from the radiator 10 to the inlet side of the outdoor unit 1 in which the refrigerant flows through the expansion valve 2, the radiator 3 and the compressor 4 as in Figure 1. Likewise as in Figure 1 , the refrigerant then flows back to the indoor unit 9 and its radiator 10 and into the primary side of the heat exchanger 14.

The secondary (heat receiving) side of the heat exchanger 14 is connected to a water circulation circuit which includes a circulation pump 15 operating to feed water heated in the heat exchanger 14 to a heating element 16. In the illustrated embodiment, the heating element is formed of an underfloor heating arrangement in which the water fed to it from the secondary side of the heat exchanger 14 flows through

piping connected between the upstream and downstream parts of the secondary side of the heat exchanger 14. It should be noted that the heating element 16 is not shown in full but represented only by four pairs of tubular connectors which convey the hot water to a corres- ponding number of tubular heater loops. Thus, the heater loops are not shown.

Figure 2 also shows an optional additional heat exchanger 17 connected in series with the first heat exchanger 14. If desired, more than one such additional heat exchanger may be included in the refrigerant circulation loop. Regardless of the number of such heat exchangers in excess of the first heat exchanger 14, the temperature sensor 13 should be placed downstream of the last heat exchanger, since it has to sense the refrigerant temperature near the expansion valve 2 (for practical purposes, the refrigerant temperature can be regarded as the same throughout the section of the refrigerant circulation loop that extends between the last additional heat exchanger and the inlet of the expansion valve 2).

Figure 2 also shows a further, likewise optional heat exchanger 20 which has its primary (heat releasing) side connected into the part of the refrigerant circulation loop C that extends between the exit of the compressor 4 and the radiator 10 of the indoor unit. A bypass conduit is connected between the inlet and the outlet of the radiator 10 of the indoor unit 9 to allow the refrigerant to short-circuit that radiator and pass directly to the primary side of the heat exchanger 14 as shown. Such a bypass conduit is shown in Figure 2 and designated by 18. In the bypass conduit 18 a bypass flow control element 19 is inserted for controlling the bypass refrigerant flow through the bypass conduit.

The bypass flow control element may be, for example, a check valve or a solenoid valve which is operated automatically or manually when the

heat pump apparatus is to be used for air-conditioning (cooling of the radiator 10, the direction of refrigerant flow through the radiators 3 and 10 and the expansion valve reversed). It may also simply be formed by the conduit 18 itself, e.g. be a piece of tubing of suitable internal cross- section which short-circuits the inlet and the outlet of the radiator 10 of the indoor unit 9, or an orifice plate or some other fixed or adjustable restrictor element. In some applications of the invention, it may be advantageous to use an adjustable shut-off valve to allow adjustment of the bypass flow rate over a suitable range, including zero flow rate.

If the optional heat exchanger 20 is provided, it may happen that at least part of the refrigerant will condense on the primary side of that heat exchanger. If the heat exchanger 20 is placed at a lower level than the radiator 10 of the indoor unit 9, so that there is a risk that any condensate will not be led to and through the radiator and beyond it on the outlet side of the radiator, the bypass conduit 18 should be mounted such that it is descending in the desired direction of condensate flow or at least non-ascending, so that the condensate will be led to that part of the refrigerant circulation loop C which extends from the indoor unit 9 to the outdoor unit 1. If it is also desired that all gaseous refrigerant will be safely led to the radiator 10 of the indoor unit, the bypass conduit should include a condensate valve, which may be of any known type.

In the embodiment shown in Figure 3 the heat pump apparatus comprises an auxiliary heat source 22, namely a wood-burning stove provided with a water tank serving as an accumulator of waterborne heat. Reference numerals 1 to 16 and 18 and 19 designate the same elements as the corresponding reference numerals in Figure 2. When the auxiliary heat source 22 is in operation, it delivers, by means of an associated circulation pump 21 , hot water to the primary side of the heat exchanger 20, the secondary side of which carries water that has

been preheated by the heat exchanger 14 of the heat pump apparatus. The circulation pump 15 feeds hot water from the heat exchanger to the heating element 16, which is formed of underfloor heating coils. Obviously, the heat pump apparatus and the auxiliary heat source 22 can independently supply hot water to the underfloor heating coils. When the heat pump apparatus and the auxiliary heat source 22 produce heat simultaneously, the heat pump apparatus adjusts, by means of its control device formed by the control units 5 and 11, its heating effect such that the heat pump apparatus operates to keep the indoor air temperature as sensed by the temperature sensor 12 at the desired value. The auxiliary heat source 22 may be activated whenever the user desires to enjoy the comfort of the flames of fire or heating by means of the heat pump apparatus is insufficient. Alternatively, or additionally, a further auxiliary heat source may be connected to the heat exchangers 17 and 20 shown in Figures 2 and 3.