FR2484065A1 - IMPROVEMENTS ON HEAT PUMPS - Google Patents

Abstract

1. Heat pump comprisign : - two moto-compressor groups (1a, 1b) for liquefiable heat-transfer fluid, capable of being operated independently of each other ; - two evaporators (15a, 15b) respectively associated individually with the compressor (2a, 2b) of each group (1a, 1b) to form the cold source of a corresponding elementary heat pump ; - a channel (16) in which these two evaporators (15a, 15b) are arranched in series so as to be able to be traversed in heat exchange by a same current of an external reheating fluid such as air, capable of containing moisture ; - a motor-driven fan (17, 18) interposed in said channel to ensure the circulation therein of the reheating fluid ; - two condensers (7a, 7b) respectively associated individually with the compressor (2a, 2b) of each group (1a, 1b) to form the warm source of the corresponding elementary heat pump ; - heat exchange means for cooling these two condensers (7a, 7b) with the help of an external fluid so that heat may be taken up by the latter, characterized by the combination of the following features : - the motor-driven fan (17) is reversible ; - means (22a, 22b) are provided for detecting individually the icing-up of one or other of the two evaporators (15a, 15b) ; - command means (24, 4a, 4b) are also provided placed under the control of the icing up detecting means (22a, 22b) and which act in such a way that in normal running the two motor-compressor groups (1a, 1b) function simultaneously, but that when icing-up has been detected on the evaporator which is at that time situated downstream in the current of reheating fluid, they stop the corresponding motor-compressor group (1a, 1b), reverse the motor-driven fan (17, 18) and then, when the evaporator of the group thus stopped has been freed of ice, set this group in operation again without stopping the other and without reversing again the motor-driven fan (17, 18).

Description

We know that heat pumps are called thermal machines

  operating in reverse, that is, to which we provide

  mechanical power to obtain heat. In fact, like these machi-

  nes work according to the Carnot cycle, their action is twofold, namely that on the one hand they transform mechanical power into heat, but on the other hand they take heat from a cold source to transfer it to a hot source , that is, they raise the temperature level of the heat thus transferred, which explains the name they were given. If the heat taken from the cold source is free, if the temperature difference between it and the hot source is not too high and if the mechanical efficiency of the machine in question is good, we can thus arrive at have at the hot source a quantity of heat much greater than that which would result from the direct transformation into thermal energy of mechanical energy

applied to this machine.

When it is possible to have water from a river or a lake, it is advantageously used to constitute the external fluid for supplying calories to the cold source, but this is rare in practice. The usual heat pumps are therefore generally designed to employ

this effect atmospheric air.

  However, this poses a sometimes annoying problem. The cold source of a conventional heat pump is in practice constituted by a heat exchanger or "evaporator" in which a suitable heat transfer fluid (freon in general) evaporates by absorbing the heat of the external fluid used (water or air) which obviously cools down. In the case where the external cold source fluid consists of ambient air, this cooling can cause a phenomenon of condensation. As long as the surface of the evaporator exposed to the air remains larger

at 00C, the water thus condensed flows and can be evacuated without difficulty.

But below this limit, there is icing. The skin in question is

covers with an adherent layer of frost which insulates it by reducing considerably

the exchange coefficient and consequently lowering the vaporization temperature of the heat transfer fluid, which in turn decreases the performance of the machine. In addition, the frost more or less obstructs the passage of the air current, thus sliding in the same unfavorable direction as the insulation of the evaporator. It is therefore essential to carry out a defrosting operation from time to time, taking care that the energy expended does not decrease too significantly the

final energy balance.

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contact of the two elements such as 7a, 7b.

The liquid freon under pressure leaving each of the elements 7a, 7b is brought by a line 12a, 12b to a pressure reducer 13a, 13b from which it arrives under reduced pressure by a line 14a, 14b to an evaporator 15a, 15b arranged so that a stream of reheating air can pass through it. Each of these evaporators can for example be produced for this purpose in the form of a flat coil or a

tube bundle joining an inlet body to an outlet body.

  The two evaporators 15a, 15b are arranged at one and at the other end of a sort of tubular box 16 inside which is provided a reversible fan 17 controlled by a motor 18 supplied by a line 19, for example three-phase, on which is interposed an inverter

20 with electric actuation.

  Of course the box 16 must have a circular section in the plane of the fan 17, but nothing prevents this section from gradually passing to a square or rectangular shape towards each end.

  to facilitate the production of evaporators.

The freon vaporized in the evaporators 15a, 15b is brought back to the

  compressors 2a, 2b by individual pipes 21a, 21b. -

  The evaporators 15a, 15b are associated with individual icing detectors 22a, 22b established in the form of electrical transducers, of known type, which send their signals by lines 23a, 23b to a microprocessor 24. The latter has three outputs, namely a first which leads to the inverter 20 and two others 26a, 26b connected to the

respective contactors 4a and 4b.

The operation is as follows In normal operation, that is to say in the absence of icing, the two groups 1a and 1b operate simultaneously, the fan 17 rotating in any direction, for example to determine a current of air according to the arrows 27. The outside air entering at a temperature tl passes through the evaporator 15a to which it gives up heat while cooling itself; it leaves it at a temperature t2 and, if one neglects the heat given off by the motor fan 17-18, it arrives at this same temperature at the evaporator 15b. It also heats it by cooling and it goes outside at a temperature t3. During this time the heat of condensation of the freon in the elements 7a, 7b of the condenser 8 heats the circulation water which leaves by 10 at the temperature a'84065

desired for the intended application, heating of premises for example.

  If the inlet temperature tl is sufficient, the outlet temperature t3 is significantly higher than 00C and any risk of icing is excluded. The machine then operates as a double pump with the small difference that the half which corresponds to compressor 2b works with an evaporator temperature slightly lower than that of the first, which implies a slightly lower coefficient of performance. But if the air flow passing through the evaporators 15a, 15b has a sufficient flow, this

  difference is practically negligible.

  When tl, while remaining above O0C, is below a certain limit (for example 50C), t3 lowers below 00C and therefore, unless the air is particularly dry, the risk of icing appears for evaporator 15b. Its heat exchange coefficient then tends to decrease, the resistance it opposes to the passage of

the air to increase and the performance coefficient of the lowering machine

ser. But as soon as the layer of frost has reached a noticeable thickness, the detector 22b operates and alerts the microprocessor 24. This is

  programmed so as to then reverse fan 17-18 by the

pourer 20 and to stop group lb by contactor 4b.

  We then pass to a defrosting phase during which the group operates alone, while the air circulates in the box 16 according to the dotted arrows 28. This air then enters at the outside temperature tl, for example 4 C ; it crosses the evaporator 15b, then at rest, without yielding heat to the heat transfer fluid which no longer circulates there and by simply gradually melting the layer of frost, which only slightly lowers its temperature. It thus arrives at the evaporator a, always in action, at a temperature slightly lower than that of entry

  (for example at 30C). It gives it the heat necessary to provide the evaporator

  tion of the freon and it comes out at a temperature even higher than 00C (by

example at 1'C), without consequently causing the formation of frost.

  As soon as the detector 22b has detected the disappearance of the frost, it sends a signal to the microprocessor 24 which restarts the group lb without reversing the fan 17-18. We thus return roughly to the initial operating conditions, with the difference however that the air flow is reversed and that it is therefore the evaporator a which receives the air at temperature t2 and which involves the risk of icing. When this risk occurs, the detector 22a comes into play and it alerts the microprocessor which stops the group and reverses the fan

248 406 D

17-18. There is again defrosting and when this is finished, the microprocessor 24 restarts the group 1a thereby bringing all

  the whole exactly at the initial conditions without any exception.

  The machine therefore operates without stopping, without the intervention of additional energy to ensure defrosting, with only relatively short periods during which one of the groups is at

  stopping, the power being temporarily halved.

  If the outside temperature tl continues to decrease, the duration of the defrosting phases increases and there comes a time when the evaporator which is alone in action during one of them frost before the other is completely defrosted. In such a case, the two detectors 22a and 22b simultaneously send an icing signal to the microprocessor 24. The latter responds by stopping the two groups and it is then possible to put an appropriate defrosting system into operating state. Alternatively the microprocessor 24 can be programmed to trigger

itself such defrosting operations and to monitor the exe

  cution thanks to detectors 22a, 22b which send them their information permanently. Fig. 2 indicates in partial view an embodiment in which two separate condensers 8a, 8b are used, each comprising an element or coil 7a, 7b traversed by the freon and a water chamber Ila, llb, these two chambers being mounted in parallel between lines 9 and 10, but with the interposition of electromagnetic valves 29a, 29b, the

  control inputs are connected to the output lines 26a, 26b of the micro-

  processor 24. The arrangement is such that when a group, such for example

  that lb (fig. 1), is stopped by microprocessor 24, the corresponding valve

  dante, such as 29b, is closed. It follows that during the operation of a single group, only the corresponding elementary condenser intervenes (ie 8a in the aforementioned example). For the water flow through the chamber 11a of this condenser, the outlet temperature is lower than during the operation of the two groups with half-flow in each elementary condenser. The coefficient of performance of the group alone in operation (group la) is thus improved, which partially compensates

the stop of the other group (lb).

  As a variant, it could be arranged so that when a group operates alone, the corresponding condenser continues to receive only half the water flow corresponding to the operation of the two groups. One could for example replace the valves 29a, 29b by two individual circulation pumps controlled by the lines 26a, 26b,

-4 84065

check valves preventing the condenser which corresponds to the pump

stopped operating as a bypass shorting the other.

It should also be understood that the above description has not

has been given only by way of example and that it in no way limits the field of the invention from which one would not depart by replacing the execution details described by all other equivalents. Thus, for example, the invention is applicable to the case where the external cold source fluid consists of water at a temperature low enough that it risks forming ice on the evaporators. It also applies to pumps which do not use the phenomenon of liquefaction and evaporation of the internal heat transfer fluid by implementing the compression and expansion of a non-liquefiable gas at the temperatures in question at the hot source.

R E V E ND I C AT I 0 N S

1. Heat pump, of the type comprising a motor-compressor unit, a condenser cooled by an external hot source fluid evacuating the calories from the pump, and an evaporator which takes calories from an external cold source fluid, characterized in that it comprises two elementary pumps, the evaporators (15a, 15b) of which are arranged in series on the same stream of external cold source fluid, while on the one hand means (17, 18) are provided to circulate this fluid at will in one direction or the other, on the other hand means (22a, 22b, 24) which, when the evaporator which is located downstream on the stream of said fluid is covered with frost or ice, stop the corresponding group, reverse the direction of circulation of this fluid (arrows 27, 28) then, when the evaporator of the group thus stopped has been cleared of ice or frost, restart this

  group without changing the direction of fluid flow.

  2. Heat pump according to claim 1, characterized in that the external cold source fluid is constituted in the known manner

by atmospheric air.

  3. Heat pump according to claim 1, characterized in that the external hot source fluid is constituted in the known manner

with water.

  4. Heat pump according to claim 1, characterized in that it ccmprend a single condenser (8) comprising a chamber (11) through which the external hot source fluid and two elements (7a,

  7b) in which the corresponding internal heat transfer fluids circulate

  respectively to one and the other of the elementary pumps,

  means being provided so that these two elements are cooled substantially-

in the same conditions.

  5. Heat pump according to claim 1, characterized in that it comprises two condensers (8a, 8b) each corresponding to an elementary pump, these two condensers being traversed in parallel by the external hot source fluid, while it means (24, 26a, 26b, 29a, 29b) are provided which, when a group (la or lb) is stopped, stop the circulation of this fluid in the corresponding condenser (8a

or 8b).

  6. Heat pump according to claim 1, characterized in that it comprises means which, when the two evaporators (15a, 15b) are both iced, stop the two groups (la, lb) and report the incident. 7. Heat pump according to claim 1, in which each evaporator are associated with auxiliary defrosting devices, characterized in that it comprises means which, when the two evaporators (15a, 15b) are both iced, stop the two groups (la, lb), start the defrosting devices, then, when defrosting is finished, stop these devices and restart these

two groups.