EP0142663B1 - Defrost control method and device for heat pumps - Google Patents

Abstract

1. A defrosting control method for heat pumps, a defrosting operation being controlled in dependence upon the temperature difference between the evaporator temperature and the ambient or supply air temperature of the evaporator, the actual temperature difference (DELTA T) being compared with a set-value temperature difference (DELTA Tm ) which varies in dependence upon the ambient or supply air temperature (Tm ), a defrosting signal being delivered on the basis of such comparison when the actual temperature difference exceeds the set-value temperature difference, characterised in that the continuously detected actual value signals of the ambient or supply air temperature (Tm ) and the evaporator temperature (Tk ) are supplied to a subtractor (60) determining the temperature difference DELTA T = Tm -Tk , the actual value signal (Tm ) is also supplied to a function former (62) which forms the set-value temperature difference (DELTA Tm ) from the actual-value signal (Tm ) and an adjusted temperature difference signal (Uv ), the actual-temperature difference signal (DELTA T) output by the subtractor (60) and the set-value temperature difference signal (DELTA Tm ) output by the function former (62) are supplied to a hysteretic comparator (61) which when DELTA T > DELTA Tm produces an output signal transmitted as first instruction signal (54) to a facility (65, 66) for triggering different defrosting signals, the actual value signal (Tm ) is also supplied to a hysteretic comparator (72) and compared therein with a signal (Uk ) corresponding to a fixed temperature above 0 degree C, more particularly 5 degrees C, and an output signal supplied as second instruction signal (56, 57 respectively) to the facility (65, 66) is produced, the actual value signal (Tk ) is supplied to a hysteretic comparator (71) and compared therein with a signal (Uo ) corresponding to a temperature of 0 degree C, and only when (Tk ) is below 0 degree C is an output signal produced which is supplied as third instruction signal (55) to the facility (65, 66) and when the first instruction signal (54) and the third instruction signal (55), the latter signalling a temperature Tk < 0 degree C, are present, the facility (65, 66) : a) if the second instruction signal (56, 57 respectively) signals a temperature Tm < Tuk , triggers a switching process whereby the heat pump drive is stopped and accelerated defrosting with heating gas is started or b) if the second instruction signal (56, 57 respectively) signals a temperature Tm > Tuk triggers a switching process whereby the heat pump drive is stopped and a normal defrosting process started.

Description

The invention relates to a method for defrosting heat pumps with features according to the preamble of claim 1.

It is known that in a heat pump working with ambient air as an energy source, the water portion of the air on the evaporator surface condenses and freezes when the evaporator temperature is below 0 ° C. It is therefore necessary to monitor this icing and to initiate defrosting if necessary. Various procedures have already been proposed for this.

The invention is based on a method for defrosting a heat pump, which is known from FR-A-2 362 350. In this method, a defrosting process is controlled as a function of the temperature difference between the evaporator temperature and the ambient or supply air temperature of the evaporator. Furthermore, the continuously ascertained actual value signals of the ambient or supply air temperature and the evaporator temperature are fed to a difference former which determines the relevant temperature difference. In addition, this actual temperature difference is compared with a target temperature difference value, which is changed as a function of the ambient or supply air temperature. On the basis of this comparison, a defrost signal is emitted when the actual temperature difference exceeds the target temperature difference. Because of this defrost signal, a defrost process is started which is the same in all cases. This can lead to the defrosting process taking too long, so that there is an undesirably long shutdown pause for the heat pump. In addition, this known regulation cannot prevent defrosting that is unnecessary per se.

It is therefore an object of the invention to provide a method for reliable defrost control of heat pumps, in which unnecessary defrosting is prevented, but necessary defrosting processes are initiated in good time and carried out according to the requirements.

This object is achieved by a method having the features specified in the characterizing part of claim 1. Details of this method are given in sub-claim 2.

The solution according to the invention is based on the knowledge that icing from the coldest outside air temperature to temperatures above 0 ° C. can occur in heat pumps operating with ambient air. Due to the approximately constant or decreasing volume flow of the refrigerant flowing through the evaporator when the outside air temperature falls, a comparatively high cooling capacity occurs at a comparatively high outside or ambient air temperature or evaporator temperature due to the vapor density, while the when the outside or ambient air temperature falls or is comparatively low, the Cooling capacity decreases sharply. For a given evaporator, therefore, the temperature difference between the outside or supply air temperature and the evaporator temperature is significantly higher at high outside or ambient air temperatures, for example 2 to 3 times, than at low temperatures.

In the method according to the invention, the target temperature difference is varied according to the ambient temperature and thus adapted to the evaporator behavior. This creates a reliable control in which unnecessary defrosting is prevented and, on the other hand, the defrosting is initiated in good time to ensure the economical operation of the heat pump. The setpoint temperature difference is preferably changed continuously as a function of the outside temperature or the ambient temperature of the evaporator. However, it is also possible to use a stepwise regulation of the target temperature difference.

In addition, the initiation of a defrosting process at evaporator temperatures above 0 ° C is blocked in the inventive method. This reliably prevents unnecessary defrosting if, at such evaporator temperatures, the actual temperature difference exceeds the setpoint value calculated from the current outside or ambient air temperature.

In addition, largely continuous operation of the heat pump is achieved due to the method according to the invention, because defrosting processes are carried out quickly and the switch-off pauses of the heat pump are thus kept small. This is achieved in that selectable different defrost signals are generated according to a predetermined criterion, with which different defrosting processes are initiated. These defrost signals are from the outside or Supply air temperature dependent and generated in such a way that in an outdoor or Supply air temperature above about 3-5 ° C, a normal defrosting process is initiated, with a defrost by ambient air, whereby only the refrigerant circuit of the heat pump is either switched off or diverted and the evaporator fans continue to operate. With outside or In contrast, supply air temperatures below 5 ° C generate a defrost signal that starts an accelerated defrosting process in which hot gas is directed to the evaporators.

• Fig. 1 ein Grundprinzip einer Wärmepumpenanlage mit Abtauregelung,
• Fig. 2 ein Temperaturdiagramm, und
• Fig. 3 ein Ausführungsbeispiel einer Abtauregeleinrichtung zur Durchführung des erfindungsgemäßen Verfahrens.

The method according to the invention is explained in more detail below with reference to the drawing. The drawing shows:

• 1 is a basic principle of a heat pump system with defrost control,
• Fig. 2 is a temperature diagram, and
• Fig. 3 shows an embodiment of a defrost control device for performing the method according to the invention.

In Fig. 1 is a compression heat pump System 10 shown, which consists of an evaporator 11 for receiving the energy Q _o from the ambient air 12 and a compressor 13 which compresses the refrigerant vapor from the evaporator 11 and feeds a condenser 14 to which a consumer 15 is connected.

It is known that an evaporator 11 exposed to the outside or ambient air 12 ices on the outside due to the water content of the air when the evaporator temperature T _k or the temperature at the evaporator surface has a value below the freezing point. In order to automatically initiate a defrosting process if required, a defrost controller 16 is provided in the heat pump system 10 according to FIG. 1, which receives the signals from an evaporator temperature sensor 17 and an ambient air temperature sensor 18 and outputs a signal to an actuator 19 when defrosting is required. With the actuator 19, the compressor 13 is switched off and thus the defrosting process is initiated. The defrosting process can also be initiated in other ways, such as. B. by reversing or redirecting the refrigerant flow.

The defrost controller 16 contains a difference generator 20, with which the temperature difference ΔT = T _a -T _{k is} formed, where T _{a is} the ambient temperature. In a hysteresis comparator 21, the actual temperature difference signal ΔT thus formed is compared with a target temperature difference signal ΔT _s , which is generated by a setpoint generator 22 as a function of the ambient temperature. If the measured temperature difference .DELTA.T is higher than the correspondingly calculated target value .DELTA.T _s , then the comparator 21 outputs a signal 23 with which the actuator 19 is activated to initiate the defrosting process. When the defrosting process continues, the defrosting process ends when the temperature difference ΔT has decreased accordingly.

On the basis of measurements and calculations, the function .DELTA.T = f (T _a ) of the temperature difference between the evaporation temperature and the outside temperature could be determined as a function of the outside temperature, which has a curve marked with the number 30 in FIG. 2. In _a first approximation, this function ΔT = f (T _a ) can be replaced by a straight line 31 (ΔT '= kT _a ). The straight line 31 thus corresponds approximately to the function ΔT = f (T _a ) if there is no icing on the evaporator 11. By frosting the evaporator, however, the temperature difference ΔT increases by a value a, which also increases approximately linearly with the ambient temperature T _a . A linear change in the target value T _a with the outside temperature T _a can thus be provided, which is shifted by a temperature difference value _a with respect to the straight line 31 and may have a greater increase. This straight line is shown at 32 in FIG. 2.

In contrast, FIG. 3 shows a block diagram of a defrost controller 40 with which the method according to the invention can be carried out.

Defrost regulator 40 is fed by a direct current source 41, the output voltage U _{r of which is} stabilized and filtered in a reference voltage generator 42. Defrost controller 40 is also connected to ambient air temperature sensor 18 and evaporator temperature sensor 17. The sensors 17 and 18 are fed by a voltage source 47, which transforms the reference voltage U _r into a predetermined sensor voltage U ^S.

The temperature signals 43 and 45 generated by the sensors 17, 18 are adapted and standardized to the voltage level of the circuit in amplifiers 50 and 51, respectively. The amplified voltage signals 52 and 53 are processed to form four command signals 54, 55, 56, 57 in the defrost controller 40. On the one hand, as in the case according to FIG. 1, the temperature difference ΔT = T _ä- T _{k is} formed with a difference generator 60, which is compared with the target temperature difference ΔT _s to generate the first command signal 54 by means of a hysteresis comparator 61. The setpoint temperature difference ΔT _s is generated by a function generator 62, specifically from the current ambient air temperature signal 52 and an externally adjustable voltage value 63 from a potentiometer 64, which corresponds to the temperature difference adapted for the respective application.

The setpoint temperature difference ΔT _s is formed in a first approximation as a linear function according to curve 32 in FIG. 2. The slope K of this function is variable here. The command signal 54 generated in this way gives the commands “switch on defrost process” or “end defrost process _” .

However, the switching process for a defrosting process is triggered by means of two logic AND gates 65 and 66 as a function of further signals, namely the command signals 55, 56 and 57.

The command signals 55 indicate whether the evaporator temperature is greater or less than 0 ° C., and they are generated by comparing the evaporator temperature signal 53 with a voltage value U _° corresponding to 0 ° C. in a comparator 71, which is also subject to hysteresis.

The command signals 56 and 57 are outside temperature-dependent signals which are generated by comparing the ambient air temperature signal 52 with a voltage U _{k associated} with a corresponding temperature in a third, hysteresis-based comparator 72, the output signal of which forms signal 57 on the one hand and a non-signal on the other Member 73 supplied, there is reversed and forms the signal 56 starting from the latter. Different defrosting processes can be initiated with these outside temperature-dependent command signals 56 and 57, which are each input to one of the logic AND links 65 and 66, respectively. For example, at different outside temperatures, either only the heat pump operation can be switched off and, at a different temperature, an additional heat switch on the air blower. In this case, U _k can be designed accordingly for an outside temperature T _uk = 5 ° C. The outside temperature T _a and the evaporator temperature T _k are measured continuously and the setpoint T _{s is} calculated continuously.

If the temperature difference AT between the measured values exceeds the target value .DELTA.T _s , the comparator 61 outputs the defrost signal 54, which is input into both logic AND operations 65 and 66. However, the logic AND link 65 will only output a switching signal 74 if it simultaneously receives the signal 55 for Tk <0 ° C. and the signal 56 for T _a <T _uk (for example T _a less than 5 ° C.). As a result, a first switching process is carried out, for. B. Switching off the heat pump operation and switching on the hot gas defrost.

If, on the other hand, the outside temperature Ta is higher than the given value T _uk , the second logic AND link 66 will output a switching signal 75 with which, for example, only the heat pump operation is switched off.

If the evaporator temperature exceeds the freezing point, then there is no danger of the evaporator 11 icing up. In this case, no signals 55 are output, so that the logic AND logic operations 65 and 66 are not switched through and defrosting does not take place, even if the temperature difference ΔT is greater than ΔT _s .

Claims (2)

1. A defrosting control method for heat pumps, a defrosting operation being controlled in dependence upon the temperature 'difference between the. evaporator temperature and the ambient or supply air temperature of the evaporator, the actual temperature difference (ΔT) being compared with a set-value temperature difference (ATm) which varies in dependence upon the ambient or supply air temperature (T_m), a defrosting signal being delivered on the basis of such comparison when the actual temperature difference exceeds the set-value temperature difference, characterised in that

the continuously detected actual value signals of the ambient or supply air temperature (T_m) and the evaporator temperature (T_k) are supplied to a subtractor (60) determining the temperature difference AT = T_m-T_k,

the actual value signal (T_m) is also supplied to a function former (62) which forms the set-value temperature difference (ΔT_m) from the actual- value signal (T_m)- and an adjusted temperature difference signal (Uy),

the actual-temperature difference signal (AT) output by the subtractor (60) and the set-value temperature difference signal (ΔT_m) output by the function former (62) are supplied to a hysteretic comparator (61 ) which when ΔT > ΔT_m produces an output signal transmitted as first instruction signal (54) to a facility (65, 66) for triggering different defrosting signals,

the actual value signal (T_m) is also supplied to a hysteretic comparator (72) and compared therein with a signal (U_k) corresponding to a fixed temperature above 0 °C, more particularly 5 °C, and an output signal supplied as second instruction signal (56, 57 respectively) to the facility (65, 66) is produced,

the actual value signal (T_k) is supplied to a hysteretic comparator (71) and compared therein with a signal (U_o) corresponding to a temperature of 0 °C, and only when (T_k) is below 0 °C is an output signal produced which is supplied as third instruction signal (55) to the facility (65, 66), and when the first instruction signal (54) and the third instruction signal (55), the latter signalling a temperature T_k < 0 °C, are present, the facility (65, 66) :

a) if the second instruction signal (56, 57 respectively) signals a temperature T_m < T_uk, triggers a switching process whereby the heat pump drive is stopped and accelerated defrosting with heating gas is started or

b) if the second instruction signal (56, 57 respectively) signals a temperature T_m >T_uk triggers a switching process whereby the heat pump drive is stopped and a normal defrosting process started.

2. A method according to claim 1, characterised in that the first instruction signal (54) and the third instruction signal (55) are each supplied to an AND-gate (65, 66 respectively), one (66) of which is supplied directly with the second instruction signal (57) while the other (65) is supplied with the second instruction signal (56) in negated form, and one AND gate (65) produces a signal (74) to trigger accelerated defrosting when the first instruction signal (54) signals a temperature difference(AT > ΔT_m),thesecondinstructionsignal (56) signals a temperaure T_m < T_uk and the third instruction signal (55) signals a temperature T_k < 0 °C, while the AND gate (66) outputs a signal (75) triggering normal defrosting when the first instruction signal (54) signals a temperature difference(AT > ΔT_m),thesecondinstructionsignal (57) signals a temperature T_m > T_uk and the third instruction signal signals a temperature T_k < 0 °C.

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