WO2007062738A1

WO2007062738A1 - Method for the operational control of a cooling system and system operating according to such method

Info

Publication number: WO2007062738A1
Application number: PCT/EP2006/010798
Authority: WO
Inventors: Stefano Bernardinello; Andreas Bottazzo; Emanuele Mortin
Original assignee: Mta S.P.A.
Priority date: 2005-12-02
Filing date: 2006-11-10
Publication date: 2007-06-07
Also published as: ITPD20050354A1

Abstract

A method for the operating control of a cooling system is described in which when operating with a heat pump provision is made for automated control of the stage of defrosting an evaporator (6) located in an external environment through monitoring the change in an indicator (MTD) measuring the difference between the external ambient temperature and the evaporation temperature of the cooling fluid with which the evaporator (6) is working, including the stage of acquiring values for the following variables directly or in correlated form: the temperature of the environment in which the evaporator is located, the evaporation pressure (Pe) , and/or the condensation pressure (Pc) comprising the further stages of : foreseeing (19, 19a, 19b) an expected change in the indicator calculated on the basis of the variables, comparing (20, 21) the expected change with the actual change in the indicator, governing operative control of the defrosting stage (15) on the basis of the difference between the expected change and the actual change in the indicator.

Description

Method for the operational control of a cooling system and system operating according to such method.

DESCRIPTION Technical field This invention relates to a new method for the operational control of a cooling system, in particular a heat pump system, to optimise the defrosting cycle in the corresponding evaporator.

During the normal operation of a heat pump cooling system the function of evaporator for the condensed cooling fluid is performed by the exchanger (finned unit) in contact with the outside environment; this is subject to a consequent fall in temperature which can and generally does result in the periodical formation of ice which builds up on the unit, progressively obstructing it. For proper functioning and appropriate energy efficiency of the heat pump the ice formed has to be periodically removed through suitable evaporator defrosting cycles. Technological background

It is known that cooling systems can be operatively controlled in such a way that defrosting of the evaporator takes place automatically whenever the need arises. Typically the evaporator operating parameters (pressure and/or temperature) are monitored to activate a defrosting cycle when these diverge from the normal operating parameters for a period of time longer than a threshold value. Incidental or foreseeable factors which affect the operation of the evaporator and which have to be borne in mind when activating the defrosting cycle may however come into play. On the other hand excessively frequent repeated defrosting will in turn adversely affect energy efficiency and the performance of the system, as a result of which it is desirable that these cycles should be managed through strict control.

A typical example of such control is described in EP0881440 which is based on control of the difference between the temperature of the outside air and the evaporation temperature measured by monitoring the evaporation pressure. Other control methods are described in US 4884414, US 4573326, US 4563877, and are essentially based on control of the temperature difference between the outside air and the evaporator. However all these known methods make provision by way of precaution for carrying out defrosting when any change whatsoever in normal parameters is observed and are thus unable to maximise the efficiency of the system by avoiding defrosting cycles which more thorough monitoring could prove to be superfluous. Description of the invention The technical problem underlying this invention is that of providing a method for the operational control of a cooling system and a system operating in accordance with that method so as to ensure that all the disadvantages complained of in connection with the cited prior art are overcome. This problem is resolved by the invention through a method and system constructed in accordance with the following claims.

Other advantages and characteristics of the present invention will become clear from the following detailed description which is given with reference to the appended drawings which are provided purely by way of non- limiting example and in which: Brief description of the drawings

Figure 1 is a diagrammatical view of a system according to the invention, Figure 2 is a flow diagram illustrating the cycle controlling defrosting of the system in Figure 1. Preferred embodiment of the invention

In Figure 1, indicates as a whole a heat pump cooling system for heating an internal environment 2 at the expense of heat drawn from an external environment 3. Very diagrammatically, system 1 comprises a compressor 4, a first exchanger 5 which when operating as a heat pump (winter) acts as a condenser, in heat exchange contact with the indoor environment, and a second exchanger 6, acting as an evaporator, a finned unit, in heat exchange contact with the external environment and through which an airflow is forced through a possible fan 8. Evaporator 6 is periodically subjected to defrosting cycles which provide for temporary heating of the finned unit through operations which may for example comprise the use of external heaters, or direct diversion of all or part of the compressed hot fluid originating from the compressor to the evaporator, bypassing the condenser through a valve member 9, or other arrangements which are in themselves known and/or are not the subject of this invention.

A control unit indicated diagrammatically by C effects operating control of system 1 and in particular operating control of the cycles defrosting evaporator 6. The method of controlling system 1 in the context of this invention is based on evaluating the change in an indicator, identified as MTD (Maximum Temperature Difference) which when the heat pump is in operation measures the temperature difference between the outside environment, that is the air in contact with the finned unit of the evaporator, and the evaporation temperature of the cooling fluid with which the evaporator is operating. The change in this indicator measures the degree of obstruction of exchanger 6 in such a way that when the unit is clean indicator MTD has a minimum value which increases as ice forms until a set-point value at which it is considered desirable that defrosting should be performed. Methodological management is the responsibility of complex circuitry and processing programs resident in control unit C and described below with reference to the diagram in Figure 2.

When the system is started up, as shown by stage 10 in the figure, the possibility of starting defrosting cycles (stage 11) is initially inhibited for a preset time, and data which may allow them to proceed are detected. The logic governing this choice is based on the fact of avoiding detecting parameters during the transitional stage of starting up system 1 which may cause incorrect actions to be undertaken by the system which is not yet under normal operating conditions. During the start-up transient there may occur phenomena such as a requirement for immediate defrosting action and/or the perception of potential alarm conditions, for example, if the unit should be accidentally clogged by snow deposits, leaves or other materials behind the external unit. In such circumstances system 1 could be shut down for attention shortly after start-up, for example by a low pressure alarm in the low pressure branch of the cooling circuit.

The following checks are carried out to avoid this disadvantage. Node 12 checks whether the delay inhibiting defrosting is active. If this is the case, or if the period during which defrosting is inhibited has not yet expired, the time which has elapsed since the compressor was last switched off is checked against a significant reference value for the maximum stop time permitted by the system (maximum stop time or tstop - node 13 in the drawing). If this comparison shows that the stoppage has continued longer than the maximum permitted time a comparison is made (node 14) between the evaporation pressure (PE) measured at the evaporator and a reference value (set point) for the evaporation pressure (Pevapsetpoint). If the value of the evaporation pressure (Pe) detected at the evaporator is less than the reference value (set point), as may happen if the finned unit is accidentally obstructed, a defrosting cycle will start immediately (stage 15); if this is not the case the count of the inhibition time (node 12) will continue.

Once the transitional stage of system start-up has been completed, or the defrosting inhibition time has expired (node 12), operating variables will begin to be acquired (stage 16). A double calculation will then be performed: the instantaneous MTD will be determined (stage 17) and the mean value will be calculated (stage 18); in parallel with this the expected MTD value will be calculated (stage 19, 19a) and its corresponding mean value (stage 19b) will be calculated through polynomials and relationships which will be discussed below. If comparison with the mean MTD value measured shows that it is greater than the mean value expected less a predetermined safety coefficient (node 20; see also stage 19a) a check is made to see whether this situation persists for a longer time than a preset time (node 21), and if this is the case it enables the defrosting stage (stage 15), which is consequently started up. If this is not the case, that is if the mean value found for the MTD is less than the expected mean value or it is found at the next node that the situation does not persist for the set time, a return is made to the stage of data acquisition 16. The test start-up of the defrosting stage performed at nodes 20 and 21 not only detects the existence of conditions which require defrosting but also checks whether it is actually needed by observing the persistence of the request for defrosting for a time in excess of a minimum value (comparison A>B for Time>Tset, where A is the instantaneous MTD and B is the expected MTD plus the safety coefficient).

When a defrosting cycle is activated, which corresponds to an affirmative response to the comparison in node 21, the event is placed in memory in stage 22, and after the system has exited from the defrosting stage when the preset defrosting time (node 22a) has been reached the method according to this invention provides that the effectiveness of defrosting be evaluated by using the number of defrosting operations performed per unit time (for example in one hour) as an indicator. The comparison is performed in node 23 by checking whether the number of defrosting operations per hour is greater than or less than the maximum number of permitted defrosting operations. Where the response is negative, any alarm in stage 24 is set and any longer defrosting time is also set. Where the response is positive a check is made in node 25 to see whether the event recurs or not. If it is reported for the first time, or if no preceding ineffective defrosting signal is received, a first occurrence of ineffective defrosting is placed in memory in stage 26 for subsequent comparisons. If this is not the case, or if an ineffective defrosting alarm has already been activated, the control attempts to overcome the problem by setting a double or otherwise increased defrosting time in comparison with the preset value. This is performed in comparison node 27 which checks whether the increased defrosting time is already active and if that is not the case increases (doubles) the defrosting time (stage 28), or, if that is the case, indicates an alarm condition (stage 29).

Among the causes which might give rise to such an anomaly the most likely is the possibility that the ice obstructing the external unit has only been partly melted. If this is the case another defrosting cycle will be quickly needed so as to resolve the problem.

If instead the request for defrosting is brought about by other causes such as a loss of cooling fluid, accidental obstruction of the unit, an error in the setting of the temperature and pressure transducers, etc., the system will generate a further request for defrosting which if the situation is not resolved in the next cycle despite a doubled defrosting time will activate an alarm condition alerting the maintenance section.

The following functions are used to calculate the expected MTD value (MTDhp) in stages 19, 19a, 19b: first of all the values of the instantaneous evaporation pressure (Pe) and condensation pressure (Pc) are measured by sensors which are in themselves known. Once these values have been obtained the instantaneous cooling performance (RF) of the unit is calculated in watts. The value of the cooling performance RF is obtained by multiplying a first third order polynomial for the volumetric throughput of the compressor and a fractional algebraic function in which the variables are Pe and Pc. The coefficients for the polynomial and the algebraic expression are calculated through appropriate interpolation.

Once the refrigeration performance (RF) has been obtained MTDhp is then calculated by dividing two polynomials: a first first order polynomial for the cooling performance (RF) as the numerator and a third order polynomial again for the cooling performance (hereinafter referred to as BETAhp) as the denominator.

The polynomials used are shown below:

RF=[(AχVS)-(BχVS²)+(CχVS³)]χ{l-[(DχPe)-(ANDχPc)+(F/(Pe/Pc)]} BETAhp = P+(QxRF)-RxRF²)

MTDhp=TχRF/BETAhp where:

RF= cooling performance in W

VS= volumetric throughput of the compressor in m³/s Pe and Pc= evaporation pressure and condensation pressure respectively in bars

BETAhp= characteristic for the exchanger when evaporating (operation of a heat pump) in W/°C

MTDhp= Difference between the ambient temperature and the evaporation temperature in ⁰C A, B, C,...., T= constants.

The expected MTDhp values are calculated at preferably regular time intervals (for example every two seconds) and the corresponding values so obtained are saved to a first table placed in memory in the first internal memory (not shown in the figures), for example a FIFO memory, present in the control unit. The first table include the values MTDhpi MTDhp_N calculated at times ti,...,t_N. After a prefixed time

(for example t_M= 3 minutes) a mean of the MTDhp, values saved in the first table is

1 ^N obtained, preferably an arithmetic mean, i.e. MTDhp _mean= — ∑MTDhp, , to which the safety coefficient is added in order to obtain value B.

At the same time interval ti,...,t_N values for the instantaneous measured MTD obtained from the difference between the external ambient temperature Ta and the condensation temperature Tc, a difference measured at every instant t, through suitable sensors, is calculated (stages 17 and 18). These values of the difference AT are then saved in a second table placed in a second memory, for example also of the FIFO type, which therefore includes the Values Of MTD mstantaneous I-- MTD mstantaneous N calculated at time ti,...,t_N. After the same preset time t_M for which MTD _mean was calculated, the mean of the MTD _ms_ta_ntaneo_us . values saved in the second table is obtained, preferably an arithmetic mean, i.e. MTD mstantaneous mean=

— 1 ∑ ^N Mri>_utantaneo, = A .

The cycle for calculating the two means and comparing the values obtained (node 20), as previously indicated, is then repeated obtaining new values

Of MTDhp mean and MTD mstantaneous mean- The invention thus solves the problem posed, and provides many advantages, including: an appreciable saving of electrical energy, because it reduces defrosting actions to the cycles which are actually necessary only, other factors being equal (volume of the accumulator tanks, thermostatic algorithms, general dimensioning of the system), improved comfort can be obtained for the user, the system independently attempts to resolve any temporary slight anomalies by temporarily increasing the defrosting time and/or setting expectations to permit possible normalisation of the fault without requiring action by specialist assistance, the system makes use of low processing power, with consequent low cost and improved simplicity of the control electronics the system makes use of simple transducers and sensors normally provided on machinery, and does not require sophisticated and expensive specialist sensors.

Claims

1. A method for the operative control of a cooling system in which, when operating with a heat pump, automated control of the stage of defrosting an evaporator located in an external environment is provided through monitoring the change in an indicator (MTD) which measures the difference between the external ambient temperature and the evaporation temperature of the cooling fluid with which the evaporator (6) is working, including the stage of acquiring values for the following variables, directly or in a correlated way: - the temperature of the environment in which the evaporator is located (Ta), the evaporation pressure (Pe), and/or the condensation pressure (Pc), characterised in that it comprises the further stages of: - providing an expected change in the said indicator calculated on the basis of the said variables, comparing the said expected change with the actual change in the said indicator, governing operative control of the said defrosting stage on the basis of the difference between the said expected change and the said actual change in the said indicator.

2. A method for the operational control of a cooling system according to claim 1 in which in the said stage of governing operating control of the defrosting stage the difference between the said expected change and the said actual change in the said indicator (MTD) is compared and a safety coefficient is subtracted.

3. A method of control according to claim 1 or 2 in which the further stages of: placing a sequence of instantaneous actual values of the said indicator (MTD) and the expected corresponding values in memory, obtaining a mean value for the actual instantaneous values and the corresponding expected values in order to determine the corresponding expected and actual changes which are to be compared in the said comparison stage are provided.

4. A method of control according to one or more of the preceding claims, in which the said expected value is calculated through the division of two polynomials: a first order polynomial for the cooling performance as the numerator and a second third order polynomial for the cooling performance as the denominator, where the said cooling performance is obtained by multiplying a third third order polynomial for the volumetric throughput with a fractional algebraic function in which the variables are the said evaporation pressure and the said condensation pressure.

5. A method of control according to claim 4, in which the coefficients of the said first, second and third polynomials and the said algebraic function are obtained through interpolation.

6. A method of control according to one or more of the preceding claims in which an initial transitional stage in which provision is made for temporary inhibition of start-up of the said defrosting stage.

7. A method of control according to claim 6 in which during the said transitional inhibition stage the following sub-stages are carried out: - comparison between the time which has elapsed since the compressor was last switched off and a significant reference value for the maximum stop time permitted for the system, if the stoppage continues beyond the maximum permitted time, a comparison is made between the evaporation pressure (Pe) measured at the evaporator and a reference value for the evaporation pressure (Pevapsetpoint), if the value of the evaporation pressure (Pe) found at the evaporator is less than the reference value, a defrosting cycle is activated, - if this is not the case the state of temporary inhibition of defrosting is maintained and counting the time of that inhibition continues.

8. A method of control according to one or more of the preceding claims in which, if the actual value of the said indicator is greater than the expected mean value for the same, provision is made for a further stage of checking whether this situation recurs in a time greater than a preset time and, if such a further stage is found, the said defrosting stage is started.

9. A method of control according to one or more of the preceding claims in which the performance of the defrosting stage is evaluated by comparing the number of defrosting stages performed in a period with a maximum permissible number of defrosting stages in that period.

10. A method of control according to claim 9 in which, if the number of defrosting operations in the period is greater than the maximum number of defrosting operations permitted, a check is made to see whether occurrence of the event is repeated or not; if it is recorded for the first time a first occurrence of ineffective defrosting is placed in memory for subsequent comparisons, if not an ineffective defrosting alarm is activated, an increased defrosting time in comparison with the preset value is set, and when the increased defrosting time is already active provision is made for the indication of an alarm condition.

11. A cooling system comprising a control unit with resident processing programs for control of the defrosting stage, characterised in that when the system is operating together with a heat pump the said resident programs effect control of the said defrosting stage according to the method in one or more of the preceding claims.