NZ229840A - Refrigerator defrost operation; time between defrosts varied to maintain operating period within acceptable range - Google Patents

Description

22 9 8 4 0 c Priority Date(s): Complete Specification Filed: Cfass: . J?. 2-.$. P.^l I Q, S "§ Publicat . r. Date: .. ?.?. MAY. igji P.O. Journa', Wo: . 1344*-.

N. Z.

NEW ZEALAND Patents Act 1953 COMPLETE SPECIFICATION DEFROST METHOD FOR EVAPORATOR OF REFRIGERATION DEVICE We, White Consolidated Industries, Inc., a Corporation of the /V OtUvOftft Vr* State of Dnl wnrr organized under the laws of the State of Delware i ("IV with a place of business at 11770 Bera Road, Cleveland, Ohio 44111 United States of America do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- - 1 - (Followed by 1A) I 1 2 , 3 4 6 7 8 9 11 12 13 14 16 17 *XO 19 21 22 24 26 27 28 229840 DEFROST METHOD FOR EVAPORATOR OF REFRIGERATION DEVICE BACKGROUND OF INVENTION The present invention relates in general to a method of automatically defrosting an evaporator of a refrigeration device, and more particularly to a method of regulating the actual time or the accumulated compressor run time between defrost operations to optimize the defrosting of the evaporator, such actual time or accumulated compressor run time being varied or maintained as a function of the length of time of the previous defrost cycle.

A typical refrigeration device, such as an air conditioner, refrigerator, freezer, or heat pump, includes a hermetic compressor connected in series relation with at least two heat exchangers, one of which functions as a refrigerant condenser, the other functioning as a refrigerant evaporator.

Operation of such a refrigeration device in the presence of humidity usually results in the formation or condensation of frost on the evaporator if the evaporator is operated at a temperature below the freezing point of water. In cases where the evaporator and areas immediately around it remain at a temperature below freezing most of the time, each operation of the compressor results in further accumulation of frost. This build-up of frost is a direct function of compressor run time and humidity at the evaporator site (which are in turn functions of such factors as ambient conditions, desired operating temperatures, I 1 ♦2 3 4 6 7 8 9 11 12 13 14 16 17 18 19 ->1 22 23 24 ?6 y 27 28 29 2 22 9 8 4 0 thermal insulation, air leaks, and, in the case of refrigerators and freezers, door openings). The build-up of frost progressively interferes with the transfer of heat to the evaporator by thermally insulating the evaporator, and may eventually physically block, or at least inhibit, airflow through the evaporator, thus resulting in inefficient and improper operation of the refrigeration device.

In response to this undesirable build-up of frost, it is well known in the art to intermittently heat the evaporator to melt the accumulated frost and then drain the resultant water away from the evaporator site. This may be done with electric heaters or other external heat sources, or in a heat pump, the heat pump may be reversed. In many cases a fixed actual elapsed time or a fixed accumulated compressor run time is provided between defrost operations. The problem with such fixed period defrosting of the evaporator is that it does not take into account the actual frost build-up. As a result, a defrost operation may be initiated when there is little or no frost, or after frost has already built up to the point of interfering with the proper functioning of the refrigeration device. In the former case, substantial energy is wasted in both raising the evaporator temperature sufficiently to defrost it and restoring temperatures to their pre-defrost values, and, in the latter, substantial energy is wasted in inefficient operation of the refrigeration circuit prior to the defrost operation.

The length of time it takes to defrost the evaporator is directly proportional to the amount of 2 <« 3 4 6 7 8 9 11 12 13 14 16 17 18 19 21 ll 23 24 26 27 28 29 31 22 9 8 4 3 0 frost build-up. Because of this, the defrost time (e.g. the time it takes to raise the evaporator to a predetermined defrost temperature) is a measure of frost build-up, and hence, a measure of the humidity and compressor run time as well. One way of attempting a better match of the defrost process to the actual frost build-up is to make the assumption that, between any two defrost operations, the humidity and the portion of the time the compressor runs (duty cycle) are largely the same. Under this assumption, the previous defrost time becomes a predictor for the operating conditions that will be encountered during the time before the next defrost operation, that is, the humidity the evaporator will encounter and the duty cycle of the compressor. Under this assumption, the time before the next defrost operation (actual elapsed time rather than the accumulated compressor run time due to the predictability of the compressor duty cycle) can then be manipulated to produce any desired amount of frost build-up before the next defrost operation. U.S. Patents 4,156,350 and 4,251,988, the entirety of which are incorporated herein by reference, are based on the above assumption.

In the first patent, the length of time the defrost heater is energized during a defrost operation is measured and a look-up table (a decoding matrix) is used to determine the actual elapsed time before the next defrost operation. The actual elapsed time from the look-up table is to be inversely proportional to the defrost time. If the operating conditions remain 1 "S2 3 4 6 7 8 9 11 12 13 14 16 17 18 19 n 22 23 24 "6 27 28 29 31 32 22 9 8 4 0 4 constant long enough, the defrost system will eventually reach an operating point where the actual frost build-up will be equal to that predicted from the known previous frost build-up. In this way, the period of time between defrost operations is made to vary with the amount of frost build-up. This system seeks an equilibrium operating point (predicted frost = actual frost). However, this operating point may still be at a point of little, or no, frost build-up, or at a point of excessive frost build-up under certain operating conditions.

In the second aforementioned patent, the length of time the defrost heater is energized during a defrost operation is measured. A weighted difference (positive or negative) between the measured time and a fixed optimum time (corresponding to an optimum amount of frost) is then added to the actual elapsed time between the previous two defrost operations. The result is then used as the time until the next defrost operation. In this way, the actual time between defrost operations is adjusted under the earlier noted assumption to try to produce exactly the predetermined optimum amount of frost. This method suffers from potentially serious system response problems. When the refrigeration device's operating conditions change rapidly (e.g. refrigerator's owner leaves for a vacation or returns from same), the use of a weighted-difference error signal is likely to produce either, a large over-correction with resultant instability, or an extremely slow response.

In both of the aforementioned prior art patents, because of the earlier noted assumption, the *», 2 3 4 6 7 8 9 11 12 13 14 16 17 18 19 "A 22 23 24 "6 27 28 29 31 22 9 8 4 0 system response is dependent on both the humidity and the compressor duty cycle. For the assumption to be correct, not only must the humidity at the evaporator be more or less constant over two defrost cycles, but also, the compressor duty cycle must satisfy this same condition. A predictable duty cycle is what allows the substitution of the actual elapsed time between defrost operations for the actual frost build-up factor—compressor run time. In any predictive control system, as the number of variables to be estimated increases, the performance of the system decreases.

U.S. Patent No. 4,680,940, the entirety of which is incorporated herein by reference, assumes only that the humidity encountered by the evaporator from one defrost operation to the next is sufficiently similar to allow prediction of future frost build-up from previous frost build-up. No assumption is made about compressor duty cycle. Instead, actual accumulated compressor run time is taken into account in the prediction of frost build-up. This avoids errors introduced by using past frost build-up to predict not only the future humidity, but also the future compressor duty cycle (which allows interpolation from actual elapsed time to accumulated compressor run time). However, the defrost method of this patent still suffers from a system response standpoint in that over-correction and under-correction can occur when unusual operating conditions invalidate the basic operating assumption.

All of the cited prior art patents provide what is known in the art as adaptive defrost control ( 1 2 3 4 6 7 8 9 11 12 13 14 16 "7 pf' 18 19 21 J2 23 24 229840 6 in that the actual compressor run time fixed, but rather prior defrost time defrosting.

SUMMARY OF THE INVENTION The present invention provides a method for adaptive defrost control that is simple to implement, extremely stable, and capable of being used without modification across an entire product line.

Using a frost build-up prediction, the present invention seeks an operating point within an acceptable range of frost build-up as opposed to a specific point of optimum frost build-up. This acceptable range establishes a zone or dead band in which no adjustment of the defrost system is made. This enhances stability by not requiring adjustments for small changes in frost build-up. Furthermore, by using this dead band of acceptable values, only a minimum number of different controls need be made for different products. For example, only a single control for all the different refrigerators in a product line need be made.

If the frost build-up measured during a defrost operation is found to be outside the acceptable range, the compressor run time (or, if desired, the actual elapsed time) between defrost operations is increased by a fixed increment to increase frost elapsed time or the accumulated between defrost operations is not is regulated as a function of a in an attempt to provide optimum 1 s 2 3 4 6 7 8 9 11 12 13 14 16 17 18 19 21 22 ->3 ^4 26 27 28 229840 build-up or decreased by a fixed increment to decrease frost build-up. This adjustment process takes place after each defrost operation, unless the built-up frost is within the acceptable dead band range.

In addition, the method disclosed herein may optionally include additional steps that further ensure stable operation of an adaptive defrost system. These steps impose limits on minimum and maximum values of compressor run time between defrost operations, and in the case of substantial predictive error, return the system to a worst case regimen to guarantee that the evaporator will not be frost-blocked prior to the next defrost operation. Furthermore a step to limit the maximum length defrost period may be added to protect against a failure of the defrost heating system and the resulting unrealisti-cally long defrost operation.

BRIEF DESCRIPTION OF THE DRAWINGS A fuller understanding of the invention may be had by referring to the following description and claims taken in conjunction with the accompanying drawings.

FIG. 1 is a flow chart of one embodiment of the invention; FIG. 2 is a flow chart of an embodiment of the invention of FIG. 1, further including a step to reset the compressor run time if excessive frost has been forming; 2 3 4 6 7 8 9 11 12 13 14 16 17 18 19 21 22 ?3 "24 26 27 8 22 9 8 4 0 FIG. 3 is a flow chart of an embodiment of the invention of FIG. 2, further including a step to sense completion of a defrost operation by sensing the temperature at the defrost site; FIG. 4 is a flow chart of an embodiment of the invention of FIG. 3, further including a step to abort a defrost operation after a maximum defrost time; and FIG. 5 is a flow chart of an embodiment of the invention of FIG. 4, further including a step of limiting the compressor run time to be between a minimum and a maximum value.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT It is well known in the art that a basic refrigeration system consists of a compressor, a condenser, an evaporator, and a defrost means. Control of the temperature in the area to be refrigerated is typically accomplished by a thermostat that energizes the compressor when additional cooling is needed (and no conflicting requirement exists, such as a defrost operation). This off and on cycling of the compressor generates one of the two inputs (compressor on signal) required for the operation of the adaptive defrost method of the invention, which in turn controls the defrost means. The other signal required is from some form of frost sensor, whereby the presence or absence of frost about the evaporator is sensed. As will be later described, the preferred method of I 1 2 3 4 6 7 8 9 11 12 13 14 16 17 18 19 21 22 23 24 26 ¥ 27 28 29 229840 implementing this sensor may be a temperature switch at the evaporator along with an appropriate step added to the adaptive defrost method.

Referring to FIG. 1, a flow chart of an adaptive defrost method according to the present invention is shown. When the refrigeration device is first activated, the adaptive defrost control method begins at the start point 10. This then results in two operations: the total time the compressor will run to the next defrost operation, as contained in run time register 12, is set to a reset value (e.g., 8 hours) and the compressor run time timer 14 is set to zero. The reset value for register 12 is most advantageously chosen to be the worst case (smallest) necessary time between defrost cycles.

As the compressor of the refrigeration device is cycled on and off in response to cooling needs, the compressor "on" signal 16 is used by timer 14 to measure the total time the compressor has run since the last reset.

The time from register 12 is supplied to equal test 18 and to adjuster 20. Equal test 18 compares the time in register 12 to the accumulated time from timer 14. This comparison process continues until the two values are equal. When the value in register 12 equals the value in timer 14, timer 14 is reset to zero, a defrost enable signal 22 which energizes a heater or other defrosting means is output, and the defrost timer 24 is activated.

Defrost timer 24 measures the time to defrost the evaporator. Frost gone test 26 repeatedly checks the frost sensor 28 until the built-up frost is ( 1 s 2 3 4 6 7 8 9 11 12 13 14 16 17 18 19 ">1 "22 23 24 °6 "27 28 29 31 32 22 9 8 4 0 gone. Upon detecting no frost, frost gone test 26 sends out a defrost disable signal 30 to disable the defrosting means and sends the time from defrost timer 24 to below range test 32.

If the time from defrost timer 24 is below a range of acceptable values (e.g., 9-13 minutes), the below range test 32 provides an input to the add time input 34 of adjuster 20; otherwise, the time from defrost timer 24 is passed along to within range test 36. If the time from defrost timer 24 is within the range of acceptable values, the within range test 36 provides an input to the no change input 38 of adjuster 20; otherwise, the within range test 36 provides an input to the subtract time input 40 of adjuster 20.

The adjuster 20 takes the current value in register 12 and, depending on whether an input has been received on input 36, 38, or 40, increases, leaves unchanged, or decreases, respectively, the value from register 12 in order to produce an adjusted value. The amounts of increase or decrease are preferably fixed increments (e.g., 2 hours), equal or otherwise, or they may be values calculated in relation to measured values such as the time from defrost timer 24.

The adjusted value from adjuster 20 is then input to register 12 and the cycle starts over, with equal test 18 waiting for the value from timer 14 to equal the value in register 12.

FIG. 2 is an embodiment of the invention shown in FIG. 1 with the additional step of resetting register 12 to the reset value if a defrost operation . 2 3 4 6 7 8 9 11 12 13 14 16 17 18 19 21 ■s12 23 24 "?6 -z7 28 29 31 32 22 9 8 4 0 li takes too long. This has the effect of restoring the defrost system to a known stable state when major perturbations have occurred in the basic assumption of a well-behaved level of humidity at the evaporator.

In FIG. 2. a less than reset trigger test 42 is added between within range test 36 and the add time input 34. If within range test 36 fails, the time from defrost timer 24 is passed along to less than trigger test 42. If the time from defrost timer 24 is less than a known trigger value greater than any value in the acceptable range (e.g., 16 minutes), less than trigger test 42 provides an input to subtract time input 40. However, if the time from defrost timer 24 is not less than the trigger value, less than trigger test 42 provides a reset input to register 12 and register 12 is reset to the reset value rather than to a value from adjuster 20.

FIG. 3 shows an embodiment of the invention in FIG. 2 with a possible implementation of the frost gone test 26 and frost sensor 28 shown in FIGS. 1 and 2. It should be understood that this implementation could as easily be practiced with the embodiment of FIG. 1.

In FIG. 3, frost gone test 26 is replaced by hot enough test 26* and defrost sensor 28 is replaced by defrost temperature sensor 28'. Defrost temperature sensor 28' measures the temperature on or about the evaporator. When the temperature measured is warm enough to indicate that the evaporator has been defrosted (e.g., 47°F), hot enough test 26' sends out a defrost disable signal 30 to disable the defrosting means and sends the time from defrost timer 24 to . 2 3 4 6 , 7 8 9 11 12 13 14 16 17 18 19 21 «2 23 24 26 28 29 12 22 9 8 4 0 below range test 32. Until the evaporator temperature passes the hot enough test 26', the test 26' repeatedly tests the defrost temperature sensor 28' waiting for the evaporator temperature to rise to the proper level.

FIG. 4 shows an embodiment of the invention in FIG. 3 with an improvement consisting of an additional step to abort a defrost operation if it appears that the defrost means has failed. This step consists of aborting a defrost operation if it has taken too long for the evaporator to reach the temperature indicating a successful defrost operation. This step avoids the case where the defrost means has failed and, as a result, the entire refrigerated area would have to reach the defrost complete temperature to complete a defrost operation, rather than just the localized heated area about the defrost means-evaporator combination. It should be understood that the additional step of this embodiment could as well be practiced in the embodiments of FIGS. 1 and 2.

In FIG. 4, too long test 44 has been added between defrost timer 24 and hot enough test 26'. In addition, too long test 44 has an output parallel to the output of hot enough test 26'. When defrost timer 24 is activated, too long test 44 repeatedly checks to see if defrost timer 24 has run longer (e.g., greater than 21 minutes) than possibly required for a normal (defrost means operating) defrost operation. If not, hot enough test 26' is then checked as described above. However, if too long test 44 finds that the defrost operation has gone on too long for normal ( 1 X 2 3 4 6 7 8 9 11 12 13 14 16 17 18 19 I 2 2 23 24 " S 27 28 29 13 22 9 8 4 0 operation, too long test 44 sends out a defrost disable signal 30 to disable the defrosting means and sends the time from defrost timer 24 to below range test 32, much the same as in the case of a normal defrost termination as described above.

FIG. 5 shows an embodiment of the invention in FIG. 4 with the added steps of limiting the possible time that the compressor may run between defrost operations to be between a minimum and maximum value. This step further improves the stability of the defrost method of the present invention. By limiting the maximum compressor run time between defrost operations, it is less likely that a change in the refrigeration device operating conditions, such as a rapid increase in humidity after extended low humidity (for example, a home refrigerator when the family returns after several days' absence and again starts to open and close the door) will force the defrost system to reset to the worst case reset compressor run time between defrost operations. By limiting the minimum compressor run time between defrost operations, no defrost operations at unrealistically frequent intervals are permitted. In addition, FIG. 5 shows a possible implementation of the defrost enable signal 22 and defrost disable signal 30 shown in FIGS. 1-4. All of the additional steps in FIG. 5 over FIG. 4 and the particular embodiment of the defrost enable/-disable signals could as easily be practiced in the embodiments of the invention shown in FIGS. 1-3.

In FIG. 5, the adjusted compressor run time from adjuster 20 is processed by the additional steps 46, 48, 50 and 52 prior to being used to set run time I 1 2 3 4 6 7 8 9 11 12 13 14 16 17 18 19 1 22 23 24 s 27 28 29 31 22 9 8 4 14 0 register 12. The adjusted run time from adjuster 20 is tested by over maximum test 46. If the adjusted time is greater than the maximum allowable run time (e.g., 48 hours), the adjusted time is set to the maximum time 48 and this adjusted time is then tested by under minimum test 50. If not, the adjusted time from over maximum test 46 is directly tested by under minimum test 50. If the adjusted time applied to under minimum test 50 is less than the minimum allowable run time (e.g., 8 hours), the adjusted time is set to the minimum allowable time 52 and this adjusted time is then used to reset run time register 12. If not, the adjusted time from under minimum time test 50 is directly used to reset register 12.

In addition, FIG. 5 shows a possible implementation of defrost enable signal 22 and defrost disable signal 30 of FIGS. 1-4. Defrost enable 22 is implemented by defrost heater enable 22a and compressor disable 22b. Defrost disable 30 is implemented by defrost heater disable 30a, delay 30b and compressor enable 30c. In the case of a refrigeration device with a defrost means consisting of an evaporator heater, it is necessary to disable the compressor from running to allow the heater to melt the frost instead of competing with the refrigeration cycle. Activation of defrost heater enable 22a and compressor disable 22b is the same as that described above for defrost signal 22. Upon completion of a defrost operation, the heater must be turned off and the compressor allowed to run again. However, if the evaporator is allowed to cool off for a period before attempting 1 r- 2 3 4 6 7 8 9 11 12 13 C G 22 9 8 compressor operation, the starting load on the compressor will be considerably less. For this reason, when defrost heater disable 30a is activated (in the same manner as described above for defrost disable 30), delay 30b is activated and a delay occurs (e.g., 5 minutes) before delay 30b activates compressor enable 30c.

Although the preferred embodiments of this invention have been shown and described, it should be understood that various modifications and rearrangements of the steps may be resorted to without departing from the scope of the invention as disclosed and claimed herein. 1 2 3 4 6 7 8 9 11 12 13 14 16 17 18 19 220840 16

Claims (10)

WHAT WE CLAIM IS:

1. A method for varying the time between defrost operations to ensure acceptable frost build-up before initiating a defrost operation in a refrigeration device having a compressor, a condenser, an evaporator, and a defrost means, said method comprising the steps of: measuring the period of time required for operation of the defrost means to melt built-up frost on the evaporator; comparing the measured time period to a known acceptable range of time; maintaining the value of the said time between defrost operations if said measured period is within said acceptable range; or increasing the value of the said time between defrost operations if said measured period is below said acceptable range; or decreasing the value of the said time between defrost operations if said measured period is above said acceptable range. 17 220840

2. A method according to claim 1, wherein the value of the said time between defrost operations is decreased if said measured period is above said acceptable range and less than a known reset trigger time that is beyond the acceptable range; or the value of the said time between defrost operations is set to a known reset value if i'"v said measured period is greater than said known reset trigger time.

3. A method according to claim 1 or 2, wherein the value of the said time between defrost operations is increased by a known first increment if said measured period is below said acceptable range; or the value of the said time between defrost operations is decreased by a known second increment if said measured period is above said acceptable range.

4. A method according to any one of claims 1,2, or 3, wherein said time between defrost operations is the actual elapsed time. G

5. A method according to any one of claims 1,2, or 3, wherein the said time between defrost operations is the compressor run time.

6. A method according to claim 3, wherein said first increment equals said second increment.

7. A method according to any one of claims 1,2, or 3, wherein the step of measuring the period of time -^IKg^EEPtxed for operation of the defrost means to melt built-up froSSI on the evaporator further comprises the steps of etecxing when the temperature on or about the evaporator "Succeeds a known maximum temperature value, and terminating 2 8 MAR I991&J ie defrost operation in response thereto.

A method according to claim 7, wherein the 18 step of measuring the period of time required for operation of the defrost means to melt built-up frost on the evaporator further comprises the step of terminating a defrost operation if the temperature on or about the evaporator does not exceed the known maximum temperature value within a known maximum allowable defrost time.

9. A method according to any one of claims 1,2, or 3, further comprising the additional step of limiting the time between defrost operations to no more than a known maximum time and no less than a known minimum time, said additional step taking priority over other steps.

10. A method for varying the time between defrost operations substantially as herein described reference to the accompanying drawings. WHITE CONSOLIDATED INDUSTRIES, INC. By Their Attorneys HENRY HUGHES LIMITED