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(11) | EP 0 067 523 A1 |
| (12) | EUROPEAN PATENT APPLICATION |
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| (54) | Frost sensing apparatus |
| (57) The frost sensing apparatus senses the presence of frost on a refrigeration coil,
the apparatus including an electrical resistor (20) mounted on a surface of the coil
and having a positive or negative temperature coefficient of resistance. A control
circuit (12), including a microprocessor (33), analogue to digital converter (25)
and control switch (40), periodically connects the resistor to a source of electrical
power for a fixed interval of time and then senses the resistance of the resistor
whereby to detect the magnitude of change of resistance during the time interval and
hence the presence or not of frost on the coil. |
[0001] This invention relates to frost sensing apparatus and has particular application in defrost control systems for refrigeration equipment.
[0002] According to the invention, there is provided apparatus for sensing the presence of frost on a refrigeration coil, said apparatus being characterized by an electrical resistor mounted on a surface of the coil, the resistor having a positive or negative temperature coefficient of resistance; and a control circuit including means periodically connecting the resistor to a source of electrical power for a fixed interval of time and means sensing the resistance of the resistor whereby to detect the magnitude of change of resistance during said time interval and hence the presence or not of frost on the coil.
[0003] An embodiment of the invention will now be described by way of example, with reference to the accompanying drawings, in which:-
Figure 1 is a block diagram of a simple defrost control system using frost sensing apparatus according to the present invention,
Figure 2 is a resistance versus temperature curve of a resistance heater-sensor of the system of Figure 1,
Figure 3 shows a pair of curves showing the presence or absence of frost or ice, and
Figure 4 is a flow chart of a heat pump system using the defrost control system of Figure 1.
[0004] Referring to Figure 1, the defrost control system 12 is connected to a power supply by conductors 13 and 14. The system 12 is further connected in use by conductor 15 to a heat pump 16. The heat pump 16 receives a defrost command on the conductor 15. An electric resistor 20 is mounted on a refrigeration or evaporator coil of the heat pump 16, (indicated by dotted line 17), and is subject to becoming covered with frost or ice as the heat pump operates. The evaporator coil typically is mounted external to the building to which the heat pump supplies heat. The electric resistor 20 is also identified as R. Typically the electric resistor 20 would be a negative temperature coefficient resistor, but it could be a positive coefficient resistor. The only requirement of the electric resistor is that it has a coefficient of resistance that is not zero. In other words, the resistor 20 must have a resistance value that changes positively or negatively as a change in temperature occurs.
[0005] The resistor 20 is connected by a pair of conductors 21 and 22 to a pair of terminals 23 and 24 which allow the interconnection of the resistor 20 to the defrost control system 12. The connector 23 is directly connected to the supply line 13, while the connector 24 connects the conductor 22 to an analog to digital converter 25 that is included within the defrost control system 12. The analog to digital converter 25 further receives power on conductor 26 from the conductor 13 along with a further power connection 27 which is connected to the ground conductor 14. The analog to digital converter 25 is connected by a conductor 30 to the connector 24. The resistance of the resistor 20 is capable of being measured at the analog to digital converter 25 and is supplied as a digital output signal on a group of conductors generally disclosed at 31. The group of conductors 31 is connected at 32 to a microcomputer 33 which typically would be a microprocessor type of device. The microcomputer 33 could be any type of analyzing circuitry, even in discrete component configuration, which is capable of providing functions which will be explained in connection with Figure 3 and the flow chart of Figure 4.
[0006] The microcomputer or microprocessor 33 is powered from conductors 34 and 35 which are in turn connected to the supply conductors 13 and 14. Only the functions of the microprocessor 33 which are directly related to the present invention have been disclosed and will be described. This microprocessor 33 could be similar to that disclosed in United States Patent No. 4,232,530. Only two outputs ports 36 and 37 from the microprocessor 33 are of interest in connection with the system disclosed in Figure 1, and they will be described in some detail. The output port 37 is connected to conductor 15 and supplies a defrost command signal from the microprocessor 33 to the heat pump 16 telling the heat pump to reverse its operation for a defrost cycle.
[0007] The output from port 36 is connected to a power control means 40. The power control means 40 is specifically disclosed as a solid state switch having a pair of terminals 41 and 42 and a gate or control electrode 43. This control means could be an electromechanical relay. The gate or control electrode 43 is connected to port 36 and is capable of receiving a gating signal to cause the power control means 40 to conduct between terminals 41 and 42. The terminal 41 is connected to conductor 30, while the terminal 42 is connected to the group or conductor 14. The control system 12 is completed by the addition of a resistor 44 that is connected across the power control means 40 and provides for the necessary voltage division for control and measuring of the resistor 20.
[0008] Before the operation of the system of Figure 1 is described, the graphs of Figures 2 and 3 will be explained. In Figure 2, a curve 50 shows the resistance versus temperature characteristics of the electric resistor 20. This is a typical resistance versus temperature curve for a negative temperature coefficient resistance means. The significance of this curve and its application to the system of Figure 1 is explained in connection with Figure 3.
[0009] In Figure 3, two curves show the resistance of the resistor means 20 versus time after the application of power to heat the resistor 20, for frost and no frost conditions. It will be noted that at the zero point in time 53, curves 51 and 52 of the resistor 20 coincide. If power is applied to the resistor 20 for a period of time indicated at t, the curves 51 and 52 will be different in resistance by an increment indicated at 54. This difference in the resistance of the resistor 20 after applying power for a period of time t indicates whether or not frost or ice exists on the resistor 20. The reason for this will be explained below.
[0010] If it is first assumed that no frost or ice exists on resistor 20, as exemplified by curve 52, the application of electric power to the resistor 20 for the time t will cause the resistance of the resistor 20 to drop to point 55. This is due to the fact that all of the energy that is put into the resistor means 20 by energizing it for the period of time t is used to raise the temperature of the resistor 20, thereby allowing the resistance value R to drop to the point 55. If frost or ice is present on the resistor 20, the application of power to the resistor 20 for time t provides two functions. The first function is to melt some or all of the frost or ice in the region of the resistor and thereby requires some of the energy. This leaves a lesser amount of energy available to heat the resistor 20, and as a result the resistor 20 follows curve 51 and has a resistance value indicated at point 56.
[0011] It thus can be seen that if the resistor 20 is energized for a set period of time and then has its resistance measured, the value of the resistance is indicative of whether frost or ice is present or absent. This difference, as exemplified in Figure 3 by the difference 54, can be used to indicate to the system of Figure 1 the presence or absence of frost or ice to control a defrost cycle in a heat pump.
OPERATION OF FIGURE 1
[0012] Initially the power control means 40 is in a conduct mode thereby allowing energy to flow from the conductor 13 to the conductor 14 (and vice versa) through the power control means 40 and resistor 20. This allows the resistor 20 to heat along either of curves 51 or 52 disclosed in Figure 3 in dependence on the present or absent of frost on the evaporator coil. The microprocessor 33 then provides a signal at port 36 to the gate 43 of the power control means 40 to operate the control means 40 to a nonconductive state. Power is then removed from the circuit directly connecting the resistor 20 across the conductors 13 and 14. With control means 40 in a nonconductive state, the analog to digital converter 25 provides a means of sensing the resistance between the conductors 26 and 30 through the interconnection 31 to the microprocessor 33. The microprocessor 33 is capable of measuring the resistance and can determine whether the resistance of the resistor 20 is at point 55 of Figure 3 or is at 56 of Figure 3. With this arrangement, the microprocessor 33 obtains information as to whether or not frost or ice has built up on the resistor 20 to a point that requires defrosting. If such a defrost operation is necessary, the microprocessor 33 provides a signal at port 37 to the heat pump 16 indicating to the heat pump that a defrost cycle is to be undertaken.
[0013] Once the heat pump 16 enters a defrost cycle, the resistor 20 can supply continuous resistance information to the analog to digital converter 25 and then to the microcomputer means 33. By continuously measuring the temperature of the resistor 20, the microprocessor 33 can determine when the evaporator coil upon which the resistor 20 has been mounted has reached a high enough temperature to indicate that complete defrosting has occurred. Typically the temperature to which the coil would be raised is somewhere between 0 to 130 centigrade. The reason that the temperature of 130 Centigrade has been selected is to allow for an adequate period of time for water to run off of the coil upon which the resistor 20 is mounted. The temperature indicated at the coil as measured by the resistor 20 is used to allow for the complete melting of any frost or ice, and for adequate run off of the defrost water.
[0014] It will be apparent from the above description that a very simple and effective defrost control system has been disclosed. The system has been specifically indicated as being adapted to be connected to a heat pump, and the operation of such a heat pump system will be further defined in connection with Figure 4. A flow chart is disclosed in Figure 4 utilizing conventional flow chart symbols for the defrost control portion of a heat pump system as disclosed in Figure 1. The flow chart will be briefly described after certain of its terms are herein defined.
[0015] The resistance 20 of Figure 1, along with the control circuit system 12 can directly determine the presence of frost or ice on the evaporator coil of a heat pump to cause the defrost cycle, and can be further extended to provide other functions. Other functions that are readily available with this arrangement are the "permit" function and the "terminate" function. The permit function determines if the evaporator coil is cold enough to be capable of collecting frost (that is, colder than 0° Centigrade). The terminate function determines if the evaporator coil defrosting process is complete (that is, warmer than 0° Centigrade and generally up to some higher temperature to allow an adequate time for water run-off). By combining the permit function, the terminate function, and the defrost cycle a system using the combined heater-sensor resistor 20 along with the control system 12 can operate a heat pump in a very efficient manner. The defrost and related cycles including the permit function and the terminate function will be briefly described in connection with the flow chart of Figure 4.
[0016] In Figure 4 a flow chart shows a heat pump system. The system is reset and then proceeds to determine whether the system is on or off. If the system is so on, the system then determines whether or not a time interval has been reached in which to measure the resistor 20 as indicated as the resistance R. If that time has been reached, the measurement of resistor 20 is accomplished. The resistor 20 is then checked to determine whether the resistor 20 is less than or equal to the "permit" temperature. If it is not, the mechanism is reset. If it is, the system energizes the power control means 40 for the time interval t. Deenergization of the resistor 20 occurs at the end of the time t, and the resistance of resistor 20 is again measured. If no frost is present at this time, the system resets to measure a new time interval. If frost is present, the system waits for the resistor 20 to cool down to the temperature of the coil and its associated ice or frost. This prepares the resistor for the "terminate" function.
[0017] After the cool-down, the microprocessor 33 commands a defrost cycle for the heat pump 16. The function of the microprocessor 33 allows for a standard defrost cycle to proceed while measuring the resistor 20 to determine if R is greater than or equal to the "terminate" temperature. If it is not, the defrost cycle continues. When the terminate temperature is reached, it is indicative that the coil has been freed of frost or ice and has had time to drain. At this point the terminate function occurs and the defrost cycle is ended with the heat pump being put back into normal operation. The time increment measuring goes on to again measure for when frost or ice has built up on the refrigeration coil.
1. Apparatus for sensing the presence of frost on a refrigeration coil, said apparatus
being characterized by an electrical resistor (20) mounted on a surface of the coil,
the resistor having a positive or negative temperature coefficient of resistance;
and a control circuit (12) including means (33,40) periodically connecting the resistor
to a source of electrical power for a fixed interval of time and means (25,33) sensing
the resistance of the resistor whereby to detect the magnitude of change of resistance
during said time interval and hence the presence or not of frost on the coil.
2. The apparatus of Claim 1, characterized in that the control circuit (12) includes
a microcomputer (33), and an analogue to digital converter (25) connected between
the microcomputer and the resistor.
3. The defrost system including sensing apparatus according to Claim 1 or 2, characterized
in that a heat pump is provided to cool said refrigeration coil, and wherein the operation
of the heat pump is reversed to heat said coil upon receiving a signal from said sensing
apparatus that frost is present on the coil.