Patent Publication Number: US-2016238332-A1

Title: Heat Exchanger Fault Diagnostic

Description:
RELATED APPLICATIONS 
     This application is a continuation of PCT/GB2014/053524, filed Nov. 28, 2014, titled “HEAT EXCHANGER FAULT DIAGNOSTIC,” and which designated the United States, the entire contents of which are hereby fully incorporated herein by reference for all purposes. Application PCT/GB2014/053524 claims priority from GB 1320977.0, filed Nov. 28, 2013, the entire contents of which are hereby fully incorporated herein by reference for all purposes. 
    
    
     The present invention relates to the fault diagnosis of a heat exchanger in which ambient air temperature and heat exchanger temperature are measured. 
     Refrigeration may be defined as lowering the temperature of an enclosed space by removing heat from that space and transferring it elsewhere. The work of heat transport is typically driven by heat, magnetism, electricity, or other means. Refrigeration has many applications, including, but not limited to: household refrigerators, industrial freezers, cryogenics, and air conditioning. Heat pumps may use the heat output of the refrigeration process and also may be designed to be reversible, but are otherwise similar to refrigeration units. 
     The vapour-compression cycle is used in most household refrigerators as well as in many large commercial and industrial refrigeration systems. The vapour-compression cycle uses a circulating liquid refrigerant as the medium which absorbs and removes heat from the space to be cooled and subsequently rejects that heat elsewhere. A typical, single-stage vapour-compression system has four components: a compressor, a condenser or heat exchanger, a thermal expansion valve (also called a throttle valve), and an evaporator. 
     Circulating refrigerant enters the compressor in the thermodynamic state known as a saturated vapour and is compressed to a higher pressure, resulting in a higher temperature as well. The hot, compressed vapour is then in the thermodynamic state known as a superheated vapour and it is at a temperature and pressure at which it can be condensed with either cooling water or cooling air. That hot vapour is routed through a condenser or heat exchanger where it is cooled and condensed into a liquid by flowing through a coil, fins or tubes with cool water or cool air flowing across the coil, fins or tubes. This is where the circulating refrigerant rejects heat from the system and the rejected heat is carried away by either the water or the air (whichever may be the case). 
     The condensed liquid refrigerant, in the thermodynamic state known as a saturated liquid, is next routed through an expansion valve where it undergoes an abrupt reduction in pressure. That pressure reduction results in the adiabatic flash evaporation of a part of the liquid refrigerant. The auto-refrigeration effect of the adiabatic flash evaporation lowers the temperature of the liquid and vapour refrigerant mixture to where it is colder than the temperature of the enclosed space to be refrigerated. 
     The cold mixture is then routed through the coil or tubes in the evaporator. A fan circulates the warm air in the enclosed space across the coil or tubes carrying the cold refrigerant liquid and vapour mixture. That warm air evaporates the liquid part of the cold refrigerant mixture. At the same time, the circulating air is cooled and thus lowers the temperature of the enclosed space to the desired temperature. The evaporator is where the circulating refrigerant absorbs and removes heat which is subsequently rejected in the condenser and transferred elsewhere by the water or air used in the condenser. 
     To complete the refrigeration cycle, the refrigerant vapour from the evaporator is again a saturated vapour and is routed back into the compressor. 
     A refrigeration unit, such as a refrigerated beverage merchandising unit (RBMU), comprises a refrigeration system (electromechanical compressor pump, refrigerant, evaporator, heat exchanger and an expansion valve) fitted with a means of controlling when a compressor runs to control the temperature of a chilled compartment. 
     Control of a compressor may be carried out in a variety of ways. At its most simple, control is via an electromechanical device, sited inside a chilled compartment, which detects the temperature and contains a contact to switch the compressor on and off. More complex electronic devices have a temperature sensor in the chilled compartment linked to an electronic control device positioned outside the chilled compartment, which contains the mechanism for switching the compressor on and off. 
     A heat exchanger is the part of a refrigeration system that expels heat gathered from the chilled compartment. Typically, the heat exchanger is located outside the cooling compartment and is cooled with air drawn through it from the ambient surroundings via an electrical fan or via convection. 
     For a refrigeration unit, the extent to which heat transfer occurs is a function of two things: 
     The temperature difference between the ambient air surrounding the heat exchanger and the temperature of the circulating liquid/gas. 
     The mass of ambient air that can exchange with the heat exchanger. 
     Poor heat exchange occurs when the amount of air passing through the heat exchanger (and the extent of heat transfer) reduces significantly due to blockage with dust, debris etc. At such a time, the refrigeration unit usually sounds an alarm, shuts itself down and an engineer is alerted. 
     Inefficient heat exchange also occurs when the ambient temperature is too high, i.e. the temperature difference between the ambient air and the heat exchanger is too small. In such a situation, the refrigeration unit will shut down and an engineer will be called out on the assumption that the heat exchanger is faulty and/or needs cleaning. Beverages, perishable food items etc. are also removed from sale. In reality, there is no fault and so a) an engineer is called out unnecessarily and b) saleable goods are unnecessarily removed from sale. 
     Accordingly, it is desirable to measure the temperature of the ambient air in the vicinity of a refrigeration unit (ambient air temperature) and the temperature of the heat exchanger itself (heat exchanger temperature) for fault diagnostics. 
     For example, where a refrigeration unit is experiencing higher than normal heat exchanger temperatures, it is important to know the ambient air temperature to obtain the correct diagnosis of what is causing an above-normal heat exchanger temperature (heat exchanger overheating). A heat exchanger is designed to get rid of heat and is usually constructed in a way that facilitates this. There are many different designs (plate, fin and coil, static, roll) but, in essence, all have the same objective—to create a large surface area that can be used to exchange heat to a medium (air, water etc) held at a lower temperature. 
     Thus, there is a need to distinguish between a genuine fault with a heat exchanger and when inefficient heat exchange is due to an insufficient temperature difference to enable heat exchange to occur. 
     The present invention seeks to solve this problem and thus resides in a fault diagnostic for a heat exchanger in which ambient air temperature and heat exchanger temperature are measured. 
     When a heat exchanger is in operation, its temperature will rise to a steady state, usually not reaching more than around 50° C. or around +20° C. above the temperature of the surrounding ambient air. Once heat exchange is no longer required by the associated device (for example, the refrigeration unit has reached its desired chilled compartment temperature), operation of the heat exchanger ceases and its temperature drops to the temperature of the ambient air temperature. 
     If the heat exchanger is surrounded by dust and dirt, the return to ambient air temperature is impeded as the dust and dirt act as an insulator and the heat exchanger retains some of its operating heat. Alternatively, if the ambient air temperature is close to the operating temperature of the heat exchanger, the heat exchanger will not be able to cool sufficiently. 
     By measuring the ambient air temperature when the heat exchanger is not in operation, it is possible to distinguish between a genuine fault with the heat exchanger, such as a need for a clean, and a simple elevation in ambient air temperature, under which circumstances the unit simply requires additional time to cool down before restarting the heat exchange process. Distinguishing between the two situations enables the efficient use and call-out of engineers. Indeed, the majority of unnecessary engineer call-outs are to RBMUs operating in high ambient air conditions rather than heat exchanger blockages. 
     In one embodiment, the present invention encompasses a heat exchanger fault diagnostic method comprising: 
     measuring the temperature of the heat exchanger when the heat exchanger is in operation, 
     measuring the temperature of ambient air around the heat exchanger when the heat exchanger is not in operation, 
     comparing the two temperatures, and 
     when the difference between the two temperatures is greater than a selected temperature difference, sounding an alarm, alerting an engineer and/or disabling operation of the heat exchanger. 
     By measuring both the temperature of the ambient air around the heat exchanger and the heat exchanger temperature, it is possible to make a judgement on whether the heat exchanger overheating is being caused by high ambient air temperature or heat exchanger blockage, or a combination of the two. 
     In current usage, a heat exchanger temperature above a certain set point, say 80 or 100 degrees Centigrade, triggers an alarm in the unit and the unit is switched off to prevent overheating. However, according to the present invention, where the high temperature is a result of a high ambient temperature, rather than a blockage or fault, if the unit is able to self-diagnose that the high heat exchanger temperature is due to a high ambient temperature, the machine is instructed to take additional time to cool down before restarting the heat exchange process, rather than shutting the machine down unnecessarily. In this way, the refrigeration unit is able to keep itself open for business and reduces the need for an engineer to be called out. 
     Where high ambient temperature is consistently recorded, the unit may provide an alert so that the siting of the unit may be changed to allow better air flow around the heat exchanger. 
     In accordance with the present invention, the method further comprises initial set-up steps in which a maximum operating temperature for the heat exchanger is set and an acceptable temperature difference between the heat exchanger temperature and ambient air temperature (Delta) is set for an efficiently operating heat exchange system. 
     For example, the heat exchanger high temperature may be set at, say, 100 degrees Centigrade and Delta is 30 degrees Centigrade. 
     In one embodiment, an alarm may be triggered when the heat exchanger high temperature exceeds the maximum set temperature and subtraction of ambient air temperature gives a difference (Delta) of less than 30 degrees Centigrade. However, if subtraction of ambient air temperature gives a Delta reading of greater than 30 degrees Centigrade then a different alarm is sounded, an engineer is alerted and/or operation of the heat exchanger is disabled. As the heat exchanger becomes blocked with debris, for example, the heat exchanger has a reduced ability to remove heat and so the difference between ambient air temperature and the high temperature of the heat exchanger will increase. 
     In one embodiment, operation of the heat exchanger is informed by the operation of a compressor, such as a pump, associated with the heat exchanger. 
     Ideally the temperature measurements are stored in a microprocessor, wherein a microprocessor is understood to be a multipurpose, programmable device that accepts digital and/or analogue data as input, processes it according to instructions stored in its memory, and provides results as output. 
     The microprocessor is able to sense or detect whether the compressor is running or not, by way of any conventional means such as a switch, and save the temperature readings. Preferably the temperature readings are saved by the microprocessor into separate files (heat exchanger temperature and ambient air temperature). The second temperature is then subtracted from the first and a diagnosis made on whether or not the heat exchanger is functioning efficiently. 
     It will be appreciated that the heat exchanger requires time to cool down after operation and so, in a preferred embodiment, the ambient air temperature around the heat exchanger is measured after a time period that is sufficient to enable the heat exchanger to cool to, or near to, ambient temperature. For example, such a time period may be approximately 2 to 5 minutes after the heat exchanger has ceased to operate. 
     In one embodiment, the two temperatures are measured by two temperature sensors: one to measure the temperature of the heat exchanger and a second to measure the temperature of the ambient air. 
     In an alternative embodiment, the two temperatures are measured by a single temperature sensor. The temperature sensor may be mounted on or close to the heat exchanger. Because temperature is measured and recorded over time, two sensors may be replaced by a single sensor. The difference in temperature recordings is sufficient to enable the microprocessor to determine whether or not the heat exchanger is functioning efficiently and, if not, whether there is a fault or the inefficiency is due to a high ambient air temperature. 
     In another aspect, the present invention resides in the use of a single temperature probe or sensor, in combination with a microprocessor, to measure temperature emitted from a heat exchanger and ambient air temperature surrounding the heat exchanger. By measuring and recording temperature readings when the heat exchanger is in operation and when it has stopped, it is possible to establish two separate, discrete temperature measurements using the same temperature sensor. 
     In particular, the single sensor is used to assess the efficiency of the heat exchanger such that, when the difference between the two temperatures falls below a critical level, the microprocessor is able to ascertain whether the inefficiency is due to a high ambient air temperature and so keep the unit functioning, rather than raising an alarm and/or shutting down the associated refrigeration system. 
     In addition, the use of a single sensor, in place of two sensors, reduces the complexity and cost in construction of devices such as refrigerator units while retaining the diagnostic capability of two separate temperature sensors. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will now be described in detail by way of example as illustrated in the accompanying figures in which: 
         FIG. 1  is a scheme setting out the flow of instructions for the method and single sensor of the invention when installed in a refrigeration unit such as a RBMU; and 
         FIG. 2  is a scheme setting out the flow of instructions for overheating of a heat exchanger in a refrigeration unit. 
     
    
    
     It will be understood that a refrigeration unit includes a heat exchanger and a compressor in which the compressor compresses and vaporises circulating refrigerant. The unit also includes a microprocessor. 
     As shown in  FIG. 1 , on initial start-up of the unit, the microprocessor begins a two-pronged routine to enable dual sensing of heat exchanger temperature and ambient air temperature. First, the microprocessor enquires whether or not the compressor is running. 
     If the compressor is running, the microprocessor starts a High Temperature routine to ascertain the temperature of the heat exchanger. If the parameter DTS (dual temperature sensor) is equal to 1, the dual temperature sensor is enabled on the refrigeration unit. The compressor is then confirmed to be running and the temperature sensor is instructed to record the current temperature (HT) of the heat exchanger. 
     If the parameter DTS does not equal 1 and the compressor is running, the microprocessor simply instructs the recording of the heat exchange temperature because the dual temperature sensing feature is not enabled. 
     As illustrated in  FIG. 2 , when the compressor is running and the temperature of the heat exchanger exceeds a maximum pre-set threshold, for example a temperature between 50 and 125 degrees Celsius, the compressor is switched off. Following this, the microprocessor subtracts the stored ambient air temperature from the high heat exchanger temperature. If the difference between the two temperatures is less than a programmed value (for example, 30 degrees Celsius), the high ambient temperature is flagged as a warning and the heat exchanger continues to operate after an extended period of cooling down. 
     If the difference between the two temperatures is greater than the programmed value (for example, about 30 degrees Celsius), this may be used to trigger an alarm or a service request for an engineer, and/or the refrigeration unit is kept switched off until a service call is answered. 
     Returning to  FIG. 1 , if the parameter DTS does not equal 1 and the compressor is not running, the high temperature enquiry finishes. 
     If the compressor is not running, the second part of the routine is initiated. If the parameter DTS is not equal to 1, the dual temperature sensor is not enabled and the routine ends. 
     If the parameter DTS is equal to 1, the dual temperature sensor is confirmed to be enabled. The microprocessor then enquires whether the compressor is off. If the compressor is recorded as running, this part of the routine finishes and the temperature sensor simply records the temperature of the heat exchanger according to the first prong of the routine. 
     If the compressor is confirmed to be off, the microprocessor enquires whether Rest Time has expired. 
     Rest Time is the minimum amount of time for which the compressor must be off between cycles. This is to prevent the compressor cycling too often, which results in mechanical damage. Rest Time is set within the microprocessor and is dependent on the compressor and its expected load. Rest Time starts when the compressor is switched off and a typical time for a RBMU is between 2 and 5 minutes. 
     If Rest Time has not expired, the routine waits until this time period has expired. 
     Once Rest Time has expired, the microprocessor instructs the temperature sensor to take a temperature reading and to write that reading to the memory of the microprocessor as ambient air temperature. The ambient air temperature timer is also started. 
     The ambient air temperature timer is the additional time allowed from end of the pre-set Rest Time to enable the microprocessor to record ambient air temperature. If the refrigeration compartment in the unit reaches a temperature at which the compressor needs to be run, the refrigeration compartment will start the compressor and over-ride the ambient air temperature timer. The temperature sensor will then revert to the high temperature sensor routine described above where the temperature of the heat exchanger is recorded. 
     Until the compressor is restarted and/or the ambient air temperature timer expires, further ambient air temperature readings are taken. The ambient air temperature reading is only stored to memory while the compressor is off and the ambient air temperature timer is running if the temperature reading is less than the previous reading stored in the microprocessor memory. When the lower temperature reading is stored to memory, the previous value stored in the memory value is over-written with the new value. This is to prevent rogue spikes in temperature from being erroneously recorded, caused by residual heat in the heat exchanger. 
     As soon as the ambient air temperature timer finishes or is overridden by the refrigeration compartment, the microprocessor instructs the temperature sensor to take a temperature reading. This reading is stored to the microprocessor memory and over-writes the previously stored value, regardless of whether it is higher or lower than the stored value. Ambient air temperature is thus recorded as the last temperature stored. 
     Once the ambient air temperature timer has expired or been overridden by the refrigeration compartment, the routine returns to the beginning and enquires whether the compressor is on or off. Thus, the compressor and the temperature recordings act in parallel, with the compressor being thermostat controlled and the temperature sensor recording ambient air temperature as and when the opportunity allows. 
     In summary, when the compressor is running, the temperature sensor acts as a high temperature sensor and continuously records the temperature of the heat exchanger with no rules. The rules outlined above only apply when the compressor is not running and Rest Time has expired. The rules enable the microprocessor to determine the ambient air temperature around the heat exchanger from the temperature of the heat exchanger itself once the compressor and heat exchanger have not been in operation for at least the pre-set Rest Time period. 
     Various modifications and variations of the described aspects and embodiments of the present invention will be apparent to those skilled in the art without departing from the scope of the present invention. Although the present invention has been described in connection with specific preferred embodiments, it should be understood that the invention as claimed should not be unduly limited to such specific embodiments.