Abstract:
A method comprises the steps of (a) providing a refrigeration unit having a refrigerated or heated space and at least one measured operating parameter, (b) providing heat to the refrigerated space when the at least one measured operating parameter exceeds a first threshold, (c) terminating provision of heat when the at least one measured operating parameter exceeds a second threshold, and repeating steps b-c when the at least one operating parameter falls below the first threshold. In using this method, the unit can be operated continuously, with substantially higher refrigerant mass flow and evaporator pressure then in the prior art. Higher refrigerant mass flow rate and higher evaporator pressure improve oil return to the compressor, thus reducing the like hood of compressor damage due to oil pump out.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     (1) Field of the Invention  
         [0002]     The present invention relates to a method for improving oil return in a refrigeration unit.  
         [0003]     (2) Description of the Related Art  
         [0004]     With reference to  FIG. 1 , there is illustrated a refrigeration unit  10 . Typically, a refrigerant/oil mixture travels in a circuit from the compressor  11  through the condenser  13  to an expansion device  19  then to an evaporator  15  and then to suction line  21  and finally back to the compressor  11 . In instances wherein an operating refrigeration unit  10  carries perishable goods at low ambient temperatures, the amount of cooling required to maintain the cargo at a given temperature is quite minimal. In such instances, the amount of refrigerant traveling through the coil of the evaporator  15  and suction line  21  is substantially reduced. Such reduction in the amount of refrigerant results in the retention and logging of oil in the evaporator. This logging of oil in turn compromises oil return to the compressor and thus reduces the heat transfer effectiveness of the evaporator coil or in some instances can even lead to complete oil pump out from the compressor resulting in the compressor failure.  
         [0005]     One possible solution to reduce or eliminate the logging of oil in the evaporator  15  or suction line  21  would be to cycle the entire refrigeration unit  10 . Under such a scenario, the refrigeration unit  10  is periodically turned on and off. If the unit is allowed to cycle in on and off fashion, then when the unit is in the “on” mode then the amount of refrigerant traveling through the system is higher than if the unit had to operate continuously to maintain the same temperature of the refrigerated space. Unfortunately, cycling the entire refrigeration unit  10  can result in a scenario wherein it is difficult to control the temperature of the space to be refrigerated within relatively narrow bounds. In addition, if the compressor does not run continuously there are additional cycling losses due to frequent start/stops resulting in increased power consumption. Also compressor reliability may be comprised due to frequent start/stops. Continual running of the compressor  11  on another hand allows for better maintenance of the temperature of the space to be refrigerated within relatively narrow bounds, and may also improve compressor reliability and operating unit efficiency.  
         [0006]     What is therefore needed is a method for operating a refrigeration unit  10  on a continuous basis with sufficient amount of refrigerant traveling through the system to prevent logging of oil in the evaporator  15 . Preferably such a method would involve the continual operation of the compressor  11 .  
       SUMMARY OF THE INVENTION  
       [0007]     Accordingly, it is an object of the present invention to provide a method for improving oil return in a refrigeration unit while maintaining tight temperature control in the refrigerated space.  
         [0008]     In accordance with the present invention, a method comprises the steps of (a) providing a refrigeration unit having a refrigerated or heated space and at least one measured operating parameter, (b) providing heat to the refrigerated space when the at least one measured operating parameter exceeds a first threshold, (c) terminating provision of heat when the at least one measured operating parameter exceeds a second threshold, and repeating steps b-c when the at least one operating parameter falls below the first threshold.  
         [0009]     In further accordance with the present invention, a refrigeration unit comprises a compressor for circulating a refrigerant/oil mixture through a refrigeration unit, a sensor for measuring an operating parameter, a heating element for providing heat to the refrigeration unit, and a controller for receiving the measured operating parameter and turning on and off the heating element in response thereto.  
         [0010]     In further accordance with the present invention, a method comprises the steps of providing a refrigeration unit having a refrigerated space, and providing and terminating heat to the refrigerated space in a pulsed on and off mode.  
         [0011]     In further accordance with the present invention, a method further comprises providing heat when the operating parameter has exceeded a time limit associated with a refrigerant system components that can be power cycled and can affect the operating parameter.nvention, 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]      FIG. 1 A  diagram of the refrigeration unit of the present invention.  
         [0013]      FIG. 2 A  logic diagram of the method of heater cycling of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0014]     It is therefore an aspect of the present invention to provide a method for cycling the operation of heaters in proximity to a refrigeration unit  10  so as to increase refrigerant/oil mass flow through the evaporator. By turning on and off a heater  17 , the refrigeration unit  10  is forced to run at a level which requires greater refrigerant/oil mass flow than would be required in the absence of the introduction of such heat. The heater is turned on and off based upon at least one continually monitored operating parameter of the refrigeration unit  10 . The heating element  17  is turned on and off to maintain the value of the operating parameter between two predefined threshold levels. By cycling a heating element  17  on and off in this manner, refrigerant/oil mass flow through the evaporator  15  is maintained at a level sufficient to substantially eliminate the logging of oil in the evaporator, thereby increasing the efficiency of the evaporator  15 .  
         [0015]     With reference to  FIG. 1 , there is illustrated the refrigeration unit  10  of the present invention incorporating the heating element  17 . Heating element  17  may be of any construction sufficient to introduce heat into the volume of space to be cooled by the refrigeration unit  10 . In some refrigeration units  10 , there are typically installed heating elements near enough to the evaporator coils to prevent the formation of ice upon the coils. In the instance that there is provided such a heating element  17  in proximity to the evaporator  15 , it is preferable to cycle such a heating element  17  using the method of the present invention. However, the present invention is not limited to either the placement or the construction of any particular heating element  17 . Rather, the present invention is drawn broadly to incorporate any and all heating elements  17  capable of producing heat and located within a space to be cooled or monitored by refrigeration unit  10 .  
         [0016]     As noted above, there are advantages with regards to the operating efficiency and reliability which arise from the continual running of the compressor. Cycling the heating element  17  with the compressor running therefore produces the beneficial effects of improving oil return to the compressor  11  while improving the heat transfer effectiveness of the evaporator  15  and avoiding cycling losses associated with frequent start/stops of the unit As additional heat is added, it becomes necessary to increase the refrigerant mass flow to provide additional cooling to compensate for the heat addition. The mass flow of refrigerant is normally increased by increasing the suction pressure by opening the suction modulation valve that regulates the amount of suction throttling. Higher pressure in turn improves the oil return to the compressor  1 . This occurs, as higher pressure tends to drive more refrigerant into the refrigerant/oil mixture. As a result, the viscosity of the refrigerant/oil mixture is reduced as the concentration of the refrigerant in the mixture is increased. As a result both higher refrigerant mass flow and reduced oil viscosity, make it easier to drag the oil along the tubes by the refrigerant, thus the oil return to the compressor  11  is improved. The continual running of the compressor with the increased mass flow of the refrigerant additionally decreases the super heat of the refrigerant as it leaves the evaporator, as less suction throttling is required. The lower super heat also plays an important role in reducing the viscosity of the oil in the superheated region of the evaporator and adjacent suction piping, the reduced oil viscosity in this section of the system also contributes to better oil return to the compressor. The reduction of oil viscosity in the superheated section occurs as a result of less refrigerant being boiled off from the oil mixture in this superheated region. Once again, the reduced oil viscosity improves the oil return to the compressor  11 . Stated differently, the heater can be pulsed in on/off fashion to allow the unit to operate continuously without danger of logging oil in the evaporator or suction line located between the evaporator and compressor.  
         [0017]     With reference to  FIG. 2 , there is illustrated a logic diagram detailing the method of cycling the heaters of the present invention. At step  1 , the compressor  11  is turned on. In step  2 , the heater is off. Step  1  and step  2  therefore represent the initialization component of the refrigeration unit  10  wherein the compressor  11  is turned on while the heating element  17  is maintained at an off position. At step  3 , a decision is made whether to cycle the heating element  17 . At step  3 , a reading is made of an operating parameter of the refrigeration unit  10  and a comparison of the read operating parameter is made to a predefined lower threshold value. The reading of the operating parameter is performed by a parameter sensor  31  configured to read a desired operating parameter and communicate the reading to a controller  33 . Controller  33  is any digital computational device or analog circuitry configured to receive readings from at least one parameter sensor  31  and, acting upon such receipt, to cycle heating element  17 . In the present example, the operating parameter chosen is the discharge pressure from the evaporator  15  and lower threshold value is 60 psi. The selection of the discharge pressure for the operating parameter and 60 psi for the lower threshold value is illustrated for exemplary purposes only. In a preferred embodiment, the lower threshold value is chosen to be at least approximately 60 psi and the upper threshold value is chosen to be less than approximately 100 psi. While such an operating parameter and lower threshold value represent one preferred embodiment, as noted above, the operating parameter may be chosen from numerous operating parameters including, but not limited to, discharge pressure, suction pressure, discharge temperature, suction temperature, pressure ratio, evaporator saturated temperature, condenser saturated temperature, ambient temperature. The operating parameter can also simply include a predetermined time interval when the heater is operating at ON position and predetermined time interval when the heater is operating in OFF position. The predetermined time interval for ON and OFF heater operation can be changed or adjusted based on other measured unit operating conditions. The operating parameter must also be in steady state condition to avoid rapid cycling of the heaters. Also steady state must be established to avoid inappropriate initiation of heat. Similarly, depending on the operating parameter chosen, the lower threshold value and upper threshold value will vary. Returning to the present example, at step  3 , the discharge pressure is measured and compared to the lower threshold value of 60 psi. In the event that the discharge pressure is below 60 psi, the heating element  17  is cycled on at step  4  and step  3  is repeated. Step  3  and step  4  continue to be repeated until the discharge pressure reaches 60 psi. Once the discharge pressure reaches  60  psi in step  3 , step  5  is performed. At step  5 , a reading is taken of the discharge pressure operating parameter to see if it is above an exemplary upper threshold of 90 psi. Like the lower threshold value of 60 psi, 100 psi is chosen for exemplary purposes. As long as the discharge pressure is not above 100 psi, step  3  is performed. Step  3  and step  5  are then repeated in a loop until the discharge pressure reaches 100 psi. Once the discharge pressure measured in step  5  exceeds 100 psi, step  6  is performed. In step  6 , the heating element  17  is cycled off and step  3  is performed. Once the heating element  17  is cycled off, the discharge pressure will begin to decrease. In the present example, it will continue to decrease until it dips below 60 psi at step  3 . At this point, step  4  is performed and the heating element  17  is cycled on once again. In this manner, the discharge pressure is maintained between  60  and 100 psi through a process of cycling the heating element  17  on and off as required.  
         [0018]     It is apparent that there has been provided in accordance with the present invention a a method for improving oil return in a refrigeration unit which fully satisfies the objects, means, and advantages set forth previously herein. While the present invention has been described in the context of specific embodiments thereof, other alternatives, modifications, and variations will become apparent to those skilled in the art having read the foregoing description. Accordingly, it is intended to embrace those alternatives, modifications, and variations as fall within the broad scope of the appended claims.