Patent Application
20250137693
The present invention relates to refrigeration (that is, cooling, which may or may not be to the point of freezing) of products such as food products, within an enclosure equipped to provide a chilled atmosphere with which the product to be refrigerated is contacted within the enclosure.
Refrigeration apparatus, by which is meant equipment including chillers and freezers, often employ what is known as mechanical refrigeration systems to establish a temperature of the atmosphere inside the equipment that is at or below a given desired level. Product to be cooled or frozen is placed within the equipment and is exposed to the atmosphere so that the product is cooled or frozen, depending on the temperatures of the product and of the atmosphere, and depending on the length of time that the product is exposed to the atmosphere. The present invention provides ways to increase the usefulness of refrigeration apparatus of this type. wherein product is cooled or frozen inside the equipment.
One aspect of the present invention is a method of modifying refrigeration apparatus, comprising
Another embodiment of the present invention is the aforementioned method which further comprises providing a capability for defrosting, by:
Another embodiment of the present invention which comprises providing a capability for defrosting is a method of modifying refrigeration apparatus, comprising
Another aspect of the present invention comprises a method of refrigerating a product, comprising
Another embodiment of the present invention is the aforementioned method which further comprises a capability for defrosting, by:
Another embodiment of the present invention which comprises providing a capability for defrosting, is a method of refrigerating a product, comprising
Yet another aspect of the present invention is apparatus for refrigerating a product, comprising
Another embodiment of the present invention is the aforementioned apparatus which further comprises a capability for defrosting, comprising
Another embodiment of the present invention which comprises a capability for defrosting is apparatus for refrigerating a product, comprising
Some preferred embodiments utilize (a) control mechanism that enables liquid nitrogen to flow into the supplemental heat exchanger and that controls said flow by monitoring the pressure of liquid nitrogen flowing from said source, or (b) control mechanism that enables liquid nitrogen to flow into the supplemental heat exchanger and controls said flow by monitoring the pressure of gaseous nitrogen leaving the supplemental heat exchanger, or (c) control mechanism that enables liquid nitrogen to flow into the supplemental heat exchanger and controls said flow by monitoring the pressure of liquid nitrogen flowing from said source and monitoring the pressure of gaseous nitrogen leaving the supplemental heat exchanger.
The present invention is useful with any refrigeration apparatus that includes an enclosure in which product is cooled or even frozen by being exposed in the enclosure to a gaseous atmosphere that is colder than the product. The refrigeration apparatus may be a freezer, chiller, cooler, blast cell, cold room, storage room, environmental test chamber, or other equivalent equipment, and may be stationary or transportable. It may be of the type in which product is treated batchwise, that is, batches of product are placed in the enclosure, the enclosure is closed, a desired temperature in the enclosure is maintained, and the temperature of the product is reduced to a desired level by exposure to the atmosphere, following which the enclosure is opened and the product is removed. Refrigeration apparatus with which the invention may be practiced also includes equipment in which product passes through the enclosure on a movable belt or equivalent carrier, wherein the product is placed on a belt at an entrance opening into the enclosure, the belt moves and carries the product through the enclosure wherein the product is cooled or frozen, and the product is removed from the belt at an exit opening of the enclosure. Examples of such apparatus include tunnel coolers and freezers, in which the belt passes on a predetermined path through the enclosure, and so-called spiral coolers and freezers in which the belt moves in a helical path around a central axis such that the belt curves and passes over itself in successive ranks.
The products that can be treated by the present invention include any product that can be cooled or frozen by exposure to the temperatures that are typically established within the refrigeration apparatus. Preferred products include food products, including raw products such as meat (including beef, pork, poultry and fish), and including processed food products comprising combinations of raw materials some or all of which may already have been cooked or otherwise treated.
Referring to
Refrigeration apparatus 1 also includes mechanical refrigeration unit 5 which comprises mechanical refrigeration circuit 20 that is illustrated in
Returning to
Referring to
Returning to
As shown in
As shown in
As shown in
The supplemental heat exchanger 30 and its associated lines (such as 32; 37; 39; and 39A and 39B if necessary) can be installed easily into an existing refrigeration apparatus 1 having enough volume within the enclosure 2. If existing openings such as 3 and 4 are not large enough, one of them may be enlarged. Alternatively, as discussed below, it may be advantageous for reasons of heat exchange efficiency and economy to provide more than one supplemental heat exchanger 30 as a plurality of modules, each small enough to fit through an opening such as 3 or 4 into the enclosure 2, wherein the overall heat exchange capacity that can be provided by all of the modules is the sum of what is available from each of the supplemental heat exchangers 30 provided in this manner. A less preferable alternative is to disassemble or cut into pieces a single larger supplemental heat exchanger, pass the pieces into enclosure 2, and reassemble it inside enclosure 2.
Thus, the overall sequence is to install one or more supplemental heat exchangers 30, to provide a line 32 to each supplemental heat exchanger 30 from a source 31 of liquid nitrogen, to provide a connection from each heat exchanger 30 to outlet 35 to line 39 and which is connected to vent pipe 37 for gaseous nitrogen, and to provide controls 31A for the flows of liquid nitrogen as described hereinbelow.
It should be noted that in one alternative embodiment of the invention, the supplemental heat exchanger 30 is not used to provide heat exchange with the refrigerant of the mechanical refrigeration circuit, and the mechanical refrigeration circuit is not used to provide heat exchange with the liquid nitrogen in the supplemental heat exchanger (nor with nitrogen vapor formed by vaporization of liquid nitrogen in the supplemental heat exchanger); whereas in other alternative embodiments of the present invention, the supplemental heat exchanger 30 is used to provide heat exchange with the refrigerant of the mechanical refrigeration circuit, and/or the mechanical refrigeration circuit is used to provide heat exchange with the liquid nitrogen in the supplemental heat exchanger or with nitrogen vapor formed by vaporization of liquid nitrogen in the supplemental heat exchanger.
Adding the supplemental heat exchanger 30 as described herein, to a refrigeration apparatus as described herein, provides numerous advantages.
The present invention is useful in refrigeration operational situations in which the mechanical refrigeration system is running to its full refrigeration capacity and capability, or is running at a level of refrigeration that is close to its full refrigeration capacity, e.g. at least 80% or at least 90% of its full refrigeration capacity. In these operational situations, the overall refrigeration capacity that is available is supplemented by the supplemental heat exchanger described herein, that becomes active only when the heat load or operating limitations such as heat rejection outside or frosting inside the enclosure 2 being cooled requires a boost in refrigeration capacity (i.e., additional refrigeration capacity or supplemental refrigeration capacity). The nominal full refrigeration capacity of a mechanical refrigeration unit (unaided by supplemental heat exchange) is readily determined from the unit's size and refrigerant.
Common issues experienced by processors using mechanical refrigeration systems include: trying to chill products at a production rate that exceeds the capacity of the mechanical system; not reaching the desired product temperature at the end of the cycle of a batch operation or at the exit of a continuous spiral or tunnel freezer; the mechanical refrigeration system capacity diminishes with processing time or seasonally, and thereby effectively reducing available refrigeration capacity provided to chill the product due to e.g. uncontrolled frosting inside the freezer or heat rejection outside the plant due to e.g. climate conditions; or the initial heat load inside the space exceeds the capacity of the mechanical refrigeration system leading to a longer than desired chilling period to mention a few.
Using a supplemental refrigeration system heat exchanger 30 with liquid nitrogen as the refrigerant allows a processor to chill a product heat load that is not capable of being fully cooled or frozen to the desired specifications by the mechanical refrigeration system in the desired time or method when needed. If a processor has no need for additional refrigeration capacity, the supplemental refrigeration system can be deactivated, consuming neither nitrogen nor other significant utilities.
The very low refrigerant temperature of liquid nitrogen provides much higher refrigeration capacity per unit volume of the heat exchanger 30 unit than is possible in refrigeration units employing refrigerants other than liquid nitrogen. The difference can be as much as 5-10× more capacity per unit volume of heat exchanger. Even with a design air temperature decrease of as much as 20-30 F (compared to 5-15 F for conventional refrigerants), the average temperature difference between the air and the liquid nitrogen-based heat exchanger can be as high as 100 F to 200 F. Therefore, the nitrogen cooled heat exchanger can be easily designed to achieve high temperature changes for the circulated air flow in enclosure 2. This allows the nitrogen cooled heat exchanger to provide substantial amounts of supplemental cooling.
The present invention includes providing the ability to use, and using, liquid nitrogen, which preferably flows through a heat exchanger such as a finned-tube heat exchanger cooled by liquid nitrogen and by vaporized liquid nitrogen inside the tubes to cool the atmosphere in the enclosure, thereby providing additional refrigeration to supplement that provided by the mechanical refrigeration system. When supplemental refrigeration is needed, liquid nitrogen flows into the tubes of the fin-tube heat exchanger, and atmosphere of the enclosure flows across the exterior surfaces of the supplemental heat exchanger. The liquid nitrogen vaporizes, absorbing heat from the gas flow across the exterior surface (e.g. fins and tubes) of the supplemental heat exchanger, and converting some or all of the liquid nitrogen into gaseous nitrogen. The warmed but still cool gaseous nitrogen can provide additional cooling in supplemental heat exchanger 30. Eventually, this gaseous nitrogen leaves the exit of the supplemental heat exchanger and is safely vented via line 39 and vent pipe 37 to the atmosphere outside of the facility. The cooled air flow is directed within the enclosure 2 to provide a refrigerated air flow as needed for the purpose of the enclosure. The nitrogen-cooled supplemental heat exchanger 30 operates in concert with, but separately from, the mechanical refrigeration unit 5, to provide supplemental cooling only when needed.
Direct feeding of liquid nitrogen refrigerant into the supplemental heat exchanger's coil tubing can result in two zones of heat transfer—a latent zone where both liquid and vapor flow of nitrogen are present, followed by a superheat zone where only nitrogen gas is present. The latent zone operates at temperatures near the saturation temperature of nitrogen at coil conditions (nominally −300 F), while the superheat zone starts at about saturation temperature and the gas heats up as it proceeds down the tube, exiting the tube (coil) outlet at the design gas exit temperature (in the range of −80F to −50 F, preferably 20 F to 50 F colder than the supply air temperature on to the coil. Colder temperatures for the exiting gas are possible but attainment may be at the expense of the cooling efficiency of a given mass quantity of the nitrogen.
Unlike conventional operations with refrigerants, the nitrogen cooled heat exchanger coil obtains substantial refrigeration capacity from the superheat zone. Conventional refrigerants typically employ superheats of only a few degrees F. above the saturated suction temperature, and only a small part of the refrigeration load is provided by the sensible warming of the refrigerant in the superheat region. In contrast, about 40% of the refrigeration provided by the supplemental heat exchanger of the present invention fed with liquid nitrogen is provided in the superheat region, with the other 60% of the duty provided from the latent zone.
Where supplemental heat exchanger designs with coils of several tube rows are employed, it is preferred to use a single nitrogen circuit, with the nitrogen flowing counter-current to the direction of air flow. The single circuit and multiple tube rows serve to spread out the temperature variation of the superheat region over the entire air flow, promoting even cooling, and the counter-current flow of air and refrigerant increases the efficiency of the heat exchange. However, the load capacity of the nitrogen cooled coil is not substantially reduced if other flow patterns are used-for example, multiple circuits, co-current flow, or a single tube row coil.
The present invention can be based on either of the following two approaches to implementing the addition of supplemental cooling to the refrigerated enclosure 2. The approach chosen for a specific instance will depend on the needs and constraints of the existing refrigeration system and facility.
The general practice by manufacturers of freezers using a mechanical refrigeration system is to place the coil system of the mechanical system within the freezer enclosure. The mechanical refrigeration system may contain one large coil by design, but often the mechanical refrigeration system can have multiple coils acting as one system to provide cooling capacity. A heat exchanger system providing supplemental cooling capacity using liquid nitrogen can also be located within the enclosure 2 as shown in
The preferred embodiment of this invention for a shared or common air flow is as follows:
For example, in the case of one system, a 1-row deep liquid nitrogen coil added to a mechanical refrigeration unit having a 10-row coil containing ammonia as the refrigerant, configured to provide full face-area coverage of the existing coil, could provide up to 50% more refrigeration capacity with less than 10% additional air-side pressure drop. In contrast, using methods of the prior art to increase capacity, adding a second 10-row coil cooled by ammonia, downstream of the existing 10-row coil, would add only 30% to the cooling capacity, and would double the air-side pressure drop. The supplemental nitrogen-cooled coil unexpectedly provides about 17 times the refrigeration capacity per coil row, as compared with extending the number of rows of the mechanically cooled coil.
Alternatively, the supplemental nitrogen-cooled coils may be placed within the freezer enclosure but at a distance from the mechanical refrigerant coils, and may have their own independent air circulation fan or multiple fans or blowers. Freezer designs often do not have space available to allow the installation of one supplemental coil and becomes even more challenging to accommodate more than one supplemental coil using nitrogen as the refrigerant. To address the space limitations inside a freezer with a mechanical refrigeration system, this invention describes attaching compact, modular nitrogen coil units to a wall or ceiling of a freezer and using a couple of holes to direct air to and from the supplemental coil system as shown in
The preferred embodiment of this invention for an independent air flow configuration is as follows:
The modular design of the MCU system allows the cooling with supplemental heat exchangers 30 to be distributed in an optimum fashion throughout the extent of the enclosure 2. This can be determined during planning of the installation, after analyzing the spatial variation of heat loads in the space, as well as the operation and air flow of the existing mechanical refrigeration system, to determine the desired placement and distribution of the MCUs in the existing system. One variant of placing supplemental refrigeration coils on freezer walls is to locate one or more coils in the vicinity of the freezer inlet port of a spiral freezer or tunnel freezer. One advantage of using nitrogen as the refrigerant in a fin-tube coil design is that the heat exchange surface of the supplemental heat exchanger, which contacts the atmosphere in enclosure 2, is colder than the heat exchange surface of the mechanical refrigeration circuit. This gives the supplemental heat exchanger the capability of forming a region in the enclosure 2 in which region the atmosphere is colder than the air stream that is cooled only by the mechanical refrigeration system. This capability to create a colder temperature zone inside the enclosure 2 can provide greater tempering of the incoming product needing to be cooled. Particularly, this cold zone can reduce moisture loss off from incoming product. By keeping moisture on the product, the frost that would have formed due to the water leaving the product does not reach the mechanical refrigeration coil and therefore slows the rate of frost buildup on the mechanical refrigeration system coil.
The supplemental heat exchangers 30 may be placed upstream, downstream, or in parallel to the mechanical refrigerant coils, with respect to the main circulation pattern of air flow within the freezer. The supplemental coils may share a common air flow path with the mechanical coils, or may have a separate air flow path, and may have their own dedicated means of air movement such as fans. When the supplemental coils share the same air flow as the mechanical refrigeration coils, a preferred embodiment is for the supplemental coils to be downstream of the mechanical coils, i.e., the warm air would first be cooled by the mechanical coils, and then further cooled by the supplemental nitrogen-cooled coils.
For the independent airflow embodiment of this invention, the supplemental heat exchangers 30 for supplemental liquid nitrogen-based refrigeration may be installed either downstream or in parallel to the air flow pattern of the existing mechanical refrigeration system. “Downstream” means positioned in the air flow after it is cooled by the mechanical coils, but before the air returns to the mechanical refrigeration apparatus. “In parallel” means that the air flow to the nitrogen cooled supplemental units is obtained upstream (before being cooled) of the mechanical coils. For example, in a spiral freezer, a parallel flow supply would entail warm air returning from the food load to be provided into the air supply of the MCU, and the cooled air leaving the air return of the MCU being directed back to the food load.
The objective of the independent air flow embodiment of the present invention is not that the MCUs will provide an increase in the overall air circulation over the food product or other load within the refrigerated space. Instead, the air flow of the MCUs is meant only to remove air from the space, provide additional cooling, and return that air to the space, while the existing air flow of the mechanical system continues to provide required air circulation to the enclosed loads. The MCUs need only be positioned where additional cooling capacity can be beneficial in the refrigerated space, where the air flow of the MCU will not cause substantial disruption of the base mechanical air flow, and where the cold exit air of the MCUs will not adversely impact the operation of the mechanical refrigerant coils. Since the MCUs can provide substantial (10-30%) increases in cooling capacity with only small independent air flows (5-15%), any potential adverse impacts of the MCU airflow can be easily avoided.
If the objective is to lower the air temperature supplied to the heat load to below that normally provided by the mechanical refrigeration system, then positioning the nitrogen cooling system downstream of the mechanical coils is preferred. On the other hand, if the objective is to provide additional cooling capacity at the same air temperature normally achievable by the mechanical refrigeration system, then parallel air flow for the MCUs can provide higher supplemental capacity. Either parallel or downstream air flow patterns will work well to modify the bulk air flow stream in the enclosure 2.
As mentioned above, the very large temperature difference between air and refrigerant in the nitrogen coils allows efficient operation of the system with substantial air temperature drops (20 F to 30 F), and this allows the system to provide substantial supplemental refrigeration even when the affected air flow is a small fraction of the overall circulation rate. For example, if the existing mechanical system operates with a 10 F air temperature drop across the mechanical coils, and a 30 F drop across the nitrogen coils, the nitrogen cooled MCUs can provide a boost in refrigeration capacity of 15% to the system while processing only 5% of the system air flow.
Another aspect of this invention is that each coil of the supplemental refrigeration system acts independent of the other coils when more than one coil is active. For a supplemental refrigeration coil, especially one using nitrogen as the refrigerant, allows for different refrigeration capacity levels in the different active coils. For example, if two supplemental coils were active and each having the same overall refrigeration capacity, one coil may provide 100% of the rated capacity while the second coil might be contributing 70% of the rated capacity. In a different scenario, these two coils can be operated in unison with both providing 85% of the rated capacity. Independent flow control of nitrogen through a coil provides the capability to deliver a precise amount of refrigeration cooling capacity to satisfy a demand.
The supplemental refrigeration heat exchanger 30 using liquid nitrogen as the refrigerant can be turned on or made active, and the flow of liquid nitrogen once activated, can be controlled using any of several different methods. In any method, it is preferred that the mechanical refrigeration system is operating at full capacity before activation of the flow of liquid nitrogen into the supplemental refrigeration heat exchanger using nitrogen. However, the control practices can also be carried out while the mechanical refrigeration system is operating but before the mechanical refrigeration system has reached full capacity, such as when it has reached a preset level such as at least 80%, or at least 90% or at least 95%, (but, as will be apparent, preferably greater than 0%) of the full refrigeration capacity of the mechanical refrigeration system without any supplemental heat exchange.
One method is to monitor the operation of the mechanical refrigeration system, and to activate the flow of liquid nitrogen into the supplemental refrigeration heat exchanger, when the mechanical system is at maximum load or is at a level that is a preset percentage of maximum load. Various signals or sensors may be used to indicate the operating capacity of the mechanical system, including suction pressure, speed, or slide valve level of the mechanical refrigerant compressor, from which the maximum heat exchange capacity can be determined depending on the specific design of the mechanical refrigeration system. When the sensors indicate that the mechanical refrigeration system is operating at full capacity, the flow of liquid nitrogen into the supplemental refrigeration heat exchanger (coils) is activated to provide additional refrigeration capacity to cool the atmosphere in enclosure 2. The supplemental heat exchange system would then continue to operate as long as the mechanical refrigeration system remains at full load, or as long as it remains at a load that is at or above the preset percentage of full load, depending on how the operator chooses to operate the overall system. Once the utilization of the mechanical refrigeration system's capacity diminishes to less than maximum utilization, or to less than the aforementioned preset percentage of maximum capacity, then the supplemental refrigeration system can be reduced in capacity (such as by reducing the flow rate of liquid nitrogen through the supplemental heat exchanger), or can be turned off fully. The supplemental refrigeration system would provide capacity linked to the monitored capacity parameter of the mechanical system. This method of activating the supplementary heat exchanger requires integration of sensor signals from the mechanical refrigeration system, and may be complicated when the mechanical system serves multiple freezer loads.
Another method, which is preferred, of controlling actuation and operation of the supplemental refrigeration heat exchanger is to rely on one or more characteristics of the mechanical refrigeration system which are correlated with the refrigeration performance of the mechanical refrigeration system, to indirectly determine when conditions have been established which indicate that the supplemental heat exchanger is to be activated by starting the flow of liquid nitrogen into the heat exchanger, as well as the conditions which indicate when the flow of liquid nitrogen is to be lessened or shut off.
One embodiment of such a method is to use a combination of monitoring the temperature of the atmosphere within enclosure 2 and monitoring the pressure difference of the liquid nitrogen flow through the tube of the supplemental heat exchanger. For a given mechanical refrigeration system, the suction pressure and resulting air temperature inside the enclosure 2 will be known. As the temperature inside the enclosure 2 warms and the utilization of the refrigeration capacity of the mechanical refrigeration system approaches or reaches full capacity, the mechanical refrigeration system will become less able and eventually unable to chill the product to the extent desired by the operator. As a result, the temperature inside the enclosure 2 rises. When that temperature has risen sufficiently to and reaches a set point that has been set by the operator, which corresponds to a given percentage of the maximum refrigeration capacity of the mechanical system having been reached, the flow of liquid nitrogen into the supplemental heat exchanger system is turned on by opening of appropriate valves between source 31 and inlet 34, and supplemental heat exchange capacity is provided to the system.
The amount of supplemental refrigeration capacity provided will continue to be provided as long as the temperature inside the enclosure 2 remains warmer than the temperature that the atmosphere would reach when the mechanical system again has sufficient capacity to maintain the desired temperature without the aid of the supplemental refrigeration heat exchanger. When the temperature of the atmosphere in the enclosure 2 begins to cool and returns closer to the target temperature of the atmosphere in enclosure 2, the amount of refrigeration capacity provided by the supplemental refrigeration system is reduced, such as by reducing the flow rate of liquid nitrogen into the supplemental refrigeration heat exchanger. When the temperature in enclosure 2 returns to the desired set point temperature, the supplemental refrigeration system is turned back or shut off into an idle mode until the refrigeration demands of the mechanical refrigeration system again overwhelms the mechanical refrigeration cooling capacity.
When the supplemental refrigeration heat exchanger system is active, control of the liquid nitrogen flow into it is controlled by maintaining the pressure difference predicted by the current temperature difference between the desired temperature of the atmosphere in enclosure 2, and the actual temperature of the atmosphere. As this temperature difference becomes larger than desired, the predicted pressure difference for the supplemental heat exchanger increases and the flow of liquid nitrogen into the heat exchanger is increased. Similarly, if this temperature difference becomes smaller, the pressure difference target is reduced, and this action reduces the nitrogen flow rate into the supplemental heat exchanger. For some installations, the temperature difference between the desired atmosphere temperature in the enclosure 2 and the actual temperature may include an offset to prevent overlapping control zones for nitrogen flow. When the temperature difference becomes negligible (meaning the desired temperature of the atmosphere in enclosure 2 and the actual temperature there including any offset are essentially equal), the supplemental refrigeration heat exchanger system is put into a standby condition. In this standby condition, no nitrogen flows into the heat exchanger and the coil sits idle not providing any additional refrigeration capacity. Other characteristics that may be detected and used to control operation of the system include measuring the temperature or thermal state of the product as it exits the enclosure 2; a temperature or thermal state that is above a preset set point temperature or thermal state of the product would cause the system to be activated to provide additional supplemental refrigeration that would lower the temperature or thermal state of the product as it exits the apparatus.
Nitrogen flow in aid of operations can be measured in other techniques as well, such as by use of a flow meter or equivalent apparatus and methodology.
The flow of nitrogen refrigerant into the supplemental heat exchanger can be controlled in any of a variety of ways, including expanding or reducing the flow of liquid nitrogen into the heat exchanger, or expanding or reducing the flow of nitrogen vapor leaving the heat exchanger, using suitable valves. Alternatively, the flow of nitrogen may be controlled in an on-off fashion using suitable solenoid-controlled valves either upstream or downstream of the supplemental heat exchanger. In the preferred embodiment, the flow is controlled using a control valve on the nitrogen gas flow leaving the supplemental heat exchanger (to line 39). It is further preferred to control the flow in a continuous fashion, over a range of flow rates, in concert with the control strategy described herein, to vary the supplemental refrigeration load as needed.
Refrigeration apparatus that relies on heat exchange between a cold surface of a heat exchanger and the surrounding atmosphere, such as mechanical refrigeration systems employing either ammonia or freon as the refrigerant, and other systems such as the supplemental heat exchanger described here that employs liquid nitrogen, generate frost on the heat exchange surfaces which is formed by condensation of water vapor from the atmosphere and freezing of that condensed water on the heat exchange surfaces. The present invention includes an advantageous approach to removing frost from heat exchange surfaces on which the frost has formed.
In this aspect of the present invention, referring to
The impeller and heater are designed to allow adequate turbulent flow of hot gas within the circuit that includes recycle line 41 to deliver sufficient heat to fully defrost the supplemental heat exchanger in the time desired. For example, nitrogen from the impeller 46 may be heated to 200 F and supplied to the inside of the recycle line 41 at a high velocity in turbulent flow, to heat the supplemental heat exchanger from within, melting the ice within approximately 30 minutes. Hot gas may be first routed via line 51 through a defrost pan 52 situated beneath the supplemental heat exchanger, to heat the pan 52 and prevent liquid water that is formed by melting of the frost and that drains off of the supplemental heat exchange from re-freezing in the pan 52, so that it can be drained as desired during or after the defrost process.
In the operation described previously of using the supplemental heat exchanger to supplement the refrigeration capacity of a mechanical refrigeration system, valves 45 and 44 are open, and valves 43 and 42 are closed.
Defrosting of the supplemental heat exchanger 30 (that is, melting of ice from the surfaces of the heat exchanger) involves establishing a closed loop of warmed, circulating nitrogen vapor. This is established by closing valves 45 and 44 and opening valves 42 and 43 and passing nitrogen vapor under the influence of impeller 46 through heater 47, into line 32 and through supplemental heat exchanger 30, into line 39 and then into line 41. More preferably, air continues to circulate over or across the supplemental heat exchanger, providing heat to remove remaining nitrogen liquid in the heat exchanger; during this process step it continues to cool the air. Of course, one can isolate the supplemental coil immediately to allow defrost to commence in an isolated fashion, but this approach is less preferred. The next step is to reduce the pressure level in the recycle line 41 to a level below the normal operating pressure of the nitrogen circuit including the supplemental heat exchanger, to a value below the maximum operating pressure of the impeller being employed. If not already activated, heater 47 is activated, preferably to a temperature on the order of 120 F to 210 F, preferably 160 F to 200 F, as detected by temperature sensor T3 in
The nitrogen vapor that is defrosting the coil is passed through this recycle loop more than once, preferably many more times than once. The hot gas circulation continues until the ice melts from the surface(s) of the supplemental heat exchanger is defrosted (preferably, until all ice present is fully melted). The extent of melting can be determined by monitoring the temperature of the gas downstream of the supplemental heat exchanger (such as temperature sensors T1 in
The time is typically about 30 minutes or so for a supplemental heat exchanger 30 having refrigeration capacity of 5 tons of refrigeration.
To improve the defrost time, a preferred method is to isolate the supplemental heat exchanger, using baffles or other means known in the art, after the liquid nitrogen has boiled out of the heat exchanger. By isolating the heat exchanger, a supplemental heat exchanger can be defrosted asynchronously to the time when the mechanical refrigeration system is defrosted and reduces the parasitic heat load back into the mechanical freezer interior volume. Since the defrost step for a supplemental heat exchanger coil by design can be any time when needed, a supplemental heat exchanger can be defrosted at the same time that the mechanical refrigeration system defrost is occurring if that makes the defrosting of the supplemental heat exchanger more convenient for an operator.
Referring to
When more than one supplemental heat exchanger is active at the same time, and to allow for an additional supplemental heat exchanger to asynchronously go into defrost mode relative to the mechanical refrigeration system defrost mode, a pattern of activity can be established so each of the supplemental heat exchangers is active for a period of time, then proceeds through a defrost sequence, and then becomes active once again or is put into a standby mode depending on freezer conditions. For example, a supplemental refrigeration system can be designed for each supplemental heat exchanger to be in operation for 90 minutes (three contiguous thirty-minute periods), followed by one 30-minute defrost period. Other patterns and segment lengths may be utilized, depending on the capacity of the system, the system design, and the needs of the process.
The present invention is more efficient than systems employed in the prior art as no additional nitrogen is needed from storage to implement the defrost operation.
Other techniques for providing the desired defrosting, to melt ice from a surface of a heat exchanger, may be utilized or incorporated as well with apparatus and/or methodology comprising any of the embodiments or features described herein. Examples of suitable techniques include directing a stream of hot gas onto the ice; incorporating electrical heating elements such as heating wires or heating rods, interspersed among tube rows; spraying liquid, such as warm water, onto the ice surface; blowing ambient air across the ice surface; periodically discontinuing flow of refrigerant through the heat exchanger, enabling the ambient conditions to cause the ice to melt; and incorporating tubes interspersed among the refrigeration coils, and flowing heated liquid such as heated brine or glycol through the incorporated tubes.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/US2023/060867 | 1/19/2023 | WO |
| Number | Date | Country | |
|---|---|---|---|
| 63301543 | Jan 2022 | US |
