Patent Application
20080016886
The detailed description particularly refers to the accompanying figures in which:
A freezer 10 comprises a cabinet 12 which defines a storage space 14 configured to receive racks 16 and drawers 18 as shown in
Freezer 10 further comprises a gasket 24 secured to door 20 and which engages a surface 26 about the perimeter of an opening 28 in cabinet 12. Opening 28 provides access to storage space 14 when door 20 is in an opened position. Gasket 24 comprises a flexible material and engages surface 26 when door 20 is closed. During engagement with surface 26, gasket 24 compresses under load and fills any irregularities between door 20 and surface 26 so as to generally seal storage space 14 relative to the ambient atmosphere. In the illustrative embodiment, gasket 24 comprises a polyvinyl chloride material. In other embodiments, any of a number of sealing materials may be utilized to seal storage space 14 when door 20 is in a closed position. Also, in some embodiments, gasket 24 may be positioned on cabinet 12 and may seal against a surface of door 20 when door 20 is in a closed position. Door 20 is held in a closed position by a latch 48 which has two apertures which are configured to receive two posts 52 on a handle assembly 54 coupled to door 20.
The operating temperature of about (−30)° C. of freezer 10 results in frosting within storage space 14. Freezer 10 may be used in a clinical or laboratory environment having an ambient temperature of about 21-23° C. and about 50% relative humidity. Thus, when door 20 is opened, relatively warm and moist air is introduced into storage space 14. Moisture in the ambient air condenses in the relative cold storage space 14 and freezes creating frost which attaches to structures within storage space 14. The nature of the operation of freezer 10 further exacerbates the frosting problem. For example, when door 20 is closed, storage space 14 becomes a generally closed volume. The well known equation PV=RT models the volume. Generally, pressure (P), volume (V) and temperature (T) are interdependent variables. When door 20 is closed, volume (V) becomes constant. Thus, pressure (P) is dependent upon temperature (T). Chiller 22 operates intermittently to maintain the temperature within storage space 14 at a target temperature. Every time chiller 22 operates, temperature (T) is lowered slightly. As discussed above, pressure (P) is directly related to temperature (T) in a closed volume and, therefore, a slight reduction in temperature (T) results in a proportional reduction in pressure (P).
When door 20 is first closed, the pressure within storage space 14 approximates the ambient air pressure outside of freezer 10. However, lowering of the temperature within storage space 14 results in a lower pressure within storage space 14 as well. It has been found that in operation the pressure within storage space 14 may be lowered by as much as about 0.25 inches of water to even about 0.50 inches of water below the ambient pressure outside of freezer 10. If gasket 24 does not completely seal storage space 14 from the ambient atmosphere outside of cabinet 12, then as the system attempts to reach equilibrium, ambient air flows through the unsealed portion of gasket 24. This ambient air includes moisture which condenses and freezes near the unsealed portion of gasket 24. Every time chiller 22 operates, the temperature (T) within storage space 14 is lowered slightly and thereby pressure (P) is lowered creating a pressure gradient between storage space 14 and ambient air. Thus, frost continues to form around areas in which gasket 24 does not completely seal storage space 14 from ambient.
Frost within storage space 14 is removed automatically by regular defrost cycles within freezer 10. More specifically, freezer 10 operates under the control of a controller 110, shown diagrammatically in
As shown in
Additionally, monitor unit 200 is coupled to a battery 114, shown in
The interaction of controller 10 and the various input and output devices is shown diagrammatically in
Controller 110 is also in communication with chiller 22 and operable to turn chiller 22 on and off. A defrost system 118 includes resistive coils which provide heat when powered. Controller 110 is configured to activate defrost system 118 as necessary. Defrost system 118 powers the resistive coils so that frost which has accumulated may be melted. By providing localized heating, the frost is melted relatively quickly without introducing excessive heat to the storage space 14.
Coil heater 138, embodied as a resistive heating element, is part of defrost system 118 and is located adjacent to coils 32 of chiller 22 and operable to defrost coils 32. A door heater 134 is coupled to door 20 and operable to defrost the inside of door 20. Yet another heater, a drain line heater 136, is coupled to a drain line 40 (best seen in
Controller 110 also communicates with a temperature sensor 44 to receive a signal from the temperature sensor 44, the signal being indicative of the temperature within the storage space 14. Controller 110 processes the temperature signal to control chiller 22 and monitor for the need to defrost. Additionally, controller 110 receives a signal from a door switch 120 which provides an indication that door 20 is closed. Door switch 120 is located on cabinet 12 and is activated when door 20 is closed. Controller 110 has an interlock which, in normal operation, requires door 20 to be closed before activating defrost system 118.
Referring again now to
Plenum 62 includes an outlet 66 which provides a path for fluid to flow out of plenum 62. A conduit 68 is coupled to plenum 62 in fluid communication with outlet 66. Conduit 68 is coupled to drain line 40 through a connector 70 and is thereby in fluid communication with drain line 40. Drain line 40 communicates through back surface 46 of cabinet 12 to drip tray 42 such that fluid accumulated in drip tray 42 is urged by gravity to flow into drain line 40. Drain line 40 is j-shaped and includes a lower portion 72 positioned above evaporator tray 60. An end 76 of drain line 40 is configured to allow fluid to escape through end 76 into evaporator tray 60.
The structure of drain line 40 is such that pressurized air from positive pressure source 58 is limited to flow through drain tube 40 through fluid in drip tray 42 and into storage space 14. Fluid accumulates in the lower portion 72 of drain line 40 so as to create a barrier to the escape of pressurized air through end 76 of drain line 40. The pressure developed by pressure source 58 is not sufficient to expel the fluid from drain line 40 and therefore, the fluid acts as a seal and pressurized air is directed into storage space 14.
The flow of pressurized air into storage space 14 tends to pressurize storage space 14 at a pressure higher than atmosphere. Pressurization of storage space 14 results in the reduction and near elimination of flow of ambient air into storage space 14. Storage space 14 is at a higher pressure than atmosphere and therefore air from storage space 14 tends to be urged to escape any areas in which gasket 24 does not completely seal storage space 14 from ambient This reduces the potential for ingress of ambient air and the subsequent development of frost near gasket 24.
Positive pressure source 58 draws in ambient air from around freezer 10 as shown by the arrows in
Positive pressure assembly 56 operates such that after a maximum pressure head is developed within storage space 14, losses within positive pressure assembly 56 prevent additional pressure from being developed within storage space 14 and there is little or no flow from positive pressure assembly 56. This results in limiting the amount of ambient air introduced into storage space 14 and limits the amount of frost developed on coils 32. In the illustrative embodiments, positive pressure assembly 56 is sized such that the pressure developed within storage space 14 is about 0.25 inches of water above atmosphere. It has been found that 0.25 inches of water provides an appropriate pressurization to limit frosting near gasket 24 as well as on coils 32.
While the illustrative embodiment utilizes drain line 40 as a conduit for the flow of air from positive pressure assembly 56, it should be understood that a positive pressure source 58 may communicate through any of a number of conduits or apertures. For example, in some embodiments a conduit dedicated to the flow of pressurized air into storage space 14 may be utilized. In other embodiments, the conduit may be omitted and positive pressure source 58 may be in fluid communication with an aperture in cabinet 12 such that air flows directly from positive pressure source 58 into storage space 14.
In some embodiments, the flow of ambient air into storage space 14 may be directed to structures other than coils 32 to control the development of frost. For example, in some embodiments ambient air may be directed over a removable structure in storage space 14. The removable structure may then be removed so that frost can be removed from the structure external to freezer 10.
In the illustrative embodiment, positive pressure source 58 operates continuously. In other embodiments, controller 110 may selectively operate positive pressure source 58 based on other conditions of operation of freezer 10. For example, positive pressure source 58 may be turned off whenever door 20 is opened.
Although certain illustrative embodiments have been described in detail above, variations and modifications exist within the scope and spirit of this disclosure as described and as defined in the following claims.
