Air-cycle air conditioning system for commercial refrigeration

A refrigeration and air-cycle air conditioning system is provided for a body defining interior space, preferably a stand alone refrigeration cabinet. The air conditioning system includes a unique combination of a compressor, intercooler, expander, motor and plenum to increase the performance of the system making it practical for use in commercial refrigeration.

FIELD OF THE INVENTION

The present invention relates generally to refrigeration and air conditioning systems, and more particularly relates to air-cycle or Brayton cycle air conditioning systems.

BACKGROUND OF THE INVENTION

Present day commercial refrigeration systems typically employ a two-phase cycle with a refrigerant that alternates between gaseous and liquid states. The systems utilize refrigerants such as Freon or R-134a to provide large cooling capacities. The refrigerant cycle for these systems include the use of a compressor, an evaporator, a condenser and expansion device and numerous refrigerant hoses. Such systems require a closed piping circuit to isolate the pollutional refrigerant from the environment. Further, these refrigeration systems at their end transfer heat through air or another gas, and hence require both a primary cooling medium, i.e. the refrigerant, and a secondary cooling medium, i.e. the air that transfers the heat from the object to be cooled. Not only are these systems expensive and require stringent containment of the refrigerant, the refrigerant level must also be monitored and refilled during the life of the system.

Open air-cycle air conditioning systems, also known as a Brayton cycle system or open reverse Brayton cycle, are known. With such a system the air (or another gas) itself is the working medium in the refrigeration cycle, and there is no need for a special refrigerant that is cycled through the system. Unfortunately, the air-cycle air conditioning system typically has a very small cooling capacity in comparison with a conventional refrigerant based system. These open air-cycle air conditioning systems are employed mainly in airplanes, where there is a ready availability of compressed air which can be tapped directly off the main engine super charger or jet engine compressor. Unfortunately, in stand alone refrigeration systems there is no such supply of compressed air, and hence the small cooling capacity in combination with a low overall efficiency or coefficient of performance (COP) of open air-cycle for refrigeration systems have made them commercially impracticable. Accordingly, there exists a need to provide a commercial refrigeration system for conditioning air utilizing an open air-cycle air conditioning system.

BRIEF SUMMARY OF THE INVENTION

The present invention provides refrigeration using an air-cycle air conditioning system for a body defining interior space, preferably a stand alone refrigeration cabinet. The air conditioning system includes several features which increase the overall COP of the system making it practical for use in commercial refrigeration.

In one embodiment, a unique plenum is employed. The system includes a compressor receiving air from the interior space, an intercooler receiving pressurized air from the compressor, and an expander receiving cooled pressurized air from the intercooler. The expander removes work from the air by reducing its pressure, and provides the cold air to a mixing chamber defined by the plenum. The plenum also includes an inlet port and an outlet port in communication with the mixing chamber and the interior space. The inlet port receives air from the interior space for mixing with the cold air from the expander to provide mixed air to the interior space through the outlet port.

According to more detailed aspects, a fan is preferably provided in communication with the mixing chamber for flowing air through the plenum, and most preferably is positioned proximate the inlet port of the plenum. The plenum may also define an intake chamber which is separate from the mixing chamber. The intake chamber is in communication with an inlet of the compressor, and preferably includes a baffle to reduce the noise generated by the compressor's intake of air. The plenum preferably is an enclosure defined by opposing side walls and opposing end walls connected by a bottom wall. The plenum is structured to collect moisture condensing from the mixing of interior air and cold air in the mixing chamber. The collected moisture may be provided to the compressor, preferably at an inlet of the compressor.

The air conditioning system preferably forms a cartridge defined by a housing enclosing the compressor, intercooler and expander. In an alternate embodiment, the plenum may be integrally formed with the housing and defined in part by various walls of the housing and an internal wall extending through the housing.

An electric motor is preferably utilized to drive the compressor or expander. The electric motor is of high efficiency, and the motor and controller sub-system are preferably at least 90% efficient. The electric motor may directly drive one of the compressor and expander by an integral combination of the drive shafts. Alternatively, a belt and pulley system may be utilized to link the expander and compressor to the electric motor. An expandable shroud preferably is utilized to cover the belt and pulley system.

In another embodiment, the air conditioning system includes a compressor, an intercooler and an expander operating as noted above. The expander and compressor preferably have an efficiency of at least 85%, and are preferably screw-type machines. The electric motor and controller sub-system preferably has an efficiency of at least 90%. Further, the intercooler preferably has an effectiveness of at least 90%.

In another embodiment, the air conditioning system includes a compressor, an intercooler, and an expander operating as noted above. The system preferably also includes a temperature sensor positioned upstream of the compressor. The temperature sensor detects the temperature of air going into the compressor from the interior space of the body. A controller is operatively connected to the motor to control the speed of the compressor based on a control algorithm. The algorithm preferably utilizes the temperature of the air going into the compressor and regulates the pressure of the air exiting the compressor. The pressure may be regulated by controlling the exhaust valve of the compressor. Alternatively, the algorithm may control the speed of the compressor, which determines the mass flow rate of air through the system as well as exiting the expander for mixing with air from the interior space. In this manner, the speed of the compressor, preferably controlled by the electric motor, can be regulated based on the air pressure exiting the compressor to vary the amount of cooling and work performed by the system.

DETAILED DESCRIPTION OF THE INVENTION

Turning now to the figures,FIG. 1depicts a schematic view of a refrigeration body20having an interior space22to be cooled by an air-cycle air conditioning system24constructed in accordance with the teachings of the present invention. The body20can represent numerous refrigeration structures, but in the preferred embodiment is a stand alone refrigeration cabinet having an interior space22containing a product to be cooled.

The air conditioning system24is an air-cycle system or open Brayton refrigeration cycle system having a compressor26, intercooler28and expander30. The compressor26and expander30are preferably driven by an electric motor76. Air from the interior space22is drawn into the compressor26as indicated by arrow34. It should be noted that the supply air could be supplied by a combination of air sources and not just from the interior chamber22intended for cooling. For example, the air supply could be supplemented by ambient air from outside the body20. The compressor26can also receive air from the plenum32, and hence air indirectly from the interior space22.

The compressor26compresses and pressurizes the air, thereby increasing its temperature. The compressor26raises the pressure of the air to a suitable pressure ratio that can be set based on the particular application and cooling requirements. The compressor26has a discharge port which routes compressed air to the intercooler28. The intercooler28receives the pressurized air, indicated by arrow36, from the compressor26. The intercooler28is preferably a down-flow or cross-flow intercooler, with a series of axial passages used to route air through the intercooler28in order to cool it, as is known in the art. If required, the intercooler28will have a condensate purging feature that acts to eliminate any condensation of water vapor that may occur in the intercooler28. The condensate that is removed from the intercooler28can be routed to the compressor26intake port and allowed to enter the compressor in order to enhance the compressor efficiency.

The intercooler28removes heat from the air and supplies pressurized cooler air to the expander30as indicated by arrow38. The expander30removes work from the air by reducing the pressure of the air, thereby cooling the same. As the air passes through the expander30, a shaft coupled to the expander30is forced to rotate. This rotating shaft provides some work back to the system24and assists with the rotation of the compressor26. A common shaft31has been depicted inFIG. 1. Any condensation that forms on the exterior of the expander30or that may exit the expander discharge port will be managed in a similar manner in which the condensate within the intercooler28was managed, i.e. it can be routed to the compressor intake for enhanced performance.

The cold air, indicated by arrow40is provided to a plenum32which is utilized to mix and eject cooled air, indicated by arrow44, to the interior space22. A plenum, as used herein, refers to an enclosure defining a space for holding air, and the plenum32will be described in more detail below. The plenum32receives secondary air indicated by arrow42from the interior space that is mixed with the cold air40exiting the expander30and ejected to provide cooled air44to the interior space22and body20.

In order to further increase the COP of the air conditioning system24of the present invention, special components have been selected for the particular application previously described. Preferably, both the compressor26and expander30are screw rotor devices having an efficiency of at least 85%. A preferred screw-type compressor or expander is described in U.S. Pat. No. 6,599,112, the disclosure of which is incorporated herein in its entirety by reference. Further, the intercooler28preferably has an effectiveness (i.e. ejected heat per unit area) of at least 90%. The motor76is preferably a small motor having a high efficiency, preferably at least 90% efficient. One preferred motor is a pancake-type motor constructed in accordance with U.S. Pat. No. 5,744,896.

The air conditioning system24is preferably formed as a cartridge46as is shown in the perspective view ofFIG. 2. The cartridge46includes a housing defined by four side walls48, a top wall50and a bottom wall52. The top wail50preferably defines a inlet56and outlet54leading to the plenum32. The top wall50also preferably defines a second inlet60which provides an air pathway leading to the compressor26. The inlet56is regulated by way of a fan58and preferably two fans58placed side by side which draw air into the plenum32. The fans58are fitted inside the inlet56as shown inFIG. 2. The higher temperature air42drawn in by fans58circulates and mixes with the cold air40from the expander30within the mixing chamber68of the plenum32(FIG. 3). The same axial fans58create an air flow through the mixing chamber68which pushes the mixed air44back into the interior space22via the rectangular shaped outlet54which may be attached to duct work within the refrigeration body20.

The air conditioning system24is shown inFIG. 3with the cover elements48,50,52removed. The compressor26is connected to the intercooler28by way of conduit61. A muffler may be interposed between the compressor28and intercooler as shown. The intercooler28is in communication with the expander30by way of a conduit62, where a muffler is also interposed. The expander30has a discharge block90which is in communication with a discharge port of the expander30and the plenum32. It can be seen that the plenum32is constructed of two curved side walls64and two opposing end walls66, all of which is connected by a bottom wall68. The plenum32is upwardly opening and frame members (shown in dotted lines) of the upper wall50of the cartridge housing46define the inlet56and outlet54in relations to the plenum32. The plenum is positioned proximate the compressor26and expander30, and preferably vertically above the compressor26and the expander30.

With reference toFIGS. 3 and 4, a pancake-type motor76can be seen which has an output shaft78driving an output pulley80. The pulley80drives a belt82which is operatively connected to the compressor26and expander30. More particularly, the compressor26includes an input pulley84and input shaft85which is driven by the belt82. Likewise, the expander30includes an input pulley86connected to an input shaft87which is also driven by the belt82. An idler system88is provided to ensure adequate tension in the belt82and to insure smooth operation of the drive system. It will be recognized that by way of this belt and pulley system, the expander30may provide power back to the system by assisting with rotation of the compressor26. As a result, the overall input power requirement for the electric motor76is reduced, since the expander30is putting some work back into the system24. The belt pulley system of the present invention is not limited to a single belt system, but can be accomplished by means of transferring a net compressor input power via any drive system, such as the common shaft31depicted inFIG. 1, or the direct drive configuration described herein below.

The plenum66also includes an L-shaped inner wall70which defines a intake chamber72leading to the compressor26. As is best seen inFIG. 5, the intake chamber72includes a baffle74which provides a torturous pathway leading to an intake block92connected to an inlet94of the compressor26. The baffle74is utilized to reduce the noise generated by the intake of air into the compressor94. The intake of the compressor26also preferably includes a filter designed for use with the compressor26.

As shown inFIG. 6, the plenum32includes a drain port96which is connected to the inlet94of the compressor26by way of a conduit98. In this manner, moisture collected inside the plenum32due to the mixing of cold air exiting the expander30and the relatively warmer air entering the plenum32via inlet56and fans58will result in condensate forming on the interior of the plenum32. The curved side walls64guide the moisture to the drain96leading to the compressor26. Additionally, the bottom wall52is slightly sloped to guide the moisture to the drain96.

Another embodiment of the air conditioning system24′ is shown inFIGS. 7,8and9. Modified components will be referred to by common reference numerals and denoted with prime. The cartridge housing46′ includes an upper wall50′ defining an inlet56but not a secondary inlet60. Fans58are provided at the inlet56to circulate air through the plenum32′ for mixing and exit via outlet54. A portion of the cover has been removed inFIG. 8, revealing that a different plenum32′ has been provided.

In this embodiment, the plenum32′ has been integrally formed and incorporated into the housing cartridge46′. That is, the plenum is formed by the three adjacent side walls48, the top and bottom walls50′,52, and an interior wall106extending between the side walls48. The internal wall106separates the compressor and expander from the intercooler. In this manner, a separate plenum structure as shown inFIG. 3need not be formed, while a sufficiently sized mixing chamber68′ can be employed. It will also be recognized that the compressor26receives air from the plenum32′ in this embodiment.

It will also be recognized that the electric motor76′ has been provided in a direct drive relationship with the expander30. As best seen inFIG. 9, the motor76′ is directly connected to an input shaft of the expander30and through belt106also drives an input shaft85of the compressor26by input pulley84. The direct drive configuration allows the system packaging, i.e. cartridge46to be more compact and less complex due to the reduction of pulleys and overall belt length. It will be apparent to those skilled in the art that the direct drive configuration can be applied directly to the compressor with a belt extending to a pulley on the expander. As shown inFIG. 8, a shroud100is utilized to cover the motor76′, belt106and input pulley84and shaft85of the compressor26. Preferably, the shroud100includes an expander portion102and a compressor portion104which are telescopically connected so that the distance between the expander30and compressor26may be varied to insure smooth operation and proper tension in the belt106.

Capacity control of the system24can be achieved by way of regulating the compressor speed through the electric motor76or a similar driving device. The speed of the compressor26relates directly to the air mass flow rate through the system24. A temperature sensor detects the temperature of air34entering the compressor26which is indicative of the temperature of the air in the interior space22. Based on a desired temperature change and the current compressor speed (i.e. air mass flow rate), a controller (not shown) can utilize the electric motor76to increase the compressor speed to attain a requisite air mass flow rate. Based on the value of the air temperature measurement, the compressor speed and thus expander speed would be adjusted accordingly to increase or decrease the mass flow within the system.

The system24of the present invention is considered a variable speed system due to the fact that the compressor speed can be adjusted from 0 rpm to any upper limit desired for the appropriate air mass flow rate. It will also be apparent to those skilled in the art that the air conditioning system24can be controlled via a fixed speed method. In this case, the algorithm would monitor the air temperature as described above, and when the air temperature approaches a predetermined set point, the algorithm would cycle off the compressor26via the motor76. The mass flow of air would therefore be controlled using a cycling approach of compressor on and off.

Another embodiment of the present invention would control the capacity of the compressor26by means of adjusting the pressure ratio of the system. A similar software algorithm would be used to determine when the compressor capacity needed to change. As the capacity requirements decrease, the software algorithm would decrease the pressure ratio. As the capacity requirements increase, the pressure ratio would be increased. Specifically, the compressor26includes an exhaust valve (such as those described in U.S. Pat. No. 6,599,112) that can be controlled to regulate the pressure of the air36outputted from the compressor26. The controller would utilize a database of predetermined pressure ratio values corresponding to air intake temperature, and control the exhaust valve to achieve the desired temperature of air in the interior space22.

In addition to the benefits of eliminating the use of a refrigerant, the utilization of the air-cycle air conditioning system24in a system having the air from a confined interior space22continually recycled through the air conditioning system24provides further benefits. Specifically, as the air in the interior space22is cooled by the air conditioning system24, the temperature of the air entering the compressor (indicated by arrow34) will be decreased and the amount of work required by the compressor26, intercooler28and expander38will be reduced as the entire system approaches a steady state condition. The coefficient of performance (COP) of the air conditioning system24of the present invention is higher as the system approaches a steady state condition compared to prior refrigeration systems utilizing refrigerants. As is known in the art, coefficient of performance refers to the refrigeration capacity (determined based on the enthalpy of the starting and ending temperatures of the air) divided by the net work performed by the system24, (which is primarily the compressor).