This invention relates generally to transport refrigeration systems and, more particularly, to a method and apparatus for optimizing the system high-side pressure in a CO2 vapor compression system with a large range of evaporating pressures.
The operation of vapor compression systems with CO2 as the refrigerant is characterized by the low critical temperature of CO2 at approximately 31° C. At many operating conditions, the critical temperature of CO2 is lower than the temperature of the heat sink, which results in a transcritical operation of the vapor compression system. In the transcritical operation the heat rejection occurs at a pressure above the critical pressure, and the heat absorption occurs at a pressure below the critical pressure. The most significant consequence of this operating mode is that pressure and temperature during the heat rejection process are not coupled by a phase change process. This is distinctly different from conventional vapor compression systems, where the condensing pressure is linked to the condensing temperature, which is determined by the temperature of the heat sink In transcritical vapor compression systems, the refrigerant pressure during heat rejection can be freely chosen, independent of the temperature of the heat sink However, given a set of boundary conditions (temperatures of heat sink and source, compressor performance, heat exchanger size, and line pressure drops) there is a first “optimum” heat rejection pressure, at which the energy efficiency of the system reaches its maximum value for this set of boundary conditions. There is also a second “optimum” heat rejection pressure, at which the cooling capacity of the system reaches its maximum value for this set of boundary conditions. The existence of these optimum pressures has been documented in the open literature. For example, maximum energy efficiency is attained in U.S. Pat. Nos. 6,568,199 and 7,000,413, and maximum heating capacity is attained in U.S. Pat. No. 7,051,542, all of which are assigned to the assignee of the present invention.
Given a set of boundary conditions (temperature of heat source, compressor performance, heat exchanger size, and line pressure drops), the value of the optimum heat rejection pressure depends primarily on the temperature of the heat sink Conventional control schemes for CO2 systems utilize the refrigerant temperature at the heat rejection heat exchanger outlet or the heat sink temperature or any indicator of these as the control input to control the heat rejection pressure. However, in systems designed for an operating envelope which covers a large range of heat source temperatures (e.g. −20 F to 57 F), such as transport refrigeration units, it may not be sufficient to correlate the optimum high-side pressure only to the temperature of the heat sink
In accordance with one aspect of the invention, in systems having a relatively large range of heat source temperatures, the control of the system high-side pressure in a CO2 vapor compression system is made dependent not only on the condition of refrigerant on the high pressure side (i.e. in the cooler), but also on the condition of refrigerant on the low pressure side (i.e. at the evaporator).
By another aspect of the invention, in addition to temperature conditions sensed at the cooler, various sensed pressure or temperature conditions at the evaporator may be used in various combinations to determine the optimum system high-side pressure.
While the present invention has been particularly shown and described with reference to the preferred mode as illustrated in the drawings, it will be understood by one skilled in the art that various changes in detail may be effected therein without departing from the spirit and scope of the invention as defined by the claims.
Referring now to
Primarily for environmental reasons, the “natural” refrigerant, carbon dioxide is used as the refrigerant in the vapor compression system 10. Because carbon dioxide has a low critical temperature, the vapor compression system 10 is designed for operation in the transcritical pressure regime. That is, transport refrigeration vapor compression systems having an air cooled refrigerant heat rejection heat exchanger operating in environments having ambient air temperatures in excess of the critical temperature point of carbon dioxide, 31.1° C. (88° F.), must operate at a compressor discharge pressure in excess of the critical pressure for carbon dioxide, 7.38 MPa (1070 psia) and therefore will operate in a transcritical cycle. Thus, the heat rejection heat exchanger 13 operates as a gas cooler rather than a condenser and operates at a refrigerant temperature and pressure in excess of the refrigerates critical point, while the evaporator 16 operates at a refrigerant temperature and pressure in the subcritical range.
It is important to regulate the high side pressure of a transcritical vapor compression system as the high pressure has a large effect on the capacity and efficiency of the system. The present system therefore includes various sensors within the vapor compression system 10 to sense the condition of the refrigerant at various points and then control the system to obtain the desired high side pressure to obtain increased capacity and efficiency.
As shown in the embodiment of
Referring now to
If the sensed S4 senses the evaporator inlet temperature TEI, then that value is sent to the controller 17 which then enters a lookup table to find the corresponding evaporator inlet pressure PEI, and the remaining steps are then taken as described hereinabove.
A further embodiment is shown in
A functional diagram for the various sensors and the control 17 is shown in
While the present invention has been particularly shown and described with reference to the preferred mode as illustrated in the drawings, it will be understood by one skilled in the art that various changes in detail may be effected therein without departing from the spirit and scope of the invention as defined by the claims.
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/US09/58543 | 9/28/2009 | WO | 00 | 6/13/2011 |
Number | Date | Country | |
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61101782 | Oct 2008 | US |