GB2233080A - Refrigeration apparatus - Google Patents

Abstract

A refrigeration apparatus designed to be capable of operation either in a vapour-compression or a thermosyphon mode comprises valve means (5, 6) for selectively bypassing the compressor (4) in the thermosyphon mode. Fluid to be cooled passes through the tube-side (A, B) of a shell and tube chill or (2), and refrigerant passes through the shell-side. The outside surface of the tubing has a layer of sintered metallic powder for assisting cooling by nucleated evaporation of condensed refrigerant in the thermosyphon mode. The condenser (1) is positioned above the chiller (2) so as to allow thermosyphon circulation of the refrigerant when the compressor is bypassed. Valves (5 and 6) control the modes of operation. <IMAGE>

Description

REFRIGERATION APPARATUS The present invention relates to a refrigeration apparatus capable of operation by direct atmospheric cooling when ambient temperatures are low, or by conventional vapour compression at higher ambient temperatures.

The power and size of present day computers means that they generate considerable quantities of heat, which has to be removed by suitable refrigeration apparatus.

Moreover, the computer environment must be air conditioned for the comfort of operators. Conventional systems meet these two requirements by the use of a single refrigeration apparatus employing a vapour compression system. However, lower temperatures are required for the air conditioning part of the load than for the cooling of the computers themselves. Splitting of the load into two components has therefore the potential for cost savings.

The efficiency of any refrigeration system depends on the difference between the temperature at which the heat is rejected and the temperature at which the heat is extracted. Efficiency is improved by extracting the heat at as high a temperature as possible and rejecting it at as low a temperature as possible.

In the computer systems mentioned above, the air conditioning load requires chilled water to be cooled to typically 50C, whereas the computer cooling part of the load can be met by water at about lloC. Splitting the load allows a significant portion of it to be dealt with at higher temperatures and therefore at higher efficiencies. Moreover, there is a possibility that the lloC water could, at certain times of the year, be produced by direct atmospheric cooling, thereby resulting in cost savings.

It is an object of the present invention to provide a refrigeration apparatus using a conventional vapour compression cycle, but which can employ direct atmospheric cooling at times of low ambient temperature.

The present invention provides a refrigeration apparatus which comprises a condenser for condensing refrigerant medium; a chiller connected to receive condensed refrigerant from the condenser, and having an inlet for fluid to be cooled and an outlet; a duct connecting the chiller to the condenser for re-cycling refrigerant to the condenser; a compressor for compressing refrigerant from the chiller; and valve means disposed in the duct for directing refrigerant flow through the compressor when the apparatus is to operate in a compression mode, and for bypassing the compressor when the apparatus operates in a thermosyphon mode; the condenser being positioned above the chiller and the duct being so dimensioned and arranged as to allow thermosyphon circulation of the refrigerant around the apparatus when the compressor is bypassed.

Preferably, the chiller is arranged to operate at low temperature differences between the refrigerant medium and the fluid to be cooled (e.g. water). In a shell and tube chiller, the fluid to be cooled generally flows through the tubes and the refrigerant medium is in the surrounding shell. Heat transfer may be promoted by the use of high flux tubing intended to promote the boiling of refrigerant on the outside of the tubing when fluid flows through the tube. The effectiveness of the high flux tubing results from the treatment of its outer surface with a thin layer of sintered metallic powder. The metal has a high heat conductivity and has for example, a high copper content (see U.S. Patent 3384154). The metallic powder produces a textured surface which consists of large numbers of inter connected re-entrant cavities.The effect of these cavities is to retain small pockets of vapour during the boiling process. These small pockets of vapour act as starting points for nucleate boiling. The benefit of this treatment in connection with the production of low temperature chilled fluid is that the refrigerant boils steadily at very low temperature differences. This overcomes the difficulties experienced when conventional tubing is used for heat transfer at small pressure differences. With conventional tubing when the temperature differences decrease boiling is suppressed and the evaporating temperature decreases to compensate. When boiling is re-established at a relatively large temperature difference the high heat transfer coefficients can result in rapid freezing of the fluid in the tubes.

This is less likely to happen if steady boiling can be maintained at small temperature differences by using high flux tubing.

It is particularly preferred to employ a number of chillers arranged in series, such that there is concurrent flow of refrigerant and fluid to be cooled. In this way, the maximum temperature difference between refrigerant and fluid is encountered at the upstream end of the first chiller, thereby maximising heat transfer. In a particularly preferred arrangement,two or three refrigeration apparatus are connected in series, each having its own condenser, chiller and compressor, such that fluid to be cooled passes successively through the chillers arranged in series. A computer is programmed to switch any of the apparatus into the thermosyphon mode when high refrigeration capacity is not required e.g. at low ambient temperatures.

The valve means is generally a three-way valve which selectively allows flow directly through the duct or via the compressor. This is preferably a motorised valve controllable by the computer.

In the thermosyphon mode, condensed liquid flows by gravity from the condenser to the chiller and gaseous refrigerant is drawn upwards from the chiller via the duct into the condenser, due to the low pressure in the condenser. The size of the duct and other interconnecting pipework is arranged so as to provide minimum resistance to flow of refrigerant when the apparatus operates in the thermosyphon mode.

An embodiment of the present invention will now be described by way of example only with reference to the accompanying drawings wherein; Fig 1. is a schematic drawing of a refrigeration apparatus and Fig 2. is a more detailed representation.

The apparatus comprises a condenser 1 connected upstream of a chiller 2. Refrigerant is recirculated from the chiller to the condenser either directly through duct 3 or via compressor 4, depending on the position of three way valve 5.

The arrangement is shown in more detail in Fig.2 and further comprises a two way valve 6 and an oil separator 7. Water cooling flow enters the chiller at B and exits at A . The water passes through high flux tubing within the chiller 2. Temperature and pressure sensors are indicated by T and P respectively. Expansion valve 8 is provided for use when the apparatus is in the vapour compression mode. Dryer 9 contains dessicant for removing water (which would otherwise be precipitated as ice, blocking the tubing) from the refrigerant. The apparatus is controlled by a micro computer (not shown) which controls the system operation dependent on the various temperature and pressure parameters and in particular controls motorised valves 5 and 6, as well as providing all the conventional protective features.

The evaporative condenser 1 is provided with two speed fans which give a significant power saving when the lower speed is adequate.

The system operates as follows.

In the conventional vapour compression mode, valves 5 and 6 are as shown. Liquid refrigerant from condenser 1 passes through pipes 10 and 11 to expansion valve 8 where it becomes vapourised and cooled. The cooled refrigerant then passes into the chiller via pipe 12, where it cools water passing through the high flux tubing. Refrigerant leaves the chiller via pipe 13 and is compressed by compressor 4 and passes to oil separator 7 via pipe 14. The gaseous refrigerant under pressure then passes through pipe 16, three-way valve 5, and pipe 17 back to compressor 1 where it is cooled and liquified once again.

In the thermosyphon mode,valves 5 and 6 are switched to the orientation shown in dotted lines. Thus liquified refrigerant from condenser 1 passes through pipe 10, valve 6 and pipe 20; before entering the chiller via pipe 12. Refrigerant leaves the chiller via pipe 21, bypassing the compressor 4 before returning to the chiller via three-way valve 5 and pipe 17. Thus, in the thermosyphon mode, the expansion valve 8 and compressor 4 are bypassed.

The evaporative condenser 1 is positioned so that a good head of liquid is available for the thermosyphon mode.

The apparatus is under control of a microcomputer. In the normal refrigeration cycle control of the refrigerant flow is by means of a low pressure float switch attached to the chiller and controlling the solenoid valve in the liquid line. Control of compressor capacity is from the chilled water outlet temperature with an overriding capacity reduction based on suction pressure followed by a low pressure cutout if the pressure falls still further. The computer is programmed so that when duty is satisfied the refrigerating system reverts to a thermosyphon mode.

Claims (8)

1. A refrigeration apparatus which comprises a condenser for condensing refrigerant medium; a chiller connected to receive condensed refrigerant from the condenser, and having an inlet for fluid to be cooled and an outlet; a duct connecting the chiller to the condenser for re-cycling refrigerant to the condenser; a compressor for compressing refrigerant from the chiller; and valve means disposed in the duct for directing refrigerant flow through the compressor when the apparatus is to operate in a compression mode, and for bypassing the compressor when the apparatus operates in a thermosyphon mode; the condenser being positioned above the chiller and the duct being so dimensioned and arranged as to allow thermosyphon circulation of the refrigerant around the apparatus when the compressor is bypassed.

2. An apparatus according to the preceding claim wherein the chiller is a shell and tube chiller, and the fluid to be cooled passes through the tubing; the tubing having on its outer shell-side surface a layer of sintered metallic powder so as to assist cooling by nucleated evaporation of condensed refrigerant.

3. An apparatus according to any preceding claim which comprises a plurality of chillers arranged such that the fluid to be cooled flows through the chillers in series, each chiller having a respective valve means to allow the chillers to be individually operated in either the compression mode or the thermosyphon mode.

4. An apparatus according to claim 3 wherein the flows of refrigerant and fluid to be cooled are arranged to be concurrent.

5. An apparatus according to any preceding claim which further comprises microcomputor means operatively connected to control the valve means dependant on the required refrigeration load.

6. An apparatus according to any preceding claim which further comprises expansion valve means, and wherein the valve means also bypasses the expansion valve means when the apparatus operates in the thermosyphon mode.

7. An apparatus according to any preceding claim wherein the valve means comprises a three-way valve adapted to connect the condenser either to the compressor or directly to the chiller.

8. A refrigeration apparatus substantially according to either embodiment described in conjunction with the drawings.