Compact Cooling System and Method for Accurate Temperature Control

In an integrated two-phase accumulator controlled loop cooling system, the pumped refrigerant is employed to cool the supply of refrigerant in the accumulator vessel. No external cooling of the accumulator vessel is required, and a standard heater in the accumulator is sufficient to regulate the boiling pressure. This allows to provide a cooling system in which sub-cooling in the pump is guaranteed by the laws of nature, and which is hence more reliable, structurally simpler, better to control and cheaper.

The invention will now be described for the specific example of a two-phase evaporative CO2cooling system that improves on, but is otherwise similar in design and functionality to the conventional 2PACL CO2cooling system described above with reference toFIGS. 1 and 2.

The integrated 2PACL cooling system200shown inFIG. 3comprises an accumulator vessel202that is similar in design to the accumulator vessel102described above with reference toFIG. 1. In particular, the accumulator vessels202comprises an electrical heater204, such as a thermo siphon heater, for heating and hence evaporating a supply of refrigerant206stored in s the accumulator vessel202, as well as a cooling spiral208for cooling and hence condensing said supply of refrigerant206.

The accumulator vessel202is connected, via a branch line210, to an inlet fluid pipe or inlet fluid tube212in which a condenser214is provided. In the context of the present invention, the terms “pipe” and “tube” may be used interchangeably. The condenser214shown inFIG. 3is generally identical or similar to the condenser110described previously with reference to the 2PACL cooling system ofFIG. 1and may be any condenser as conventionally employed in cooling systems.

The condenser214is connected, via an input line216and an output line218, to an external chiller220. The external chiller220may be any conventional chiller as employed in cooling systems or any other cold source, and in general may be similar to the external chiller108described with reference to the conventional 2PACL system ofFIG. 1. However, in contrast to the configuration ofFIG. 1, the external chiller220is merely connected to the condenser214, and does not also serve to cool the accumulator vessel202. Compared to the external chiller108, the external chiller220according to the present invention may hence be smaller, and the amount of piping may also be reduced. No interference between the multiple cooling connections is present anymore.

The condenser214serves to sub-cool the CO2supplied to the condenser214from the accumulator vessel202via the branch line210and inlet fluid pipe212. Sub-cooled CO2leaves the condenser214and is supplied, still via the inlet fluid pipe212, to an inlet222of a liquid pump224. The liquid pump224may be similar to the liquid pump112described with reference toFIG. 1, and can in general be any pump suitable for pumping liquid CO2(or other fluids if used instead of CO2in the invention). An outlet226of the liquid pump224is connected to an outlet fluid pipe228, which supplies the pumped CO2to an object to be cooled (not shown).

On its way to the object to be cooled, the pumped CO2traverses the cooling spiral208provided in the accumulator vessel202, and hence exchanges heat with the supply of refrigerant206stored in said accumulator vessel202. The outlet fluid path can hence be subdivided into two sections, a first section228aconnecting the outlet226of the liquid pump224to an inlet s of the cooling spiral208, and a second section228bdownstream from an outlet of the cooling spiral208. In operation, the accumulator vessel202will in general be filled with saturated liquid and vapor, and hence the pumped CO2in the outlet fluid pipe228will have been heated up to the accumulator temperature once it reaches the outlet of the accumulator cooling spiral208. At this stage, the fluid is still supplied via the outlet fluid pipe228and is not yet to boiling, although its temperature now coincides with the temperature of the boiling fluid in the accumulator vessel202, or nearly so. This is due to the higher pressure in the outlet fluid pipe228. The liquid CO2at boiling temperature is then supplied to an evaporator (not shown) in thermal contact with the object to be cooled. Once the liquid CO2at boiling temperature reaches the evaporator, the pressure is lowered and the fluid starts boiling, thereby cooling the object.

The integrated 2PACL according to the present invention is nearly isotherm during boiling from the evaporator to the inlet of the liquid pump. Only the pressure drop in this part of the system causes a small temperature gradient, much smaller than in systems using a liquid cooling flow. In the latter systems, the liquid flow is difficult to control as it is subject to heating during transfer. Local sensor control may be employed, but will usually lead to the system having a low response time. In contrast, the system according to the present invention controls the distant temperature by controlling the pressure, which is transmitted with the speed of sound, and hence almost without any delay.

CO2in a mixed liquid/vapor phase returns from the object to be cooled and is channeled through the inlet fluid pipe212to the condenser214for subsequent cooling, and hence the cooling cycle is closed.

In particular, when illustrated in a pressure-enthalphy phase diagram, the cooling cycle of the integrated two-phase accumulator controlled loop cooling system according to the embodiment ofFIG. 3is generally identical to the cooling cycle of a conventional 2PACL cooling system, and hence reference may be made to the phase diagram ofFIG. 2.

The functionality of the cooling system according to the embodiment ofFIG. 3is in general very similar to the conventional 2PACL cooling system known from the art.

However, in the invention, the cooling of the supply of refrigerant206stored in said accumulator vessel202is achieved exclusively by means of thermal contact with the pumped refrigerant via the cooling spiral208, and no external cooling of the accumulator vessel202is required. In operation, the CO2boiling pressure in the accumulator vessel202is controlled solely by means of heating via the electrical heater204. A control unit230controls operation of the electrical heater204in response to a pressure in the accumulator vessel202detected by means of a pressure gauge232. Since the control unit230is needed solely for controlling the electrical heater204, it does not require a complex programmable logic control such as the control unit120described with reference toFIG. 1.

Cooling the supply of refrigerant206in the accumulator vessel202by means of the discharge liquid of the pump224has the additional advantage that the accumulator temperature cannot fall below the temperature of the discharge liquid of the pump, which is higher than the saturation temperature at the pump inlet222. In this way, the sub-cooling of the pump is guaranteed by the laws of nature, and the risk of evaporation of the refrigerant in the liquid pump224is avoided without any additional sub-cooling control, which conventionally also had to be provided by the programmable logic control unit120.

The invention hence results in a two-phase evaporative CO2cooling system that is structurally simpler, more reliable, better to control and cheaper to build, but without compromising on the functionality of a conventional 2PACL system. The integrated 2PACL cooling system according to the present invention is similar in complexity and price to conventional cooling systems employing thermostatic baths, but has the additional advantage of accurate (isothermal) and direct temperature control on distant experiments in combination with very small cooling tubes.

The description of the preferred embodiments and the figures merely serve to illustrate the invention and the beneficial effects associated therewith, but should not be understood to imply any limitation. The scope of the invention is to be determined solely based on the appended set of claims.

REFERENCE SIGNS

1002PACL cooling system102accumulator vessel104electrical heater106cooling spiral108external chiller or cold source110condenser112liquid pump114heat exchanger116return pipe118liquid CO2with the same temperature as the boiling temperature in the accumulator120programmable logic control unit122temperature gauge124pressure gauge200integrated 2PACL cooling system202accumulator vessel204electrical heater206supply of refrigerant208cooling spiral210branch line212inlet fluid pipe214condenser216input line of condenser214218output line of condenser214220external chiller or cold source222inlet of liquid pump224224liquid pump226outlet of liquid pump224228outlet fluid pipe228afirst section of outlet fluid pipe228228bsecond section of outlet fluid pipe228230control unit232pressure gauge