This article introduces and discusses the types of cooling system available. Specific water cooling considerations and recommendations for discrete MBE systems can be found in MBE: Water cooling system.
A single pass system is by far the most simple and by equal measures the most wasteful. The standard 3-5 bar mains water is perfectly suitable for achieving 5-10 litres per minute of flow through an MBE system. The system comprises connecting a tap to the inlet of the MBE manifold and piping the outlet to the nearest drain (figure 1). Sadly this means around 7000 litres of water per day are pouring down the drain. Hence this is not a permanent cooling solution, but is a perfectly viable backup to maintain cooling temporarily whilst the primary cooling system is down.
Clearly the only sane solution is to recycling the cooling water. There are 2 main types of recycling water cooling system: open and closed systems. An open system is by definition a direct system (whereby the cooling action is performed directly on the cooling water) in contrast a closed system can be either direct or indirect (via a heat exchanger).
Each type of system is considered in detail below, though first let’s imagine a basic cooling system: The simplest recycling system comprises cooling to counteract the MBE system’s heating and a pump to maintain suitable water flow (Figure 2). This system is arguably similar to an automotive cooling system. It relies purely on convective-and-conductive cooling and hence is very energy intensive and grossly inefficient without a suitably large airflow to hand.
Refrigerant chiller types
In static systems it is therefore common to use a high vapour pressure fluid as a cooling medium and take advantage of the heat consumption in the process of evaporation to achieve local cooling. The two primary variants are absorption and vapour compression cooling, and they differ primarily in the method the refrigerant vapour is converted back into liquid to continue the cooling cycle.
Absorption cooling is more efficient than vapour-compression, particularly if you have abundant, high grade waste “heat” and “cold” that can be utilised. The heat is used to evaporate the refrigerant which is then absorbed into a nearby absorber (creating a low pressure inside the system). Heat is utilised again in another chamber where the absorber is heated to release the refrigerant, which is subsequently condensed with coolant and the aid of a heat exchanger. The whole process is shown in figure 3. Handily MBE facilities have both waste heat (racks, cooling water, air conditioning) and waste cold (LN2 phase separator exhaust gas) that can be utilised to reduce power consumption.
Figure 3: Absorber (Pending: Please check back later)
Vapour-compression cooling utilises a compressor, a condenser, an evaporator and an expansion valve. The system (figure 4) is strikingly similar to figure 2 at first glance, however here the phase change of the refrigerant (from liquid to vapour) is providing a more efficient cooling action than pure convection can achieve. The excess heat and cold of an MBE facility cannot be directly coupled with commercially available vapour-compression chiller system, however a hybrid system is proposed at the end of the article.
Heat exchange (Direct and indirect)
The systems described thus far have been closed loop systems. A closed loop system is one where the cooling water never “sees” atmosphere and therefore never has the opportunity to evaporate. Closed loops are therefore immune to external contamination (though internal contamination via corrosion of the system interior can still occur) and importantly should not require refilling. The water cooling system can be through of as a series of heat exchangers between a number of temperature zones. The terms direct and indirect refer to how the heat exchange is performed upon the cooling water itself. A direct system (figure 5) exposes the cooling water to the refrigerant pipes, whereas an indirect system keeps the cooling water contained in its own pipework (figure 6). Indirect systems are therefore less likely to introduce contamination into the cooling water. There is no particular advantage of one system over the other for MBE cooling applications.
Reservoir (buffer) tank
A reservoir tank can provide additional stability, aid modularity and enable multiple systems to be run from a single chiller. They can also enable the waste “cold” from the LN2 phase separator exhaust to be put to good use (see Hybrid System below). The chiller can be either direct or indirect. Note when connecting multiple system in parallel care must be taken to ensure the “resistance” of each system is approximately equal to ensure all receive equal flow (discussed in MBE: Water cooling system). This can be achieved by individual flow control valves and flow meters. A system with a reservoir tank and suitable pumps and valves serving two MBE systems and an XRD is shown in figure 7.
Closed and Open loop
Open loop systems are a perfectly viable alternative to closed loop systems, however they are generally more suited to larger facilities and they require extensive maintenance and suffer from certain hidden costs that will be discussed herein. The cooling in an open loop system is provided by direct action via evaporation of the cooling water in a cooling tower. Banish the image from your mind of an industrial cooling tower belching out plumes of white water vapour; a cooling tower can be a much smaller, simpler, aestheticly pleasing affair. They of course need some space, either on a rooftop or adjacent to the facility. Water cooling towers can be both open (figure 8) and closed (figure 9) loop systems. Particular attention needs to be paid to the water in the tower. The evaporation action within the tower creates a “micro-sea” with a constantly increasing salt content. Hence the water requires a regular “blowdown” cycle where saltwater is remove and new tap water is used to top the system up. The removed water can still be useful (e.g. for irrigation) so long as it is done before the salt content gets too high. Hence it must be constantly monitored. Furthermore, since the water is constantly evaporating it must be constantly topped up. It must also be “treated” and “conditioned” to prevent the build up of algae. The additional environmental costs associated with open loop systems can be offset against their lower run cost, however because of these costs and complications (I will reiterate that) they are normally only applicable to large facilities.
As the name suggest, hybrid systems combine two or more aspects of the individual systems. The gain in reduced cooling cost is offset by an initial higher installation cost. More specifically a hybrid system should utilise the waste “heat” and waste “cold” of an MBE facility in order to reduce the cooling power requirements. The final figure is not intended to be an ultimate cooling system, nor is it intended to be perfect, it is simply intended to provoke you to consider how the discrete sub-systems of an MBE facility can be coupled together to save energy and cost (figure 10).