In this month's article we complete the series of close control air conditioning articles by looking at close control for industrial and medical applications.

Precise temperature and humidity control for standard rooms and laboratories, as well as contamination control requirements in biotechnology, electronics and medical applications present special challenges. Special purpose chillers are used in both medical and industrial applications.

AC for computers in industrial environments

The main heat exchanger in any close control air conditioning system is the evaporator cooling coil, usually constructed of refrigeration grade copper tubing with aluminium fins. The air passing over this coil may contain corrosive chemicals, which encourage electrolytic action between the copper and the aluminium. One solution is to epoxy coat the coil, or alternatively - depending on the nature of the corrosive gases - use a coil constructed of stainless steel or copper tubes/copper fins.

Fans used in air conditioning equipment are usually constructed of a painted or galvanised steel impeller with similarly finished scroll. Stainless steel or epoxy-coated steel (or even polypropylene) are used in aggressive conditions.

The ambient temperature - particularly for example in steel mills in tropical countries - may reach levels at which conventional air-cooled plant cannot be used because of excessive condensing pressures.

Even with semi-hermetic compressors, there is normally a limit of 100m between the air-cooled condenser and the evaporator, and so the condenser may still have to be situated in a high temperature area.

Whilst R134a refrigerant may be used instead of R407C, if the condensing temperature is likely to exceed 70°C then a water cooled solution may have to be considered. If cooling water is in short supply an air-cooled chiller situated some distance away from the controlled site may be the only practical solution. The chilled water is then pumped directly to a cooling coil in the air-handling unit.

Specially designed industrial cooling systems are manufactured for these applications - both air-cooled, water-cooled and chilled water systems. Because of the high heat loads experienced in these applications, packaged units up to 200kW capacity are available. They have a very robust frame, epoxy coated panels and incorporate cooling coils and fans as described above. Filters are usually in stainless steel frames, and activated carbon filters may also be used.

The microprocessor control system utilises military standard components, and if humidity control is also needed, electrode boiler canister humidification is used for ease of maintenance.

Little glass house

In the early days of data centres, users were so proud of their computer room that it was often situated in the company headquarters reception area with glass walls. Visitors were encouraged to peer through these walls but were very rarely admitted to the holy of holies. As computer security became ever more important the computer room was relegated to a more anonymous location.

However the phrase glass house - used particularly in the United States - has been revived with the introduction of products designed specifically for file servers used in both industrial and medical locations.

The computer equipment itself is rack-mounted within what appears at first to be like a wine display cabinet. However the little glass house, as it is known, not only incorporates air temperature control within a secure enclosure, but also a rack-mounted uninterruptible power supply (battery backed), complete cable management - and a monitoring and alarm system.

If power is lost, a ventilation fan powered from the UPS continues to operate, eliminating the build up of hotspots within the cabinet whilst remedial action is taken.

The cooling system utilises a rotary compressor, integral evaporator, air filtration and condenser with evaporative condensate removal so all that is required is that the little glass house be plugged into the mains power supply - no special installation is needed.

Standard rooms and controlled areas

A special requirement in industry exists for rooms within which either the temperature or humidity (and sometimes both) have to be controlled within very tight limits. Just one example is the quality control testing of the mechanical properties of paper.

A very high and uniform air recirculation rate is required, and the cooling and heating should not occur in steps (as may be perfectly acceptable in most close control applications). The heating may use thyristor controlled electric heating or hot water heating coils with fully modulating control valves. The cooling system is either with a chilled water coil with fully modulating control valve or direct expansion with electronically controlled hot gas bypass. Humidification will also usually employ an electrode boiler with fully modulated output.

Clean rooms

Contamination control has become a key feature of many industrial and medical applications.
Apart from the generally very high levels of filtration required - usually involving the use of HEPA filters - airflow distribution has to be carefully controlled to prevent cross contamination between one part of the controlled space and another. Laminar flow air distribution involves a piston-like unidirectional constant velocity either from ceiling to floor (vertical flow) or from one wall to another (horizontal flow). A typical air velocity is 0.5m/sec.

It does not take very much calculation to realise that the volumes of air that have to be moved are enormous - even compared with a computer room or telephone exchange. The pressure drop across the filter banks is usually in the region of up to 500Pa (with a filter approaching full dust capacity). This results in considerable power being absorbed by the usually backward curved plug fans employed, and consequently the air conditioning heat load is mainly sensible.

Different parts of a clean room installation may have to be controlled at different pressure drops with respect to each other. For example, the cleanest part of the installation is held at the highest pressure so that there is a tendency for the very cleanest air to pass always to the less contaminated free zones.

In microelectronics facilities, chemical etching and cleaning operations are carried out in special laminar flow cabinets with localised air exhaust. The general air conditioning system has to cope with make-up air introduced to compensate for the air that is extracted.

Other parts of the microelectronics facility may require very precise temperature control. Special units have been developed to deal with these requirements. Three term PID control and high specification temperature and humidity transducers are used. The units incorporate high efficiency filters, and utilise humidifiers with stainless steel electrode boilers.

In pharmaceutical installations, localised extraction also means that the air supply system has to be designed to handle additional quantities of fresh air as required.
In certain medical facilities, air recirculation is not permitted at all, because of the risk associated with build up of micro-organisms in the system.

Packaged units used for clean facilities have to be particularly flexible in operation and application, and therefore are usually of the chilled water configuration with a high sensible heat factor coil. They operate from a central chiller which may be of the free-cooling type.

This type of chiller incorporates an additional air to water heat exchanger; if the outside ambient is sufficiently low, direct cooling of the water supply is provided. At higher ambient temperatures, the system operates as a conventional air-cooled chiller.

Process chillers

When heat densities are very high, or for other reasons it is not convenient to resort to air cooling, direct water-cooling is employed. The source of chilled water provided is extremely important to the successful operation of the system.

An example of this occurs with MRI scanners used as a diagnostic tool in hospitals. The heat generated within a magnetron must be removed immediately and over-temperature protection devices operate instantaneously if the cooling system fails - very embarrassing and potentially expensive if one is in the middle of a scan.

Other examples in industry are plastic injection moulding machines, as well as water cooled computers (in which one may be cooling the condenser of a built-in cooling system that utilises helium to cool the electronic circuits directly).

So process chillers - usually configured as run and standby - are used for these special purposes. To prevent unwanted condensation, the exit chilled water temperature is usually higher than that for conventional chillers used in 'comfort' applications. Circulating pumps are built in - again run and standby - and may incorporate stainless steel impellers. Each chiller is microprocessor controlled and water flow and temperature are continuously monitored. Alarms are signalled both locally and remotely.

Close control air-conditioning is commonly required in production processes

Controls and monitoring

Several times in this series of articles on close control air conditioning, reference has been made to the microprocessor controls that also provide remote communications facilities. If a controlled environment goes outside the limits set - whether temperature, humidity, or particle count - it is vital that those responsible are notified immediately. With the use of redundancy in systems, switching over to a standby system may give time in which to correct the problem. However it may be necessary to start shutting down the critical system in an orderly fashion - be it computer, communications system, power plant, or wafer fabrication facility.

Whilst we have concentrated on the conditioned environment, associated parameters such as power supply, security, fire risk, are all vitally involved in maintaining the critical system in an operational state. For example a loss of power can put the whole system at risk and a very large number of operational alarms can be generated in a short time.

It has become commonplace in even quite modest buildings to install building energy management systems that provide the building user with a central point to monitor all the building services, as well as a means of operating the building at maximum energy efficiency.

With critical systems, whether we have close control air conditioning, precision air conditioning, or high performance air conditioning, energy efficiency is important. However keeping that critical system running is even more important.

Great care has to be exercised in designing the interface with the BEMS, which must not (except in case of fire) override the critical control system. Some BEMS are not designed to handle the very large number of alarms occurring in a short time that may be produced by a critical system.
It is often considered advisable to introduce a buffer between the critical system and the main BEMS, and for alarms associated with the critical support system to be signalled completely separately.

Conclusion

There are an increasing number of applications for precision air conditioning installations, in spite of the fact that developments in electronics have resulted in many critical systems being less sensitive to environmental variations. However reliability levels are so important that close temperature (and in many applications - humidity) control is essential.

In addition heat densities are increasing with closer-packed microcircuits, and this trend presents particular challenges. Also the increasing importance of contamination control has resulted in a growing demand for high sensible heat ratio, ultra-reliable air conditioning units.

With thanks to Mike Creamer of Business Edge who revisits his Masterclass series of articles, updating and adding to the information which proved so useful to readers when the series was first published over ten years ago. In this reincarnation, the series will cover both air conditioning and refrigeration and serve as an on-going source of technical reference for experienced personnel as well as providing a solid educational grounding for newcomers to our industry.