Critical parameters which need to be maintained to provide basic protection for a compressor are coolant water flow or temperature, discharge air temperature and lubricating oil pressure or level (in the case of lubricated compressors). Typical minimum parameters, ie danger levels, might be:
• coolant water flow 2/3 of normal flow rate
• discharge air temperature 160°C
• lubricating oil pressure 2 bar
Temperature sensors and pressure sensors can provide cover for these. If temperature and pressure operated switches are used, they can provide automatic protection either as alarm devices or control devices, shutting down the system when a critical parameter level is reached.
Where compressors employ closed-circuit water cooling (either thermo-syphon or forced circulation), additional cooling water can be provided from the main or other cold water supply via a temperature regulator. This allows cold make-up water to be supplied to the cooling tank at a controlled rate, hot water being displaced through an overflow. A typical installation of this type is shown in Figure 10.
Automatic temperature control can be applied to mains water cooling systems, the purpose in this case being to eliminate wastage of water and to prevent excessive cooling which could cause condensation within the cylinder. A typical installation applied to a two-stage compressor is shown in Figure 11. It is important that the temperature regulator cannot shut off the supply of cooling water completely. This can be guarded against by incorporating an adjustable bleed in the valve itself (ensuring a small flow when the valve is shut) or the use of a bypass, the flow through which is controlled by a needle valve.
In the case of larger compressors operating in a closed cycle, cooling is accomplished by passing the water through a mechanical cooler or cooling tower. Again temperature control can prove advantageous, particularly to avoid too low a temperature being developed in the second stage cylinder.
An important consideration in the case of cooling towers is the possibility of freezing, with the consequent cessation of water circulation calling for the compressor to be shut down. This possibility can be avoided by using a temperature regulator valve commanding a supply of steam to a heating coil i n the sump. The regulator can be set to open when the water temperature falls below, say 2″C, so that no steam is used until near freezing conditions are approached (Figure 12). To guard against the possibility of the cooling tower freezing solid, the line from the diversion control valve can be arranged to return water from the compressor direct to the sump instead of to the top of the tower under low temperature conditions.
Temperature regulators can also be used to advantage on water cooled after coolers. The use of a temperature regulator enables the water consumption to be set to the point of maximum efficiency and water economy. An example of the use of a temperature regulator controlling main water supply to an aftercooler is shown in Figure 13.
Water cooled aftercoolers are usually the most efficient type; the lower the air temperature they produce, the better. Air cooling is also widely used, particularly where cooling water is expensive or not readily available; portable units almost exclusively use air cooling. An air blast aftercooler is usually the first choice. A motor driven fan directs a stream of air over a bank of finned tubes, through which the compressed air passes, see Figure 14.