It is always desirable to keep a stable condensing pressure with reciprocating compressors, particularly when the plant runs throughout the year in a non-tropical climate. The temperate winter experienced in the UK can result in condensing pressures which are too low for the plant to operate properly, unless steps are taken to limit the rate at which heat is rejected in the condenser.
Figure 11.1 shows the method adopted for a cooling tower associated with a reciprocating compressor. A direct control is exercised over condensing pressure: as the pressure tends to rise, pressure sensor Cl partially closes the by-pass port in the three-way valve Rl, passing more cooling water through the condenser. Sometimes the cooling water flow temperature on to the condenser is controlled (for centrifugal compressors) but this is not preferred for reciprocating machines because of the more sluggish response to changes in condensing pressure and the need for this to be kept fairly constant—flow temperatures of less than 19°C are very undesirable with this mode of control. Occasionally a two-port throttling valve is used instead of a three-port valve but it must always be controlled directly from condensing pressure, never by water temperature. Centrifugal machines prefer to operate with the cooling water temperature on to their condensers controlled at a value that is allowed to fall from about 27°C in summer to a minimum of about 18°C, in order to improve low-load performance, conserve energy and mitigate the risk of surge (see section 12.13). With centrifugal compressors the method shown as a broken line in Figure 11.1 is suitable. A motorised, two-port, butterfly valve, R3, is located in a by-pass across the condenser. The valve is sized so that the water pressure drop through it when fully open equals, or is just less than, the static lift of the tower. Upon fall in flow temperature, sensed by C3, the valve R3 is progressively opened. When the valve is fully open all the water by-passes the tower. It is essential that the valve is beneath the static water level and that it is always flooded when fully open, otherwise control will be lost at partial load.
Induced draught towers are taller than cross-draught and cooling by natural convection, with the fan off is possible to some extent. To save the running cost of the fan it may be switched on-off by an immersion thermostat, C2 (Figure 11.1) with a set-point of, say, 24 ± 2°C. When the ambient wet-bulb is low enough for the tower to produce water at 22°C by natural means the fan cycles and the flow temperature onto the condenser varies between 22° and 26°C. Meantime, the condensing pressure is stabilised by Cl and Rl.
De Saules and Pearson (1998), using the work of Braun and Diderrich (1990) and Whillier (1976), describe a way of reducing the energy consumption of cooling tower fans and their related water chillers. It is argued that set points of 26°C to 28°C, for the control of cooling water flow temperature, are unnecessarily high for much of the year and result in excessive energy consumption since compressor power is sensitive to cooling water flow temperature. Savings greater than those achieved by lowering the set-point of the water leaving the cooling tower can be achieved by increasing the fan speed in proportion to the ratio of the partial load to the design maximum load. It is claimed that scheduling the fan apeed in this way prevents the increased cooling tower fan power from exceeding the saving in compressor power. However, it is acknowledged that cooling water flow temperatures less than 19°C could adversely affect lubrication and, by implication, give other problems. While saving energy is a laudable aim it must be remembered that doing so is not the prime purpose of the installation, which is to provide satisfactory air conditioning at partial load as well as full load.
Evaporative condensers have their capacity controlled by means of a condenser pressure — sensing element, C1. This modulates the flow of air passing over the wetted coils by means of motorised dampers, as shown diagrammatically in Figure 11.2. It is customary to have auxiliary contacts on the damper motor control so that, when the dampers are fully closed, the fan is switched off. A cruder form of control, not acceptable in temperate climates when the refrigeration plant has to run throughout the winter, is just to cycle the fan. A better method is to vary fan speed in response to changes in condensing pressure. Some economy in running costs can be effected, if thought necessary, by arranging to switch off the pump when the outside dry-bulb temperature is low enough to permit the condenser to reject its heat without the aid of evaporation.
In the past air-cooled condensers have been used for smaller duties but in recent years, because of the health risks associated with the use of cooling towers and evaporative condensers, there has been a tendency to use air-cooled condensers for much larger duties. When used with small cooling loads (for example, some computer installations) it is not uncommon for the refrigeration plant to have to run in winter as well as in summer, the reason being that only a small, constant quantity of fresh air is handled by the air conditioning system. Controlling condensing pressure by cycling the condenser fan or varying its speed may not prevent excessive cooling by the wind, in very cold weather. Using motorised dampers can then also be unsatisfactory: they do not provide a hermetic seal when closed and will admit cold outside air, even if the fan is off.
An alternative control (Figure 11.6) is sometimes used. There are several variations of this, some probably patented, but in principle the intention is to reduce the capacity of the condensers by reducing the surface area available for heat transfer. The liquid refrigerant is backed-up inside the condenser by restricting its flow to the liquid receiver. This reduces the area of the condenser through which heat is transferred from the condensing vapour to the environment and thus varies the rate of heat rejection in a way that can be automatically controlled from condensing pressure. As the pressure falls (sensed by PI), the motorised valve Rl reduces the outflow of liquid to the receiver. The size of the liquid receiver needs to be increased somewhat so that the refrigerant demands of the system can be met, even though Rl is fully closed for a short while. It is also necessary to have a gas-pressure equalising line between the condenser and the receiver so that an adequate pressure is maintained on the upstream side of the expansion valve.
Fig. 11.6 Condenser pressure control by varying the level of liquid refrigerant inside the condenser.
Posted in Engineering Fifth Edition