Book - School of Science and Technology

Building management systems 653often allow varying combinations of compressor (rather than whole chillers) to besequenced.. Although a series arrangement may permit control of water flow temperature to bemore stable, the hydraulic resistance of the system will increase with consequent higherpumping costs. (It is not usual to provide arrangements where more than two machinesoperate in series.)Normally, with reciprocating machines, capacity would be adjusted by proportionalcontrol, sensing the temperature of the return water. With screw or centrifugal machines,PI control would normally be used.Cooling towersA typical arrangement for a condenser water circuit, connecting chiller condensers toeither cooling towers or an equivalent, is shown in Figure 22.33. The cooling effect of thetowers is controlled either by sequence switching or speed control of the fans. A bypassvalve is normally provided in the heat-rejection circuit, which operates in conjunctionwith the fan controls on start-up and sometimes during colder weather, in order tomaintain a constant return water temperature to the condensers. To ensure economy inenergy, the condenser water temperature may be varied with respect to the imposed loadand thus retain a practical minimum between the condensing and evaporating temperaturesof the chillers. Attention must be given to the hydraulics of the condenser watercircuit to avoid unintended cavitation and all water based cooling plant must be providedwith frost protection.Where water is used for condenser cooling, a number of chilling machines may beconnected together, in parallel, into a single circuit to be pumped over multiple coolingtowers or their equivalent. Where air cooled condensers are used, however, each chillingmachine will have a dedicated cooler. Fan speed control, fan switching or modulatingdampers may be used to control either the condensing pressure or temperature. Normally,PI action would be installed for control purposes.Where water cooled chillers are used, the option exists to use the heat rejection devicealone during periods of sufficiently low outside ambient, in order to provide `free cooling'.This is expanded upon in Chapter 19 (p. 544).Building management systemsThe introduction of computer and data communication technologies has given rise todramatic developments in control and management systems, probably the most significantsingle change in the heating and air-conditioning industry in the past decade.Advances in microelectronics, and the reduction in cost of components coupled withimproved reliability, has led to widespread use of these systems, from quite small installationsto multiple site applications and with varying levels of sophistication. Flexibility inavailable systems has led to different approaches to application and this, in turn, hasgiven rise to descriptions such as energy management system, building energy managementsystem, building automation system, supervisory and control system; the authors preferbuilding energy management system (BEMS) as the generic title.The basic functions of building energy management systems may include:. initiation of systems control functions. continuous monitoring of systems