By K. M. Letherman, N. S. Billington
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The Psychrometer is then swung round for a minute or so and the two thermometer readings taken. The wet-bulb should be read first, as it starts to rise immediately. The process is repeated until a steady value of wet-bulb temperature is obtained. When swinging the Psychrometer round, an airspeed of about 5 m/s should be aimed for, which means that it should rotate at about 4 rev/s. The aspirated or Assmann Psychrometer also has a pair of mercury-in-glass or alcohol-in-glass thermometers, but the air is drawn over the bulbs at the correct speed by a fan driven either by a clockwork motor or an electric motor.
Dependence on a single computer processor is reduced by the use of remote stations each with its own microprocessor and hence capable of some independent operation, but under the control of a central computer for higher-level functions. The installation of a building automation system can be justified on the basis of the wide range of functions and services which are available, related not only to heating and air conditioning but also to monitoring and control of lighting, electric power, communications, lifts, fire detection and alarm, fire fighting, cold-water services, plant management for energy conservation, maximum demand monitoring of electricity use and planned and programmed maintenance schedules.
M: motor actuator. 2 modulating. When an individual boiler is shut off a motorised valve in the return is closed after a delay of 2 or 3 minutes to avoid cut out on the high-limit thermostat. It is desirable also to arrange for a motorised flue damper to close, conserving the heat stored within the metal of the boiler. A corresponding delay must be arranged before the burner is fired to allow time for the flue damper and the return water valve to be driven open. A reverse return arrangement for the piping is desirable to ensure that the water-flow rate is the same through each boiler.
Automatic Controls for Heating and Air Conditioning. Principles and Applications by K. M. Letherman, N. S. Billington