Document 99DJ47rY2KGRL58jyay7okkK5

American Society of Heating and Ventilating Engineers Guide, 1936 apparatus; or the actuator may be a diaphragm, bellows, or tube filled with a volatile liquid (Fig. 2) in such way that expansion and contraction with changes in temperature will operate the controlled apparatus by a direct mechanical, electric, or pneumatic connection. A room or wall thermostat in its simplest form contains a single tempera ture-sensitive element which is so set that it maintains, by actuating the controlled system, a single temperature. A variation of this is the twotemperature or dual thermostat having two temperature-sensitive ele ments, one of which is set for a higher temperature than the other. Such a thermostat is used on day-night systems where the night temperature Fig. 1. Typical Bi-Metallic Thermostatic Elements Fig. 2. Diaphragm Type Thermostat is to be lower than that maintained during the daytime hours. Switching the control from one element to the other is accomplished by an external or an internal switch, which can be operated manually or by a time device. Duct type thermostats are used in systems where the equipment must, respond to changes in the temperature of the air passing through a duct. In their usual form, these thermostats are so constructed that their switching mechanism is outside the duct, while the temperature-sensitive element projects inside into the air stream. . Thermostats which operate in liquids have the same general construc tion as duct thermostats except that the sensitive element is usually 270 Chapter 14--Temperature and Humidity Control enclosed in a tube to keep it from direct contact with the liquid. They are used in pipes, vats, and tanks, and are called immersion thermostats. Sometimes surface thermostats are used in place of duct or immersion thermostats. These devices, so constructed as to respond to changes in temperature of the surface of the duct or vessel containing a fluid, are clamped or screwed to such surfaces in a manner which will provide as rapid as possible heat transfer between the surface and the sensitive element. Other forms of thermostats are available for specialized purposes, but those described previously are the ones in general use for heating, venti lating and air conditioning systems. Apparatus Sensitive to Relative Humidity Devices which are responsive to changes in the relative humidity of the surrounding air, and which will motivate equipment to compensate for the changes, are called humidistats or hygrostats. These may vary, con siderably in their sensitive elements, but they all operate through con necting equipment which automatically causes humidifying apparatus to supply more or less moisture as required. Some of the more complicated ones contain essentially two thermostats, one working on a dry-bulb temperature and the other on a wet-bulb temperature; by proper inter connection of th$ parts they operate to maintain a definite relation be tween these two temperatures. Other devices use elements, directly sensitive to humidity, made of special wooden blocks, human hair, fiber, membranes, or strips of prepared paper. Hygrostats are available for, use with both electric and pneumatic control systems. Apparatus Which Operates Valves Apparatus which is so mechanically or electrically equipped that it will open and close valves, and possibly give them intermediate positions in any pipe line of a heating, ventilating, or air conditioning system, is termed a valve operator. The function of a valve operator is, essentially, to move the plunger of a valve in a manner required by its type and construction. For instance, in a single-seat valve, the disc is moved against the seat and held there with sufficient pressure to prevent flow. A three-way valve, however, requires a valve operator that will place the double disc, as required, between the two seats. Each type of valve has special characteristics to which a valve operator must be adapted. When a valve is used in shut-off service the valve operator simply opens the valve or closes it completely, as required. When the valve is to provide throttling service, a different type of valve operator is used so that the valve may be held at any intermediate position between open and closed. Valve operators use as their power source either compressed air (pneumatic system), electricity (motor-driven type or solenoid type), or a volatile liquid (direct-connected type). Apparatus Which Operates Dampers Apparatus which is so mechanically or electrically equipped as to open .and close dampers, and possibly give them intermediate positions, in accordance with the purposes of the system using the dampers is termed 271