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Unformatted text preview: 1-1 1-2 Disturbance variable-QL (heat lost to surroundings); other possible sources of disturbances are the loss of gas pressure and the outside door opening. Specific disturbances include change in outside temperature, change in outside wind velocity (external heat transfer coefficient), the opening of doors or windows into the house, the number of people inside (each one generating and transmitting energy into the surrounding air), and what other electric lights and appliances of any nature are being used. 1.3 The ordinary kitchen oven (either electric or gas), the water heater, and the furnace (Ex. 1.2) all work similarly, generally using a feedback control mechanism and an electronic on-off controller. For example, the oven uses a thermal element similar to a thermocouple to sense temperature; the sensor's output is compared to the desired cooking temperature (input via dial or electronic set-point/display unit); and the gas or electric current is then turned on or off depending on whether the temperature is below or above the desired value. Disturbances include the introduction or removal of food from the oven, etc. A non-electronic household appliance that utilizes built-in feedback control is the water tank in a toilet. Here, a float (ball) on a lever arm closes or opens a valve as the water level rises and falls above the desired maximum level. The float height represents the sensor; the lever arm acting on the valve stem provides actuation; and the on-off controller and its set point are built into the mechanical assembly....
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This note was uploaded on 04/10/2008 for the course CHE 242 taught by Professor Cummings during the Spring '08 term at Vanderbilt.
- Spring '08