By Roger W. Haines
There are the explanation why now we have a brand new version each 4 or 5 years. the 1st is that expertise alterations. bankruptcy 10, on computer-based con trols, has needed to be nearly thoroughly rewritten. basics do not switch, however the instruments to be had to us do switch. evaluate and correct use of these instruments makes it much more central that we comprehend basics. a lot of our regulate difficulties stem from using new units as an answer to difficulties which are, in reality, regulate layout error. New devices, for instance, Direct electronic Controls (DDC), won't remedy uncomplicated difficulties and will even compound them. None-the-Iess, you will discover an intensive dialogue of DDC simply because i feel it's the possible "future" in HVAC regulate. however it needs to be utilized with an excellent figuring out of basics. the second one cause is that I continue to learn and want to cross on my new and more suitable figuring out to my readers. therefore you will discover a few small yet vital revisions, a dissertation on keep an eye on "modes," and a way more targeted dialogue of ways digital keep an eye on units paintings. There are a number of locations the place i've got corrected what I now understand to be blunders. I say sorry for those. i've been a lot inspired by means of the popularity of this e-book some time past, and that i desire that this re-creation might be beneficial. thanks to your support.
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Extra resources for Control Systems for Heating, Ventilating and Air Conditioning
2 Safety Controls are used in HVAC systems for the detection of abnormally high or low temperatures and for smoke detection. High temperature sensors or smoke detectors are required in most systems by NFPA and local codes. Low temperature sensors are used to prevent freezeup. A high temperature sensor will usually have a bimetal or rod-and-tube element designed for insertion in the supply or return air duct. Factory temperature settings of 125°F to 135°F are provided. If the air temperature exceeds the control setting a switch will open and remain open until the device is manually reset.
05 sec to 15 min or more. Pneumatic timers use the familiar solenoid coil principles but the movement of the solenoid is delayed by the diaphragm cover of a pneumatic chamber with a very small, adjustable leakport (Figure 3-9). In the on-delay sequence, when the coil is energized the solenoid pushes against the diaphragm, forcing the air out of the chamber. The leakport governs the rate of escape, and therefore the time delay. When the coil is deenergized, a check valve opens to allow the chamber to refill rapidly.
It is loosely pivoted in the center so that when it turns past center the weight of the mercury running to the low end causes it to pivot farther. The mercury bubble acts as a conductor to connect the electrodes. The mercury tube may also be used with a bourdon tube pressure sensor. The diaphragm movement of bulb and capillary sensors can be used to trip an electric switch. Bellows sensors can be used in the same manner. The tripping action can be direct or through a linkage. A bistable spring or overcenter mechanism is required to provide the snap action, as described above.