Room temperature control system

3 Examples of control systems Room temperature control system 

Steady eonditions will exist when the aetual and desired temperatures are the same, and the heat input exaetly balanees the heat loss through the walls of the building. 

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Fig. 1.7 Block diagram of room temperature control system.

In the on-off device, the sensing element is connected to the spirally wound tube (O) and changes in pressure cause the tube to move such that a lever (P) moves the indicator (L), which indicates the temperature of the system being measured (about 150°F in the figure). Attached to indicator (L) is a contact (A). Placed either side of (L) are moveable arms, (M) and (N), which indicate the minimum and maximum temperature deviation before some corrective action is taken. These two arms are connected to (B) and (C) which also carry electrical contacts. Thus if the temperature drops to 125°F then the contact (A) (on L) makes contact with (B) and presumably results in some form of heat being supplied to the system. On-off systems result in limit cycling, in which the controlled quantity oscillates between the upper and lower limits (many room temperature control systems still work on this principle, and if the upper and lower limit are too far apart one is alternately too hot and then too cold). 

Individual room temperature control is difficult with warm air systems, as any temperature control damper is likely to pass air at all times. Air thermostats are not accurate detectors of temperatures and temperature overshoot occurs, with consequent increases in energy consumption... 

Are vaporizers provided with automatic gas line shutoff valve, downstream pressure-reducing valve, gas flow control valve, temperature control system and interlocks to shut down gas flow on low vaporizer temperature, and appropriate alarms in a continuously manned control room ... 

Generally, the gel time is measured on a small amount of catalyzed resin. The sample is heated at a controlled temperature and probed until the resin does not exhibit any flow. The time between the point the temperature was applied (or the catalyst and resin mixed, if a room-temperature setting system) and the gel state is recorded as the gel time. 

A Constant Temperature Control System for the Range, Room Temperature to -320 F (1) 126... 

For room temperature control that is provided by a central heating system, the system consists of a room and heating system from a control point of view. The temperature measurement is located in the room whereas the manipulated variable is the gas valve of the heating furnace. 

In a typical experiment, the test sample and a suitable reference material are contained in two separate, identical ampoules kept at constant temperature in separate, identically constructed wells of the calorimeter. Ideally, the reference material is identical or very similar to the test sample in mass, heat capacity and thermal conductivity, but, unlike the test sample, it is thermally inert (i.e. the reference material will not undergo changes that result in heat production or absorption under the conditions of the experiment). One example is a small quantity of ordinary glass beads in air at room temperature used as reference for the same amoimt of a hydrated ceramic material which is expected to lose water under the same conditions. Consequently, most of the noise arising from temperature fluctuations is removed when the reference data are subtracted. A feedback temperature control system between the wells (a) serves to ensure that the temperature difference between the weUs is zero and (b) provides an output that measures any difference in electric power requirement of one well relative to the other, needed to keep the temperature of both weUs the same. This power difference, as a function of time, is the output from the calorimeter, which is recorded continuously or intermittently over the duration of the test. 

The SWNT ropes used were the same as those used for bundle strength measurements discussed earlier. The matrix material used was Epicote 1006 epoxy resin, a room temperature curing system. 32 dog-bone SWNT/epoxy specimens with dimensions of 40 mm x 3.5 mm x 0.4-0.6mm were obtained. The gauge length of die specimens was about 15 mm, and the length of SWNT ropes embedded in the epoxy was about 20 mm. Details of the fabrication method can be foimd elsewhere. The volume fraction of SWNT ropes in the composite was controlled widiin die range of 0.1-0.9%. Specimens were cyclically tested by an Instron 8800 Microforce Tester under tension-tension at 5 Hz, using a sinusoidal wave function at R ratio (ratio of minimum to maximum cyclic stress) of 0.1. 

Fermentation. The term fermentation arose from the misconception that black tea production is a microbial process (73). The conversion of green leaf to black tea was recognized as an oxidative process initiated by tea—enzyme catalysis circa 1901 (74). The process, which starts at the onset of maceration, is allowed to continue under ambient conditions. Leaf temperature is maintained at less than 25—30°C as lower (15—25°C) temperatures improve flavor (75). Temperature control and air diffusion are faciUtated by distributing macerated leaf in layers 5—8 cm deep on the factory floor, but more often on racked trays in a fermentation room maintained at a high rh and at the lowest feasible temperature. Depending on the nature of the leaf, the maceration techniques, the ambient temperature, and the style of tea desired, the fermentation time can vary from 45 min to 3 h. More highly controlled systems depend on the timed conveyance of macerated leaf on mesh belts for forced-air circulation. If the system is enclosed, humidity and temperature control are improved (76). 

This is used when (1) the room needs heating instead of cooling or (2) for reheat as described above. It is vital in close control systems that its capacity is sufficient to maintain room temperature under these conditions, otherwise the system may fall into a loop, with the controls continuing to see high rh due to temperature. Using only part of the cooling coil for dehumidification will alleviate this situation. A heater capacity of the sensible heat extracted during dehumidification plus half the peak winter fabric loss is recommended where the room load could be nil in winter such as a start-up situation. 

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