Thermometer pressure differential

An exhaust fan in a constant-area well-insulated duct delivers air at an exit velocity of 1.5 m/s at a pressure differential of 6 cm H2O. Thermometers show the inlet and exit temperatures of the air to be 21.1°C and 22.8°C, respectively. The duct area is 0.60 m. Determine the actual power requirement for the fan. 

The great majority of automatic process control systems involve one or more of only five process variables, namely, pressure, temperature, flow rate, composition, and liquid level. Many of these variables are measured by the same kind of instrument, and indeed, all of them under certain circumstances can be evaluated in terms of pressures. Thus temperature can be measured by the pressure exerted by a confined gas in the gas thermometer the differential pressure across a restriction in a flow line is a measure of flow rate the pressure exerted by a boiling liquid mixture... 

The apparatus must be adequately insulated and the column jacket provided with the requisite heat compensation (section 7.7.3). The use of an automatic column head (section 7.5.3) allows the distillation to be performed with a minimum of attention. In the present case it would be useful to place a contact thermometer, connected to a bell via a relay (section 8.2.2), in the column head. The thermometer would be set to the boiling point of benzene, and when a signal was heard, the initial reflux ratio of 9 would be increased, say, to 20. In the same way it would be progressively raised to the final value of 50. The load may conveniently be regulated by means of a contact manometer actuated by the pressure differential (section 8.4.2). In accordance with the amount of the charge the still pot may be a pilot-plant flask (section 7.6.1) or a round-bottomed flask of 4 to 10 litres capacity, heated by a flask heater (section 7.7.1). 

A differential manometer of the Rayleigh type ( 14.VII A) was used in accurate measurements of the vapour pressure of water, and a simple apparatus for measuring the difference in vapour pressures of two liquids was described by Lord Kelvin. Stock s vapour pressure thermometer ( 19.V1 Q can be used to measure the vapour pressures of liquids, including those at low temperatures. Special techniques are necessary for very small vapour pressures,... 

Figure 3 shows the test section and instrumentation. Ten wall temperatures on the tube external surface were measured with 0.5 mm diameter calibrated type E thermocouples electrically insulated from the aluminium. Fluid inlet and outlet temperatures were measured with 1 mm diameter calibrated type K thermocouples. Cah-bration was carried out with a Rosemount 162-CE platinum thermometer. Due to the high thermal conduchvity of the aluminium and the low thickness of the tube walls the measured temperature is very close to the wall temperature in contact with the fluid (the difference less than 0.01 K). The inlet fluid pressure was measured with a calibrated Rosemount type 11 absolute pressure sensor. Two calibrated differential pressure sensors measured the pressure loss through the test section. A Rosemount Micro-motion coriolis flowmeter was used to... 

Filled-bulb temperature sensors are also widely used. An inert gas is enclosed in a constant-volume system. Changes in process temperature cause the pressure exerted by the gas to change. Resistance thermometers are used where accurate temperature or differential temperature measurement is required. They use the principle that the electrical resistance of wire changes with temperature. 

In practically all modem apparatus for laboratory distillation certain functions are now automatic. Heating is often controlled by contact thermometers, and the rate of evaporation of the charge with differential manometers. Reduced pressures are also frequently kept constant with automatic units. In fact, the present trend is towards fully automatic control of the conditions of distillation and simultaneous recording of the data measured, an object that has already been reached in many industrial installations. The ideal procedure is for the mixture to be merely introduced into the apparatus and the latter switched on it then automatically yields the required products of distillation and at the same time prepares a diagram of the top and bottom temperatures and other data that may be needed. 

Ensure all flow and differential pressure indicators etre zeroed and calibrated. All control room pressure indicators need to be checked with a calibrated gage. All dial thermometers and outside pressure gages should be removed and checked for accuracy. The unsatisfactory ones should either be replaced or removed prior to the test and read with calibrated thermocouples, thermometers, or gages during the test. These calibrated devices should adequately fit in the existing fittings. 

Connecticut) vapour pressure thermometer (up to 6(X) F), which could be supplied with recorder 4) a device based on differential expansion of metals (up to 1500 F), typically based on copper and iron, and available from H. W. Bulkley, Schaffer Budenberg, Edward Brown Co. (Philadelphia), and (Jueen Co. 5) Le Chatelier thermo-electric pyrometer, available from (Jueen Co. at 140 6) Siemens electrical resistance thermometer, as modified by H. L. Callendar (no supplier of this instrument is given) 7) and, although it did not give continuous measurements, the optical pyrometer of Nouel and Mesure, supplied by ()ueen Co. Although these were being sold by suppliers in the USA they were nearly all instruments of European origin. 

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