Throttle coefficient

THERMAL PROPERTIES OF PROPANE. HEAT CAPACITY, JOULE-THOMSON COEFFICIENT, ISOTHERMAL THROTTLING COEFFICIENT, AND LATENT HEAT OF VAPORIZATION. FROM PROCEEDINGS OF THE 4TH SYMPOSIUM ON THERMOPHYSICAL PROPERTIES, UNIV. MARYLAND COLLEGE PARK, MD. 

Because both machines operate with the same throttle coefficient H AP... 

Steam turbine, 53, 146, 282-92, 179 back pressure, 282 blade deposits, 479 condensing, 282 efficiency, 288 extraction, 282 induction-type, 282 paitial admission, 288 rating, 290 reliability, 478 selecuon variable, 275, 285 speed, 278 stage losses, 286 steam temperatures, 284 steam velocity, 288 trip and throttle valve. 479 Step unloading system, 80 Stiffness coefficients, 385 Stodola slip, 153, 155 Stonewall, 186 Straight labyrinth. seal leakage, 532... 

The normal practice in the design of forced-convection reboilers is to calculate the heat-transfer coefficient assuming that the heat is transferred by forced convection only. This will give conservative (safe) values, as any boiling that occurs will invariably increase the rate of heat transfer. In many designs the pressure is controlled to prevent any appreciable vaporisation in the exchanger. A throttle value is installed in the exchanger outlet line, and the liquid flashes as the pressure is let down into the vapour-liquid separation vessel. 

Figure 3.14. The lower ends of fractionators, (a) Kettle reboiler. The heat source may be on TC of either of the two locations shown or on flow control, or on difference of pressure between key locations in the tower. Because of the built-in weir, no LC is needed. Less head room is needed than with the thermosiphon reboiler, (b) Thermosiphon reboiler. Compared with the kettle, the heat transfer coefficient is greater, the shorter residence time may prevent overheating of thermally sensitive materials, surface fouling will be less, and the smaller holdup of hot liquid is a safety precaution, (c) Forced circulation reboiler. High rate of heat transfer and a short residence time which is desirable with thermally sensitive materials are achieved, (d) Rate of supply of heat transfer medium is controlled by the difference in pressure between two key locations in the tower, (e) With the control valve in the condensate line, the rate of heat transfer is controlled by the amount of unflooded heat transfer surface present at any time, (f) Withdrawal on TC ensures that the product has the correct boiling point and presumably the correct composition. The LC on the steam supply ensures that the specified heat input is being maintained, (g) Cascade control The set point of the FC on the steam supply is adjusted by the TC to ensure constant temperature in the column, (h) Steam flow rate is controlled to ensure specified composition of the PF effluent. The composition may be measured directly or indirectly by measurement of some physical property such as vapor pressure, (i) The three-way valve in the hot oil heating supply prevents buildup of excessive pressure in case the flow to the reboiier is throttled substantially, (j) The three-way valve of case (i) is replaced by a two-way valve and a differential pressure controller. This method is more expensive but avoids use of the possibly troublesome three-way valve.

There are two variations of the basic set-up of the Joule-Thomson experiment which both yield practical information. In the isothermal Joule-Thomson experiment the temperature is held constant with a downstream heater, and the resultant heat input for the pressure decrease permits an experimental evaluation of (8H/8P)T, the isothermal Joule-Thomson coefficient. In the other variation there is no throttling device used, and the pressure is held constant. For the steady-state flow of gas the temperature change is measured for measurable inputs of heat. This experiment, of course, yields (8H/8T)P, or CP. Thus, the variations of this constant-flow experiment can yield all three of the important terms in Equation (7.46). 

The vapor-compression cycle incorporating an expansion valve is shown in Fig. 9.1h, where line A- 1 represents the constant-enthalpy throttling process. Line 2 + 3, representing an actual compression process, slopes in the direction of increasing entropy, reflecting the irreversibility inherent in the process. The dashed line 2 - 3 is the path of isentropic compression (see Fig. 7.6). For this cycle, the coefficient of performance is simply... 

The effectiveness of a refrigeration cycle is measured by its coefficient of performance. For given values of Tc and TH, the highest possible value is attained by the Carnot refrigerator. The vapor-compression cycle with reversible compression and expansion approaches this upper limit. A vapor-compression cycle with expansion in a throttle valve has a somewhat lower value, and this is reduced further when compression is not isentropic. The following example provides an indication of the magnitudes of coefficients of performance. 

The advantages of steam are the excellent heat transfer coefficient (>6000 W/(m K)), the constant temperature over the whole exchange area and the fast and convenient regulation of heating by means of a throttle valve. Lowering the steam pressure leads to lower condensation temperatures [13]. 

The quantity dT/dP)u is called the Joule-Thomson coefficient, and is represented by the symbol mj.t. it is equal to the rate of change of temperature with the pressure in a streaming process through a plug or throttle. According to equation (9.11), dH/d2 )p is the heat capacity of the gas at constant pressure, i.e., Cp, so that (11.4) is equivalent to... 

Equation (11.6) is quite general and should apply to any gas/ for its derivation is based entirely on the first la>y of thermodynamics without assuming any specific properties of the system. However, for an ideal gas, (dE/dV)r is zero, as seen earlier, and since PV = /2T, it follows that td PV)/dP ]T is also zero hence, since Cp is finite, it is seen from equation (11.6) that for an ideal gas mj.t. must be zero.f The Joule-Thomson coefficient of an ideal gas should thus be zero, so that there should be no change of temperature when such a gas e.xpands through a throttle. J... 

Because of the variation of the Joule-Thomson coefficient with both temperature and pressure it is not easy to calculate the change of temperature resulting from a given throttled expansion, even when such data as in Table IV are available. This can be done, however, by a series of approximations. By estimating a rough average for the Joule-Thomson coefficient, some indication of the fall of temperature can be obtained. 

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