Thermodynamics

3 Throttling ValvesThrottling valves are any kind of flow-restricting devices that cause a significantpressure drop in the fluid. Some familiar examples are ordinaryadjustable valves, capillary tubes, and porous plugs (Fig. 5–29). Unlike turbines,they produce a pressure drop without involving any work. The pressuredrop in the fluid is often accompanied by a large drop in temperature,and for that reason throttling devices are commonly used in refrigerationand air-conditioning applications. The magnitude of the temperature drop(or, sometimes, the temperature rise) during a throttling process is governedby a property called the Joule-Thomson coefficient, discussed in Chap. 12.Throttling valves are usually small devices, and the flow through themmay be assumed to be adiabatic (q 0) since there is neither sufficient timenor large enough area for any effective heat transfer to take place. Also,there is no work done (w 0), and the change in potential energy, if any, isvery small (pe 0). Even though the exit velocity is often considerablyhigher than the inlet velocity, in many cases, the increase in kinetic energyis insignificant (ke 0). Then the conservation of energy equation for thissingle-stream steady-flow device reduces to(5–41)That is, enthalpy values at the inlet and exit of a throttling valve are thesame. For this reason, a throttling valve is sometimes called an isenthalpicdevice. Note, however, that for throttling devices with large exposed surfaceareas such as capillary tubes, heat transfer may be significant.To gain some insight into how throttling affects fluid properties, let usexpress Eq. 5–41 as follows:orh 2 h 1 1kJ>kg2u 1 P 1 v 1 u 2 P 2 v 2Internal energy Flow energy ConstantThus the final outcome of a throttling process depends on which of the twoquantities increases during the process. If the flow energy increases duringthe process (P 2 v 2 P 1 v 1 ), it can do so at the expense of the internal energy.As a result, internal energy decreases, which is usually accompanied by adrop in temperature. If the product Pv decreases, the internal energy and thetemperature of a fluid will increase during a throttling process. In the caseof an ideal gas, h h(T ), and thus the temperature has to remain constantduring a throttling process (Fig. 5–30).Chapter 5 | 239(a) An adjustable valve(b) A porous plug(c) A capillary tubeFIGURE 5–29Throttling valves are devices thatcause large pressure drops in the fluid.IDEALGASThrottlingvalveT 1 T 2 = T 1h 1h 2 = h 1FIGURE 5–30The temperature of an ideal gas doesnot change during a throttling (h constant) process since h h(T).EXAMPLE 5–8Expansion of Refrigerant-134a in a RefrigeratorRefrigerant-134a enters the capillary tube of a refrigerator as saturated liquidat 0.8 MPa and is throttled to a pressure of 0.12 MPa. Determine the qualityof the refrigerant at the final state and the temperature drop during thisprocess.Solution Refrigerant-134a that enters a capillary tube as saturated liquid isthrottled to a specified pressure. The exit quality of the refrigerant and thetemperature drop are to be determined.