Microsoft Word - Cengel and Boles TOC _2-03-05_.doc

3 Throttling Valves

Throttling valves are any kind of flow-restricting devicesthat cause a signif-
icant pressure drop in the fluid. Some familiar examples are ordinary
adjustable valves, capillary tubes, and porous plugs (Fig. 5–29). Unlike tur-
bines, they produce a pressure drop without involving any work. The pres-
sure drop in the fluid is often accompanied by a large drop in temperature,
and for that reason throttling devices are commonly used in refrigeration
and air-conditioning applications. The magnitude of the temperature drop
(or, sometimes, the temperature rise) during a throttling process is governed
by a property called the Joule-Thomson coefficient, discussed in Chap. 12.
Throttling valves are usually small devices, and the flow through them
may be assumed to be adiabatic (q0) since there is neither sufficient time
nor large enough area for any effective heat transfer to take place. Also,
there is no work done (w0), and the change in potential energy, if any, is
very small (pe 0). Even though the exit velocity is often considerably
higher than the inlet velocity, in many cases, the increase in kinetic energy
is insignificant (ke 0). Then the conservation of energy equation for this
single-stream steady-flow device reduces to

(5–41)

That is, enthalpy values at the inlet and exit of a throttling valve are the
same. For this reason, a throttling valve is sometimes called an isenthalpic
device. Note, however, that for throttling devices with large exposed surface
areas such as capillary tubes, heat transfer may be significant.
To gain some insight into how throttling affects fluid properties, let us
express Eq. 5–41 as follows:

or

Thus the final outcome of a throttling process depends on which of the two
quantities increases during the process. If the flow energy increases during
the 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 a
drop in temperature. If the product Pvdecreases, the internal energy and the
temperature of a fluid will increase during a throttling process. In the case
of an ideal gas,hh(T), and thus the temperature has to remain constant
during a throttling process (Fig. 5–30).

EXAMPLE 5–8 Expansion of Refrigerant-134a in a Refrigerator

Refrigerant-134a enters the capillary tube of a refrigerator as saturated liquid

at 0.8 MPa and is throttled to a pressure of 0.12 MPa. Determine the quality

of the refrigerant at the final state and the temperature drop during this

process.

Solution Refrigerant-134a that enters a capillary tube as saturated liquid is

throttled to a specified pressure. The exit quality of the refrigerant and the

temperature drop are to be determined.

Internal energyFlow energyConstant

u 1 P 1 v 1 u 2 P 2 v 2

h 2 h 1 ¬¬ 1 kJ>kg 2

Chapter 5 | 239

(a) An adjustable valve

(b) A porous plug

(c) A capillary tube

FIGURE 5–29

Throttling valves are devices that

cause large pressure drops in the fluid.

Throttling

valve

IDEAL

GAS

T 1 T 2 = T 1

h 1 h 2 = h 1

FIGURE 5–30

The temperature of an ideal gas does

not change during a throttling (h

constant) process since hh(T).