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

Assumptions 1 Heat transfer from the tube is negligible. 2 Kinetic energy

change of the refrigerant is negligible.

Analysis A capillary tube is a simple flow-restricting device that is commonly

used in refrigeration applications to cause a large pressure drop in the refrig-

erant. Flow through a capillary tube is a throttling process; thus, the enthalpy

of the refrigerant remains constant (Fig. 5–31).

At inlet:

At exit:

Obviously hfh 2 hg; thus, the refrigerant exists as a saturated mixture at

the exit state. The quality at this state is

Since the exit state is a saturated mixture at 0.12 MPa, the exit temperature

must be the saturation temperature at this pressure, which is 
22.32°C.

Then the temperature change for this process becomes

Discussion Note that the temperature of the refrigerant drops by 53.63°C

during this throttling process. Also note that 34.0 percent of the refrigerant

vaporizes during this throttling process, and the energy needed to vaporize

this refrigerant is absorbed from the refrigerant itself.

4a Mixing Chambers

In engineering applications, mixing two streams of fluids is not a rare

occurrence. The section where the mixing process takes place is commonly

referred to as a mixing chamber.The mixing chamber does not have to be

a distinct “chamber.” An ordinary T-elbow or a Y-elbow in a shower, for

example, serves as the mixing chamber for the cold- and hot-water streams

(Fig. 5–32).

The conservation of mass principle for a mixing chamber requires that the

sum of the incoming mass flow rates equal the mass flow rate of the outgo-

ing mixture.

Mixing chambers are usually well insulated (q0) and usually do not

involve any kind of work (w0). Also, the kinetic and potential energies

of the fluid streams are usually negligible (ke 0, pe 0). Then all there

is left in the energy equation is the total energies of the incoming streams

and the outgoing mixture. The conservation of energy principle requires that

these two equal each other. Therefore, the conservation of energy equation

becomes analogous to the conservation of mass equation for this case.

¢TT 2 
T 1  1 
22.32
31.31 2 °C53.63°C

x 2 

h 2 
hf

hfg



95.47
22.49

236.97
22.49

0.340

P 2 0.12 MPa

(h 2 h 1 )

¡

hf22.49 kJ/kg

hg236.97 kJ/kg

Tsat
22.32 C

P 1 0.8 MPa

sat. liquid

f

T 1 Tsat @ 0.8 MPa31.31 C

h 1 hf @ 0.8 MPa95.47 kJ/kg

(Table A–12)

240 | Thermodynamics

Throttling

valve

u 1 = 94.79 kJ/kg

P 1 v 1 = 0.68 kJ/kg

(h 1 = 95.47 kJ/kg)

u 2 = 88.79 kJ/kg

P 2 v 2 = 6.68 kJ/kg

(h 2 = 95.47 kJ/kg)

FIGURE 5–31

During a throttling process, the
enthalpy (flow energy internal
energy) of a fluid remains constant.
But internal and flow energies may be
converted to each other.

Hot

water

Cold

water

T-elbow

FIGURE 5–32

The T-elbow of an ordinary shower
serves as the mixing chamber for the
hot- and the cold-water streams.