Page 70 - 4498

throttling valve. After compression in the compressor (1–2), the gas is
successively cooled in the heat exchangers (2–3-4) and then expanded
(throttled) through the valve (4–5). Here, part of the gas is liquefied and
accumulates in a collector, and the unliquefied gas is passed into the heat

exchangers, where it cools fresh batches of compressed gas. To liquefy a
gas by a throttling cycle, the temperature of the compressed gas before
admission to the main heat exchanger H3 must be lower than the inversion

temperature. A heat exchanger with a foreign cooling agent (exchanger
H2) performs this cooling. If the inversion temperature of the gas lies
above room temperature (nitrogen, argon, oxygen), then the scheme is
basically feasible even without heat exchangers H1 and H2. The use of

foreign coolants in this case has the purpose of increasing the yield of
liquid. But if the inversion temperature of the gas is below room
temperature, then a heat exchanger with a foreign coolant is mandatory.

For example, liquid nitrogen is used in the liquefaction of helium.
For liquefaction of gases on an industrial scale, cycles with
expanders are most often used, since the expansion of gases with the

performance of
external work is the
most efficient cooling

method. Scheme and
temperature-entropy
diagram for a gas-
liquefaction cycle

using an expander: (C)
compressor, (E)
expander, and (Th)

throttling valve are
illustrated Fig. 3.4.3
Here, the liquid
usually is not obtained
Figure 3.4.3
in the expander itself
because it is
technically simpler to

carry out the liquefaction in an additional throttling stage. After
compression in a compressor (1–2) and precooling in a heat exchanger (2–
3), the stream of compressed gas is divided into two parts. Part M is

diverted to the expander where, on expanding, it performs external work
and is cooled (3–7). The cooled gas is fed into the heat exchanger, where it

70