Ulvac CRYO-U Series Instruction Manual page 94

Principle of Refrigerator Operation
G-M Cycle
Gifford and McMahon developed the concept of G-M cycle in the late 1950's. In the
driving method of G-M cycle, there are a way which drives mechanically and a way
which drives using the differential-pressure of the operational gas. Since the GM
cycle is efficient, it can make drive speed late. Moreover, the load concerning the seal
currently used for an inside is also light. Therefore, it is a highly efficient and
reliable refrigeration cycle. In this manual, the refrigeration cycle with the
mechanical driving system adopted by ULVAC CRYOGENICS INC is explained.
Figure E-2 shows the principle of G-M cycle & P-V chart (P: pressure, V: volume in the
expansion chamber).
Ａ The displacer is first positioned at the bottom of the cylinder.
The low-pressure valve is closed and the high- pressure valve is opened.
a The compressed helium is introduced into the warm end and the cold end (the
expansion chamber) of the cylinder.
Ｂ Pressure inside the cylinder increases.
b When moving a displacer up, the clod end (expansion room) is filled up with the
helium gas of room temperature, being cooled by the regenerator.
Ｃ The volume of the cold end (the expansion chamber) is now maximum.
At this time the high- pressure valve is closed and low-pressure exhaust valve opened.
c The compressed helium in the cold end (the expansion chamber) is expelled through
the regenerator causing a temperature decrease by Simon expansion.
Ｄ The cold end obtains the lowest pressure.
d The displacer is moved to the initial lower position and the low-temperature helium is
transferred to the compressor. The temperature of the gas returns to room temperature
by heat exchange between the regenerator.
Ａ One cycle of the helium gas circulation is completed.
The P-V diagram of idealized G-M cycle shows a quadrangle.
E-2 ULVAC CRYOGENICS INCORPORATED