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The closed cycle refrigerator APD DE202 is a two-stage cryogenic refrigerator that operates in the Gifford-McMahon refrigerator cycle. It uses helium gas from a helium compressor to produce the cold temperatures.

In this type of refrigerators the compressor unit is separated from the cold head and connected via flexible pressure tubings or via rigid pressure tubings. Connection is made via self-sealing screw-couplings, virtually excluding any loss of helium gas even when screwed on and off repeatedly. The refrigerators can be used to cool cryo-pumps or to cool samples in cryostats wherever temperatures in the range between 20 K and 300 K are to be achieved.

The great advantage of the Gifford-McMahon principle is that compressor unit and cold head (expansion unit) are separated. This provides higher flexibility as the lighter weight cold head can be operated in any mounting position.

The following figure is an idealized representation of the Gifford-McMahon Pinciple for a single-stage cold head. The regenerator is drawn outside the displacer while in reality it is integrated in the displacer. The high-pressure and low-pressure connections of the compressor unit are connected to the cold head via pressure tubings. By means of a valve control either high-pressure or low-pressure helium gas is led to the expansion volume via the regenerator. The regenerator is a heat exchanger where the entering warm helium gas is precooled to the temperature to which the regenerator was cooled by the expanded gas leaving the expansion chamber.

Simplified, the Gifford-McMahon principle can be divided into four steps. The displacer is on the left-hand side of the expansion Volume V which in this position corresponds to the dead volume. The low-pressure side is blocked by the respective valve position, the high-pressure side is opened. In the pV diagram this means a rise from low pressure pL toward high pressure pH at constant minimum volume Vmin The displacement piston is drawn back, with the high-pressure valve open, whereby the volume V, at constant pressure pH increases to its maximum value Vmax. Now the high-pressure valve H is closed and the low-pressure valve L is opened. The helium gas expands from pressure pH to pressure pL at constant volume Vmax. Now, at constant pressure pL, the displacer is brought to its initial position whereby the helium gas cycle is completed. The theoretical value of the transferred amount of heat during one cycle corresponds to the rectangular area in the pV diagram: Qideal = (Vmax - Vmin) (pH - pL) In the cold heads the high-pressure or low-pressure helium gas is controlled by means of a rotary valve fitted on the shaft of an electric synchronous motor. The rotary valve rotates on the control disk. The displacer with integrated regenerator is pneumatically actuated by a control piston. Through additional holes in the control disk the control chamber is supplied alternatingly with high pressure and low pressure by means of the control piston. The operating frequency is around 2 Hz.

 

Specifications:

  • Temperature range:   20 - 300 K
  • Cooldown time :   70 min
  • Weight:   6.8 kg
  • Temperature controller:  LakeShore
  • Temperature sensors: Pt100 (close to heater) + Cernox (close to sample)  

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