To: file

From: Jon Hagen

Date: 10-21-02

Subject: Gifford McMahon Cycle

Looking at CTI brochure we see

1 displacer at bottom of cylinder.

2 high pressure valve opens

3 displacer rises and high pressure gas fills space below displacer with cold gas (cooled by regen)

4 high pressure valve closes

5low pressure valve opens

6 GAS BELOW DISPLACER EXPANDS, FLOWING UP THROUGH REGENERATOR. THE EXPANDING GAS COOLS THE LOWER CYLINDER WALL AND REMOVES HEAD FROM THE REGENERATOR ON ITS WAY THROUGH.

7displacer moves back to the bottom of the cylinder, forcing the remaining low pressure gas through the regenerator.

8. low pressure valve closes (note that this could have been done before step 7).

Note that the cooling is not produced by the gas doing work against a piston, but simply by the gas expanding into the low pressure side of the pump. Cooling by free expansion. Could the GM cycle be designated a "free expansion refrigeration cycle?" CTI says "system is said to use a 'no- work' cycle."

Exhaust gas: hotter or colder than input gas?

Take the startup condition; everything is at room temperature.

The displacer is down and the high pressure valve has been opened to put high pressure gas in the space above the regen/displacer.

The displacer moves up, allowing the high pressure gas to occupy the space opened up.

No heat flow has occurred yet. No temperature differences.

Now the low pressure valve is opened, allowing gas to 'free expand' out of the space below the regen/displacer. As the pressure in this space lowers, the temp also lowers, because the fast molecules have selectively exited. This being the case, the exhause gas will be hotter than room temperature.

Eventually, when the refrigerator has cooled down to its ultimate low temperature, the amount of heat carried off by the exhaust will be small, and the exhaust gas will be only slightly warmer than the intake gas.

Now the high pressure valve closes and the low pressure valve opens