Types of cryopump

Depending on the cooling principle a difference is made between 

In the case of bath cryostats - in the most simple case a cold trap filled with LNj (liquid nitrogen) - the pumping surface is cooled by direct contact with a liquefied gas. On a surface cooled with LNj (T = 77 K) HjO and COj are able to condense. On a surface cooled to = 10 K all gases except He and Ne may be pumped by way of condensation. A surface cooled w/ith liquid helium (T = 4.2 K) is capable of condensing all gases except helium. 

In continuous flow cryopumps the cold surface is designed to operate as a heat exchanger. Liquid helium in sufficient quantity is pumped by an auxiliary pump from a reservoir into the evaporator in order to attain a sufficiently low temperature at the cold surface (cryopanel). 

The liquid helium evaporates in the heat exchanger and thus cools dovm the cryopanel. The waste gas which is generated (He) is used in a second heat exchanger to cool the baffle of a thermal radiation shield vi/hich protects the system from thermal radiation coming from the outside. The cold helium exhaust gas ejected by the helium pump is supplied to a helium recovery unit. The temperature at the cryopanels can be controlled by controlling the helium flow. 

Today refrigerator cryopumps are being used almost exclusively (cold upon demand). These pumps operate basically much in the same way as a common household refrigerator, whereby the following thermodynamic cycles using helium as the refrigerant may be employed  

---

Thus for a rocket impulse I of 250, approximately 10.5 kw of refrigeration capacity are required per pound of rocket thrust. However, this constant-flow approach may not be necessary in most applications since the rocket flow is usually of short duration and thus amenable to a heat sink type of cryopump, as described by Wallace and Rogers [2]. 

---