International
Tables for
Crystallography
Volume F
Crystallography of biological macromolecules
Edited by E. Arnold, D. M. Himmel and M. G. Rossmann

International Tables for Crystallography (2012). Vol. F, ch. 10.1, pp. 244-245   | 1 | 2 |

Section 10.1.3. Principles of cooling equipment

H. Hopea* and S. Parkinb

aDepartment of Chemistry, University of California, Davis, One Shields Ave, Davis, CA 95616–5295, USA, and bDepartment of Chemistry, University of Kentucky, Lexington, Kentucky, USA
Correspondence e-mail:  hhope@ucdavis.edu

10.1.3. Principles of cooling equipment

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There are many ways to construct a low-temperature apparatus based on the cold-stream principle that functions well, but they are all made according to a small number of basic principles.

All gas-stream crystal-cooling devices must have three essential components: (a) a cold gas supply, (b) a system of cold gas delivery to the crystal, and (c) a system for frost prevention at the crystal site.

10.1.3.1. Liquid-nitrogen-based cold gas supply

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Historically, two methods were commonly used: generation of gas by boiling liquid N2 with an electrical heater, and cooling of a gas stream in a liquid-N2 heat exchanger. The currently common methods are boiling, and cooling of the gas by means of a refrigerator.

Because precise voltage and current control are easily realized, the boiler method has the advantage of providing very accurate control of the flow rate with minimal effort. Precise control of the flow rate is typically not attained when the rate is controlled with standard gas-flow regulators, because they control volume, not mass.

In addition to control of the flow rate, precise control of the temperature requires exceptional insulation for the cold stream. The longer the stream path, the higher the requirements for insulation. As a rule, temperature rise during transfer should not exceed 15 K at a flow rate of 0.2 mol N2 min−1; preferably, it should be significantly lower. Higher cooling loss leads to excessive coolant consumption and to instability caused by changes in ambient temperature. High flow rates may also tend to cause undesirable cooling of diffractometer parts.

Appropriate insulation can be readily attained either with silvered-glass Dewar tubing or with stainless-steel vacuum tubing. Glass has the advantage of being available from local glassblowing shops; it generally provides excellent insulation. The main disadvantages are fragility and a rigid form that makes accurate positioning of the cold stream difficult. Stainless steel can provide superb insulation, given an experienced manufacturer. A major advantage is the availability of flexible transfer lines that greatly simplify the positioning of the cold stream relative to the diffractometer.

10.1.3.2. Liquid-helium-based cold gas supply

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Open-stream cooling devices that can reach temperatures around 5 K are now commercially available. In principle, the design is simpler than that for liquid-nitrogen-based devices. A basic cooling apparatus consists of a liquid-helium transfer line from a pressurized delivery tank, an evaporation chamber and a cold gas delivery tube. The transfer line is an insulated capillary tube. The delivery tube is an insulated stretch of vacuum tubing with an electrically heated nozzle at the exit. The helium flow is controlled with a needle valve. Because of thermal loss, the flow rate also largely determines the temperature in the range below 20 K. Above about 20 K, an in-stream heating element is used for additional temperature control. This is necessary, because if the flow rate is too low, the cooling stream becomes unstable and will not reliably cover the sample.

For a given setting of the flow control valve, the flow rate depends on the tank pressure. For a constant temperature, a constant pressure is required. The pressure is controlled with a pressure regulator that can reduce the tank pressure by releasing helium gas or raise it by adding helium gas from an external helium supply source. A typical delivery tank pressure is around 20 kPa. It is very important that the pressure is constant. The precise value attained is less important.

At atmospheric pressure, helium at around 40 K has the same density as room-temperature air. This means that very cold helium will rise in air, and has the capacity to seriously cool instrument parts in its way. Goniometer heads, beam stops and beam collimators are particularly vulnerable. A simple remedy is to use a small fan to mix the cold helium with room air.

10.1.3.3. Frost prevention

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Three areas must be kept frost-free: the crystal, the crystal mount and the delivery end of the transfer tube. The first successful solution to this problem was the dual-stream design of Post et al. (1951[link]). It provides for a cold stream surrounded by a concentric warm stream. If the warm stream is sufficiently dry, this will prevent frost around the outlet. The crystal will remain frost-free only if mixing of the two flows occurs downstream from the crystal. For a stream aligned with the axis of the goniometer head, an additional shield is needed to keep the goniometer head frost-free.

References

Post, B., Schwartz, R. S. & Fankuchen, I. (1951). An improved device for X-ray diffraction studies at low temperatures. Rev. Sci. Instrum. 22, 218–220.








































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