Counterflow Heat Exchanger forCryogenic NMR Spectroscopy
Many areas of NMR spectroscopy require the ability to study a sample at different temperatures. Specifically in solid-state NMR spectroscopy conducting experiments at low-temperatures allows characterizing the dynamic properties of the sample. Gas chillers are often used to create a variable temperature (VT) gas stream to cool the sample. However, the lowest temperature these devices typically can achieve is about -90ºC (183 K). To achieve lower temperatures typically a dry nitrogen gas stream that is cooled in a heat exchanger using liquid nitrogen (LN2) is used.
Different design for heat exchangers have been developed over the years, with the most common one using copper coils immersed in liquid nitrogen. But, this design has a massive drawback. At high flow rates, which require increased gas pressures, the VT gas is easily liquified inside the immersion coils. This leads to a discontinous gas flow. In the worst case scenario, LN2 will sputter out of the VT line and onto the rotor. This can easily cause instabilities of the rotor rotor crashes.
A counter-flow heat exchanger for efficient sample cooling
Unlike using immersion coils to cool the VT gas, the Bridge12 heat exchanger employes two gas streams that flow in opposite directions. In one direction, a vacuum pump pulls liquid nitrogen through a capillary from a storage dewar into the heat exchanger. The LN2 evaporates and the cold nitrogen gas cools the VT gas, which is travelling in the opposite direction. Depending on the vacuum and therefore the amount of LN2 pulled through the capillary, the temperature of the VT gas can be controlled from room temperature (RT) down to < 90 K. The storage dewar is not pressurized, and refilling the dewar can be done at any time. The entire heat exchanger assembly is thermally insulated to guarantee efficient operation and minimal cryogen consumption.
1.8 l/hr LN2 to maintain a VT temperature of 86 K
The heat exchanger is designed to operate very efficiently. At the lowest temperature of 86 K, and a VT gas flow rate of 50 scfh (23.6 l/min) only 1.8 l/hr of LN2 is consumed, corresponding to about 45 scfh (21.2 l/min) of nitrogen gas and a 50 l storage dewar is sufficient for 24+ hrs of operation. The output temperature of the VT gas can be regulated by adjusting the VT gas flow rate and the amount of liquid nitrogen pulled by the vacuum (see back page). Once a stable flow rate is established, fluctuations are very small. To accurately set and stabilize the VT gas temperature, the built-in heater element of the NMR probe is used.
The Bridge12 counterflow heat exchanger has a very flexible design to accommodate an individual researcher’s requirements. It can be equipped with a cryogenic bayonet fitting to connect individual transfer lines (as shown in the photograph). Alternately, the heat exchanger can be equipped with a transfer line directly connecting to the NMR probe. Optionally, an LN2 level sensor is available to monitor the cryogen level inside the storage dewar and to aide automatic refills for long-term operation.