CryoTel Cryocooler FAQs
Q. What is the minimum level of vibration that can be achieved?
A. System vibration levels are greatly influenced by the mass, configuration, and rigidity of the system in which the cryocooler is installed. Having established that important point we can discuss vibration of the cryocooler as a free-body. Sunpower provides a passive (reactive) balance absorber that is tuned to mitigate the primary drive frequency of the cooler. With generic production tuning, the resultant free-body vibration acceleration is approximately 300-400 milli-g (acceleration). This level of vibration is tolerable in many applications. When lower levels of vibration are required, Sunpower offers several options for further reducing vibration levels:
- In-situ tuning of the primary passive absorber - Tuning the passive absorber when the cryocooler is mounted in the customer’s system allows for addressing and mitigating the effects of the system design on the total system vibration.
- Second harmonic passive absorber – Sunpower engineering can work with system designers to tune a dual-frequency absorber assembly that includes a first and second harmonic absorber. Vibration levels can reach 200 milli-g and lower.
- Active absorber – The active (powered) absorber is an add-on unit that attaches to the back of the cryocooler. The unit can be driven by a closed loop driver that reads a vibration signal and provides a drive signal to the absorber to counteract the vibration energy. Vibration levels on the order of 40 milli-g can be achieved. Historically Sunpower has not provided the drive system for the active absorber; customers preferred to design their own system.
Sunpower is taking steps to be a leader in the low-vibration application of Stirling cryocoolers. To that end Sunpower has invested in a new vibration laboratory setup with the sole purpose of developing methods of reducing the vibration signature of our cryocoolers. We already have some exciting new developments that we believe will take us a long way towards making vibration a non-issue for the CryoTel® cryocoolers.
Q. At temperature, my steady state heat load is low. Can I use a higher capacity cryocooler to reduce the cool down time?
A. The CryoTel® cryocoolers have a minimum power operating threshold. The controller establishes the minimum level and will not allow the cryocooler to operate below this point. This is necessary in order to maintain the critical bearing system that is used for the moving parts. If the minimum cooling power (lift) of the cryocooler model at the target temperature is greater than the thermal load of the user’s device, the cryocooler system will not be able to maintain the target temperature. The load temperature will drift down until equilibrium is reached between the thermal load and the cryocooler lift. Note: the cryocooler lift reduces as the cold tip temperature gets colder.
Q. Can the cryocooler operate in a vacuum?
A. Yes the cryocooler can operate inside a vacuum chamber. The controller, however, due to some components is not rated for vacuum.
Q. The large diameter section of the cryocooler gets very hot to the touch. Is this a problem?
A. We refer to this section as the “back end” of the cryocooler and it is where the motor is housed. The cryocooler can operate indefinitely with the back end at 80ºC or below without damaging the cryocooler. However, performance of the cryocooler will be affected by elevated temperatures. We recommend that, when possible, some form of active cooling be provided for the back end; air flow, conductive, or liquid. Contact Sunpower engineering, wade@sunpower.com, for ideas about removing heat from the cryocooler.
Q. Is there a preferred mounting orientation for the CryoTel® cryocooler?
A. No, the cryocooler has been designed to mounted in any orientation.
Q. Are there any special mounting requirements for the CryoTel® cryocooler?
A. The CryoTel® cryocoolers offer several options for mounting. There are threaded hole patterns in the body of the cooler that are intended for mounting integration. For installations that will use conductive cooling for the reject heat, the cooler can be held in position by the thermal strap. The cooler can be mounted to the system via the cryostat flange either vertically or horizontally with the cryocooler cantilevered from the flange. Note: Care must be taken to not secure the cryocooler to the system structure at more than one point in such a way that will induce stress in the body of the cryocooler. Multiple attachment points can be used as long as all but one are compliant.