Sensor cooling by direct blowdown of a coolant

This article describes results of a proof-of-concept study on cooling of the optical components in a long wavelength infrared (LWIR) sensor using direct blowdown of R23 refrigerant. This technique allows on-demand cooling of optics in a sensor system with an extremely simple and lightweight system. The thermodynamic properties of certain refrigerants such as R23 allow a direct expansion from a high pressure and ambient temperature (300 K) to saturated two-phase conditions at a low pressure without precooling. Refrigerant saturation temperatures between the triple point (118 K) and the normal boiling point (191 K) can be obtained depending upon the level of vacuum available. Experimental results are presented for a 15-cm-diam primary mirror intended for use in a LWIR sensor. The mirror (weighing between 500-750 g) was cooled using direct blowdown of R23 to temperatures of less than 140 K. Cooling times were on the order of 2-3 min. A onedimensional thermohydraulic model was developed to model the cooldown process. Good correlation was obtained between the model and the experimental results. Further system optimization has been undertaken using the model to minimize system mass for a given aperture sensor optical system and cooling requirement.