TABLE OF CONTENTS The term low temperature applies to the applications covered in this chapter and embraces the temperature range of approximately ?40 to ?80 C ( ?40 to ?110 F). The description low is relative, because these temperatures are not low to the scientists and engineers who build and operate systems at temperatures approaching absolute zero. The band of temperatures addressed in this chapter is positioned below those temperatures normal for the freezing of food and well above cryogenic temperatures, which are usually defined as below ?150 C ( ?240 F). Thus, the refrigeration temperatures explored in this chapter do not extend to the technologies of liquefaction of natural gas, nitrogen, oxygen, helium, and other industrial gases. Similarly, this chapter does not encompass the bulk of industrial refrigeration activities that span the range of evaporating temperatures from about 5 C down to ?40 C (40 F down to ?40 F).
Some justification should be advanced for treating refrigeration below ?40 differently than systems operating above that temperature. Aren t the principles the same? Physical laws applicable above ?40 do not suddenly lapse at temperatures below that value. However, there are some differences in the following areas:
Refrigerants
Systems (3 stage and cascade)
Materials for pipe and vessels
Insulation
Secondary coolants
Magnitude of boiling and condensing heat-transfer coefficients
Refrigeration technologies below 0 C (32 F) differ somewhat from those above that temperature, such that a competent designer and...
