Cryogenic Heat Transfer
Introduction
The word cryogenics is defined as the study of low temperature phenomena. The temperature separating cryogenics from conventional refrigeration is -150 oC (123K) or -238 oF (222 oR). The normal boiling points of industrial gases such as helium, hydrogen, nitrogen, oxygen, and air all lie below -150 oC, whereas the common refrigerant fluids used in domestic refrigerators, air conditioners and freezer all boil at temperature above -150 oC. Because cryogenic systems operate at temperature far below ambient temperature, heat transfer is always a major concern.
The second major concern is the relatively small latent heat of vaporization associated with cryogenic gases. For example heat of vaporization of liquid nitrogen is only 199.3kJ/kg (85.7 BTU/Ib) at pressure of 1 atm, liquid helium is 20.7kJ/kg (8.9 BTU/lb). Conversely, it is 2257kJ/kg (970Btu/lb) for water at the same condition. So a very little heat-in leak will pose major problem to cryogenics medium.
Selected Properties of Some Cryogenic Liquids at their NBPa
The transfer of liquid cryogens such as liquid nitrogen over any distance in a plant, whether indoors or outdoors, requires special piping insulation. Issues that need to be considered in cryogenic liquid transfer includes the economic loss of cryogen through evaporation and quality degradation of the cryogen delivered at the point of use.
Under normal operational circumstances, the liquid in the system is constantly vaporizing into gaseous nitrogen due to constant heat leak. The accumulated gas in the pipeline will result in excessive gas within the transfer system which causes:
1.Diminished cryogen cooling capacity, warmer liquid with higher enthalpy
2.Excessive vapor phase resulted in higher gas content in liquid
3.Inconsistent delivery of liquid nitrogen due to plug and slug flow conditions
4.Variation in cool-down time from one point to another
5.Increased in operating cost due to higher heat loss
Heat-in Leak
The primary challenge for an efficient and cost-effective transfer of cryogens is to minimize heat transfer (heat-in leak). All liquid seek an equilibrium with surrounding temperature. Cryogen such as liquid nitrogen at -320°F seeks to reach an equilibrium with the atmospheric temperature surrounding the transfer pipe, resulting in heat evaporation loss.
