Impact of Operating Pressure on Cooling Capacity and LN2 Use Efficiency
Figure 4.
The absolute amount of refrigeration recovered from the LN2 will also depend on the operating pressure of the cryogenic system. As Figure 1 shows, increased pressure leads to warmer
operation. This is a strategy often used to delay or avoid freezing the HTF in the cryogenic heat exchanger.2,4,5 The main downside to this approach becomes obvious in examining Figure 2. While the warmer temperature may alleviate freezing
problems, the available latent heat of vaporization drops significantly with increasing pressure and temperature. An example
of such a system is the LN2/GN2 recirculation system with high-pressure fluid ejector technology as shown in Figure 4. The proposed 11-bar LN2 may boil almost 30 °C warmer than at atmospheric pressure, however, it also has over 40 kJ/kg less heat of vaporization to
give off. That is approximately 20% of all the available refrigeration from the phase change. Praxair has developed a system
that operates with as little as 3 bar LN2 pressure. Operating at a lower pressure helps the system recover more refrigeration, which is available for cooling, reducing
the amount of LN2 consumed in the process.
Impact of HTF Velocity
Many conventional cryogenic heat exchangers require a high HTF velocity to delay freeze up.2 A highly viscous fluid like an HTF at low temperature flowing at high velocity will generate significant frictional heat,
i.e., parasitic heat, which will add to the refrigeration demand of the system. Therefore, choosing a refrigeration system
with the lowest minimum HTF fluid velocity requirement is important.
COOLING THE CONDENSER
Figure 5.
The condenser is typically cooled by direct expansion of a refrigerant into the coils or plates (DX condenser). In the case
of a mechanical refrigeration system, the refrigerant is usually a hydrofluorocarbon type chemical or a mixture of chemicals.
Cryogenic systems often use direct expansion of LN2 and/or GN2 to cool the condenser surfaces. A simplified process-flow diagram is shown in Figure 5. In all these cases, the liquid refrigerant
vaporizes in the DX condenser, forming a two-phase flow. The significant heat transfer coefficient difference between the
liquid and gas phase refrigerant causes uneven cooling rates at different points in the condenser. The result is uneven ice
formation and nonuniform use of the condenser surface.
