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2.1.3. De-superheating and sub-cooling

De-superheating is different from condensation of a saturated vapor. The sensible heat should be removed first to de-superheat the vapor to obtain the saturated vapor. Similarly, the saturated liquid is to be further cooled down (sub-cooled) by extracting sensible heat below the boiling point. The temperature profile is shown in Figure 1.9 for the condensation of superheated vapor to obtain the sub-cooled liquid from the same exchanger. The mean temperature difference and heat transfer coefficient should be calculated individually for each section if the degree of superheat/ sub-cool is large. The weighted mean temperature difference and overall transfer co-efficient can be used to design the condensers if heat load due to sensible heat transfer in each unit about 25% of latent heat transfer. Otherwise, it is convenient to design separate de-superheater and sub-cooling exchangers. The calculations for detail study can be found out in reference [3] (page 283-285) .

Figure 1.9 . Condensation with de-superheating and sub-cooling [2] .

Practice problem:

Design a horizontal condenser for the condensation of 45,000 lb/h of almost pure normal propyl alcohol available at 15 psig. At this pressure, the boiling point of n-propyl alcohol is 244°F. Water available in the temperature range of 95 to 120°F can be as the coolant. The maximum pressure drop of 2 psi and 10 psi is permissible for the vapor phase and water respectively.