Temperature vs vapour pressure for various HCs
Temp, °F Propylene Propane I-Butane n-Butane Pentane
-40 5.6 1.5
-20
0
+20 53.6 41.0
+40 81.0 63.8 11.7
Condensers
M
Vapour
10.7 -7.6
32.6 23.6 -3.2 -7.5
Refrigerant compressor
Evaporators
pressure,
psig
-10.5 -10.4
3.1
Accumulator
-12.2
17.0
-3.2
2.9
Expansion valve
-14.3 -13.9
-13.2
-12.1 -10.4
Table 2
Figure1 Typical refrigeration cooling loop
in turn becomes the decisive factor in selecting the refrig- erant. For example, if a process requires the temperature to be maintained at 40°F, the refrigerant must be colder than 40°F, considering the achievable approach temperature in the evaporator. The lighter the hydrocarbon, the higher the pressure at which the target temperature can be achieved. As shown in Table 2 , the pressure required to achieve -40°F can vary from -14.3 psig to 5.6 psig, based on the hydrocar- bon selected. For a given target temperature of refrigerant, the lower the compressor suction pressure and molecular weight, the lower will be the operating density of the gas
process cooling systems due to their excellent thermodynamic performance, availability, and minimal environmental impact. Typical refrigerant loop and equipment are shown in Figure 1 . The number of compressors, condenser fans, and evapo- rators can vary based on various parameters, such as process duty, the target refrigerant temperature, and the achievable condenser outlet temperature. The impact of various varia- bles on system design and performance is explained below: Target temperature: The process side temperature require- ment governs the target temperature of refrigerant, which
and the higher the compres- sor capacity (ACFM). Typically, gases with lower molecular weights necessitate higher suction pressure; therefore, the combined effect of suction pres- sure and molecular weight on the compressor’s suction den- sity must be evaluated. The design of an evapora- tor for vacuum conditions can increase its manufacturing cost. It may also cause oper- ating issues related to air and moisture ingress in the evap- orator or attached instrument connections. Process duty: Services for which the required target temperature cannot be achieved with cool- ing water or fin-fan coolers may need refrigerant-based cooling systems. All such services are typically clubbed together in a parallel arrangement using com- mon refrigerant compressor(s), condensers, and an accumula- tor. The process duty dictates the required refrigerant flow rate through the evaporators to efficiently remove heat from the system. Evaporators use the latent heat of vaporisation of the refrigerant to achieve the process side cooling requirement. Some refrigerant unavoida- bly vapourises downstream of
120 100 160 140 200 180 240 220 260 320 300 340 280
Propylene Propane Iso-butane n-butane
80 60 40 20
0
-40 -20
60
65
70
75
80
85
90
95
100 105
110 115
120
125
130
135
140 145
150
Temperature (˚F)
120 100 160 140 200 180 240 220 260 320 300 340 280
80 60 40 20
0
Condenser ou t let temp (˚F) Ambient temp (˚F)
-40 -20 -20
Month
(For propane-based refrigeration system during Middle East summer months)
Figure 2 Ambient effect on vapour pressure and condenser temperature
94
PTQ Q4 2025
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