Isobutane and propane vapour fraction at evaporator inlet for different condenser temperatures
Isobutane
Propane
Condenser
Evaporator
Vapour fraction at evaporator
% extra liq
Condenser
Evaporator
Vapour fraction at evaporator
% extra liq
temp, °F
temp, °F
vapourised (vs base case)
temp, °F
temp, °F
vapourised (vs base case)
100 110 120 130 140 150
-40 -40 -40 -40 -40 -40
0.45 0.49 0.53 0.57 0.61 0.65
base case
100 110 120 130 140 150
-40 -40 -40 -40 -40 -40
0.47 0.51 0.55 0.60 0.64 0.69
base case
6.8
7.7
13.7 20.8 28.0 35.5
15.6 23.8 32.4 41.5
Isobutane
Propane
Condenser
Evaporator
Vapour fraction at evaporator
% extra liq
Condenser
Evaporator
Vapour fraction at evaporator
% extra liq
temp, °F
temp, °F
vapourised (vs base case)
temp, °F
temp, °F
vapourised (vs base case)
100 110 120 130 140 150
-20 -20 -20 -20 -20 -20
0.40 0.44 0.48 0.52 0.56 0.60
base case
100 110 120 130 140 150
-20 -20 -20 -20 -20 -20
0.42 0.46 0.52 0.55 0.60 0.65
base case
6.3
7.2
12.8 19.4 26.2 33.1
14.7 22.4 30.5 39.1
Table 3
the expansion valve as the pressure falls, leading to higher required circulation rates. The process duty can vary with ambient conditions and plant load. The required quantity of refrigerant mass flow to be circulated by the compressor is calculated from the cooling duty requirement:
propylene can result in a substantial 40 psig rise in vapour pressure and, consequently, compressor discharge pressure. This makes heat rejection to the warmer ambient air more challenging, driving up the condensing temperature. This effect is comparatively less pronounced for refrigerants such as propane, isobutane, and n-butane, in descending order of impact. The condenser, if designed for excess surface area, can compensate for reduced logarithmic mean temperature difference (LMTD) due to hotter ambient conditions, thereby helping to reduce the condensing temperature and, conse- quently, the compressor discharge pressure. Effect of relative humidity on condenser : There is a gen- eral inverse relationship between ambient temperature and relative humidity. As ambient temperature increases, the relative humidity tends to decrease. This is a common mete- orological phenomenon: warmer air can hold more moisture. Therefore, if the absolute amount of moisture remains con- stant or increases less proportionally than temperature, the relative humidity will fall. Since humid air is less dense, for a constant dry bulb tem- perature, the mass flow rate of air will slightly decrease as humidity increases. However, as water vapour has a higher specific heat, the expected benefit of more heat transfer is often negated by the reduction in air mass flow. A reduced mass flow rate of the cooling medium inherently reduces the overall heat transfer capacity of the fin-fan cooler. The primary driving force for sensible heat transfer in a dry fin-fan cooler is the difference between the process fluid temperature and the dry bulb temperature of the ambient air; hence, humidity has very minimal impact. However, if the system is designed for evaporative cooling, then the relevant driving force for cooling becomes the wet bulb temperature. In such systems, lower humidity allows
Refrigerant mass flow = process duty / (liq. fraction at evaporator inlet x latent heat)
The condenser duty is always higher than the evaporator duty because it must reject both the evaporator heat and the heat added by the compressor. In addition, the condenser not only removes latent heat, but also the superheat and provides subcooling to the condensed refrigerant. Flash gas generated at the evaporator inlet also leads to more gas in circulation and increased heat duty for condensers: Effect of ambient temperature on condenser : The perfor- mance of fin-fan condensers is strongly affected by ambient conditions. Typically, the condenser outlet temperature is 15-30ºF above the dry bulb temperature of the air. During the summer months, elevated ambient tempera- tures have a direct and significant impact on the condenser outlet temperature of a refrigeration system. The overall per- formance of the condenser is diminished in hotter weather, primarily due to a reduction in the mass flow of cooling air across its tubes. As illustrated in Figure 2 , a strong positive correlation exists, where rising ambient temperatures lead to a corre- sponding increase in the condenser outlet temperature. This phenomenon is compounded by the exponential relationship between temperature and refrigerant vapour pressure; for instance, a mere 10°F increase in condenser temperature for
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PTQ Q4 2025
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