PTQ Q4 2025 Issue

Reduction in latent heat of condensation with temperature

% Change in latent heat

Temp (°F)

Propylene Base case

Propane Base case

n-Butane Base case

i-Butane Base case

n-Pentane Base case

100 110 120 130 140 150 160 170 180 190 200

-4.3 -8.6

-5.8

-5.1

-5.0

-11.6 -17.3 -23.1 -28.8 -34.4 -40.1 -45.8 -51.4 -57.0

-10.3 -15.4 -20.5 -25.7 -30.8 -35.9 -41.1 -46.2 -51.3

-10.1 -15.2 -20.2 -25.3 -30.3 -35.4 -40.4 -45.5 -50.5

-10.0 -15.0 -20.0 -25.0 -30.1 -35.1 -40.1 -45.1 -50.1

-12.8 -17.1 -21.4 -25.7 -29.9 -34.2 -38.5 -42.8

Table 4

increases, as shown in Table 4 , the latent heat of conden- sation of a given refrigerant progressively decreases. For instance, the latent heat of condensation for C₃ and C₄ hydrocarbons decreases 20% to 30% when the condenser temperature increases from 100°F to 150°F. Even though the refrigerant’s latent heat of condensation decreases at those higher temperatures, the effect of the lower LMTD is more significant than the effect of the reduced latent heat. Hence, more surface area is required to achieve better performance. Effect of temperature on centrifugal compressor capacity Refrigeration systems in oil and gas installations frequently employ centrifugal compressors due to their capability to handle large volumetric flows. However, the flow rate of these compressors is inversely affected by the discharge pressure (see Figure 3 ). Consequently, the increased back pressure observed during hotter weather due to higher vapour pressure, while plant cooling requirements simulta- neously rise, leads to a reduction in compressor flow. This phenomenon can result in significant cooling limitations. Compressor performance, specifically the reduction in refrigerant flow as head increases due to higher condenser temperatures, is highly dependent on its operating curve. A flatter curve means a small rise in head will cause a sub - stantial drop in flow, whereas a steeper curve will result in a smaller flow reduction. Installing a variable frequency drive (VFD) on the compressor allows speed adjustment to operate more favourably within its performance curve under these increased head conditions. This can help prevent the com- pressor from operating in inefficient regions or approaching surge conditions that can occur with fixed-speed machines when pressure differentials become excessive. Effect of subcooling A subcooler, if practical, can be installed downstream of the condenser and works by cooling the liquid refrigerant after it has already condensed in the main condenser. The cooling medium in the subcooler has to be colder than the surrounding air to cool it further. The compressor discharge pressure and the condensing temperature are primarily set

for more evaporative cooling and thus better performance, as there is a greater difference between the dry bulb and wet bulb temperatures. Effect of condenser temperature on evaporator: When the outside air gets hotter, the condenser in the refrig- eration system must also work at a higher temperature. This causes more issues than just making the centrifugal compressor work harder against a higher pressure. It also directly affects how well the evaporator can cool. As the condenser gets hotter, more of the liquid refrigerant turns into gas too early, at the throttling valve outlet, even before it fully enters the evaporator. This is called ‘flash gas’, which means there is less useful liquid refrigerant available to absorb heat and provide cooling. For example, as shown in Table 3, if an isobutane con- denser goes from 100°F to 150°F (with the evaporator at -40°F), about 35.5% more of the liquid turns into gas too soon compared to the ideal situation. For propane, this problem is even bigger, with about 41.5% more liquid vapourising too early under similar conditions. This flash gas takes up space in the evaporator and compressors and does not help with cooling. Hot ambient conditions create a double problem as the compressor struggles with higher pressures. At the same time, the evaporator does not get enough cooling liquid, significantly reducing the refrigera - tion system’s overall cooling capacity. Effect of temperature on latent heat of condensation In refrigeration systems, as the condenser temperature

Figure 3 Typical centrifugal compressor performance curve Flow (ACFM)

PTQ Q4 2025