Understanding heat transfer systems

Many batch and semi-batch processes in the chemical industry require both heating and cooling from the same equipment, such as a jacketed reactor. Operational simplicity often requires the same heat transfer fluid (HTF) for both services. Some unique design considerations include:

Specific heat and density properties over the full temperature operating range.

Vapor pressure at the high end of the temperature range.

Viscosity and pumpability at the low end of the range.

Maintaining the mass balance between cold and hot loops.

Overall cost.

Plant preference.

Systems that must heat and cool often use the same HTF because using different HTFs may require separate coils, separate heating and cooling jacket zones, or outside heat exchangers, each of which complicates mechanical design. Different HTFs may be used in a common jacket, but operations are complex because the fluids must be drained before changeover from heating and cooling and back again. An examination of alternatives using a jacketed vessel illustrates some approaches and challenges.

One approach (see Figure 1) uses steam to heat and water to cool a process vessel directly. This system works, providing the required cooling temperature doesn't fall below the freezing point of water. However, the jacket must be drained when switching from cooling to heating. Additionally, the cooling water isn't normally suitable as boiler feedwater and must be segregated from the condensate return. Cooling water must be drained from the jacket first. Any remaining steam condensate after a heating cycle can be mixed with the cooling water, assuming the boiler feedwater additives are compatible with the cooling water system.

Figure 1. This direct approach uses steam to heat and water to cool a process vessel.

Other HTFs can be used in a vapor phase for heating and in a liquid phase for cooling, assuming the HTF can service the full operating temperature range. If a suitable HTF is available, then a common drain/return system could be used.

When heating and cooling services require different HTFs, they usually must be segregated physically. The system in Figure 1 also could use different HTFs for heating and cooling, but each would have to be drained before the switchover, and each must tolerate some cross-contamination. Operational delays and cross-contamination during switchover normally render such a design impractical.

Another method uses a circulating loop that passes the circulating process fluid through heat exchangers in series (see Figure 2) with one heating the fluid and the other cooling it. This solution is more mechanically complex than the first approach, but offers some advantage in that the heat exchangers may be designed without vessel geometry or jacket area acting as design limitations.

When the required heating and cooling temperatures are within the service range of a single fluid, a variation on the first approach may be used (see Figure 3). A circulating loop is fed cold HTF for cooling service and hot HTF for heating service. Because these HTF supply loops are interconnected, HTF liquid levels may vary during switchover because the fluid transfer from one loop to the other goes through the tempering loop. Most systems are naturally self-correcting because heating and cooling cycles balance the mass transfer between loops. However, an open balance line may be provided between the expansion tanks.

Figure 3. This scheme works if the required heating and cooling temperatures are within the service range of a single fluid.

Because of the range of HTFs available, design and operational approaches, and the system cost, owner and operator preferences strongly influence system design. A system using steam and water may be the least expensive solution, but may limit operations. A common non-aqueous synthetic HTF may provide the most flexibility and reliability, but may require a premium-cost HTF to achieve the desired temperature operating range. Process requirements, system cost, operability, safety, reliability and end-user preferences must be balanced.

A critical design issue is finding a single heat transfer fluid with stable, acceptable properties at both high and low temperatures. It must not boil, freeze or decompose (vapor systems are drained of liquid cooling fluid during switchover). Some synthetic HTFs with acceptable properties have low flash points and pose fire hazards. Some have unacceptable odors, which may disqualify them from use in hygienic or sterile settings, such as pharmaceutical production facilities.