Previously, we compared two methods of heating water for plant sanitation, both involving DSI (Direct Steam Injection). While the use of individual steam-water mixing tees offer many conveniences, there are some shortcomings with efficiencies and safety. The Pick alternative is the installation of a centrally located, Variable Flow heating system as a safe and controllable single source hot water utility.
Another method of generating plant sanitation hot water is the use of the indirect method of heat exchangers, such as tubular and plate-and-frame units. These units are very common as one could spend hours sourcing all the manufacturers or suppliers. Categorized, at least in this report, as indirect heat exchangers, energy is transferred across either a tube bundle or a plate pack. Steam does not come in contact with the process side, the water.
Heat exchangers make up the major share of the heat transfer market, with an extensive range of applications. They are well accepted in the industry as the method of heating or cooling, using various sources of heating for a wide range of process fluids.
Indirect heat exchangers are most efficient where the steam supply pressure is 30 psig or less. A plus since Direct Steam Injection units are required to overcome water side pressure.
The main difference to DSI technology is that only latent steam energy can be transferred across a thermal transfer barrier, plate or tube. Heating is dependent on the amount of surface area and fouling, with condensate released to drain or returned to the boiler.
In comparison, energy efficiency and savings are the main advantages of direct steam injection, where all the available steam energy is transferred into the water flow, both latent and sensible BTUs. Make sense? For many operations, this is a hot benefit: Less energy required – lower load on the boiler.
In plant sanitation, accurate temperature control is important. With DSI steam interfaces directly with water flow, heat transfer is immediate. Changes in water flow rate are reacted to quickly, as fast as the temperature control loop can recognize and respond.
Since indirect methods rely on the latent steam energy to transfer across its heat transfer barrier, the response to changes in load is not immediate. This deteriorates as surface area becomes fouled over time. The slower the response to sudden changes in demand, the poorer the temperature control. Water discharge temperature is often too high or too low, with typically a long time before returning to set point.
Other drawbacks to indirect heat exchangers include the costly maintenance requirements of trap assemblies and condensate return systems. Also, since heat transfer surface area is required, heat exchangers are typically much larger units that take up valuable floor space.