In this last in a series of Pick Blogs covering methods of heating water for plant sanitation, we take a look at direct contact hot water heaters. These systems typically incorporate a cylindrical stack with an internal diffuser near the top, a burner section and a water collection reservoir at the bottom. Water is sprayed onto the diffuser section. This is where heat transfer takes place between the water droplets and heat generated by the burner gases that move upward. Water interfaces directly with gases, thus “direct contact”. Water is then collected in a small reservoir area where it is pumped out to the plant.
Direct contact heaters are chosen as an alternative for plants that are steering away from the use of steam boilers. By eliminating the boiler, an on-site licensed operator is not required. With the use of steam with shell-and-tube or plate-frame type heat exchangers, energy efficiency suffers. The direct contact method can be very efficient, often as much as 95-98%.
There are drawbacks to gas-fired systems that should be considered. First, the upfront equipment price and installation costs can be very expensive. It is not unusual for a direct contact system to be three to four times the cost of a simple direct steam injection system. Maintenance and energy costs are compounded by the requirement of a boost pump, as well as the complexity of the design.
Sanitation hot water requirements can be very demanding, especially at times of peak load. This is where direct contact heaters run out of gas. The reservoir of hot water is not sufficient for high loads, often requiring the use of a separate, larger hot water tank. This can waste energy as water loses temperature when being held in the tank for extended periods.
As an alternative, direct steam injection heaters can be designed to handle the maximum needs for plant sanitation requirements. It is not uncommon that a direct steam injection heater is added as a trim or supplementary heater when the direct contact unit is running on empty.
Direct steam injection systems are comparably less expensive and provide the most efficient use of a boiler steam supply. Bigger is not better, as direct steam injection systems are compact, yet effectively do the job.
Contact us to find out how a Pick direct steam injection variable flow heater can reduce your energy usage and operating costs.