In new construction putting a mechanical pump on process heat exchangers can lead to savings on installation costs. For vacuum systems, net positive suction head is critical for typical electric pumps. Therefore, heat exchangers often are elevated to extreme heights to allow for proper drainage. Because they are immune to the cavitation issues that cause failures in electric pumps, mechanical pumps can reduce the skirt height required for columns or reboilers, which often can lead to thousands of dollars in construction savings.
CONSIDER THE BIG PICTURE
Condensate management requires a holistic approach to realize significant energy savings. Here're a few practical points to remember:
From a design perspective, gravity and distance play key roles in efficient recovery and return of condensate to the boiler. A typical system design involves overhead condensate return lines as well as long piping runs. Vacuum systems may require extensive elevations to drain a conventional level-control condensate pot.
Adding a device such as a pumping trap can minimize the height necessary -- sometimes to as little as four feet -- which can lead to savings of thousands of dollars in construction costs alone (Figure 3). A pumping trap also enables the heat exchanger to run at its lowest, most-efficient steam pressure, minimizing energy consumption.
The sizing of condensate return piping is another critical design factor. Lines must be large enough to accommodate the movement of liquid condensate as well as the presence of steam and flash vapor. In addition, receivers (flash tanks in open/vented system) must be sized to provide adequate separation of flash steam and condensate; failure to properly size receivers may cause upset conditions within the drainage system. Vent lines also must be sized adequately to decrease flash steam velocity, which likely will reduce the risk of condensate carryover.
A properly sized steam trap also is essential for efficient system operation. A number of variables dictate which type of trap to use but, in general, mechanical inverted-bucket traps usually prove to be the best solution because they allow continuous drainage of condensate. A rule of thumb is to locate traps at 100–300-ft intervals.
Condensate collection assemblies, which bring multiple valves together in one central location, may offer advantages by reducing the number of individual condensate collection points along a line -- cutting installation costs and space requirements and increasing accessibility to equipment for routine maintenance and repairs.
Simple management practices such as insulating distribution and condensate return lines can pay big dividends. Insulation can reduce energy losses by 90%; insulate any surface over 120°F. Establishing a routine steam-trap inspection and maintenance program also is essential for maximizing condensate recovery and return. An effective trap management program alone can achieve fuel savings of up to 10%.
STEVE ASHBY is sales manager, Condensate Management Group, for Armstrong International, Three Rivers, Mich. E-mail him at email@example.com.