An industrial facility needed to heat 2,600 gpm of water to 60°F from 35°F to prepare it for biological treating. The site had no existing steam or other thermal loads generating low-level heat. The only major sources of energy available were pipeline natural gas and electricity.[pullquote]
Both the water rate and temperature varied seasonally. In addition, contaminants made the water highly fouling. An initial experiment using a small direct-fired heater quickly failed due to high fouling rates.
Liquid velocity and liquid temperature affect fouling -- the faster the velocity and the lower the temperature, the lower the fouling rate. Usually the most important temperature in exchanger fouling in a heater, is the film temperature of the fouling fluid, not the bulk temperature. Fire-exposed tube heaters have high fireside metal temperatures, which lead to high film temperatures. The high film temperatures created extremely high fouling rates.
Water rate variations made the problem worse. Occasional low flows decreased velocity and increased residence time in the heater. Low velocity allows fouling to stick more readily. Long residence time results in rapid fouling because liquid stays longer in the high-temperature liquid film.
Techniques for raising velocity and residence time include using multiple small heaters and recirculation systems. Multiple units boost capital requirements. Recirculating some hot water from the outlet fluid back to the inlet increases velocity and decreases residence time but elevates water temperature. The benefits, or costs, of circulation systems depend on the tradeoff between velocity and time versus temperature.
The site leaned toward a steam generator system, particularly because of the operating company's experience (and comfort) with steam systems at other sites. Relatively low-pressure steam could provide the heat.
However, without infrastructure, even a modest steam system quickly becomes complex. The total system ends up including:
• water make-up pumping (from a drinking water system);
• water softening, including two skid-mounted units plus a brine tank, salt basin and associated equipment;
• boiler feedwater system;
• steam generators (two 50%-capacity skids);
• chemical treating (one skid); and
• steam condensers (four shells, to allow on-line cleaning) to heat the water.
One quick way to measure system complexity is to count the number of piping and instrumentation diagrams (P&IDs). The steam-driven heating system sprawled over 14 P&IDs. Even with most control done via programmable logic controllers (PLCs) integrated with the skid modules, the system would require a fair amount of field construction.
The prospect of a highly complex unit for an apparently simple task leads to a natural question: "Is there a simpler and cheaper way to do this job?" We quickly checked other options.
A hot oil system became a leading contender. It's relatively simple and requires little maintenance. The key equipment include a surge drum, circulating pump, fired heater and hot-oil/water exchanger. The entire system requires only four P&IDs. The controls also are much simpler and reside in the PLC on the unit's single major module. Plus, fuel costs are slightly lower.
However, the hot oil system gives a higher average film temperature in the water heater. Because the hot oil uses sensible heat, inlet temperature exceeds steam condensing temperature. So, fouling rates are higher.
The choice becomes (A) a cheaper and simpler system with higher fouling rates or (B) a more expensive and complex system with minimum fouling rates. The unknown maintenance costs of the two options further complicate the decision.
The final recommendation was to use the hot oil system -- but to design the final water exchangers to both minimize fouling and make them as easy as possible to clean.
Andrew Sloley is a contributing editor to Chemical Processing. You can reach him at [email protected].