Recover Low Level Heat, Part I

Most raw materials and final products usually are stored at ambient temperature. Because the driving force for heat transfer is the difference in temperature between hot and cold streams, it theoretically isn’t possible to perfectly match heating and cooling loads. Hence, heating and cooling have become the two important steps carried out independently — and heaters, boilers, cooling towers and air-fin coolers represent the most commonly used equipment.

All modern process plants are designed with maximum possible heat integration between the heating and cooling streams. (What’s really operationally feasible depends upon more than heat transfer issues — see, for example, “Control Challenges Can Pinch Energy Savings.” ) But plants designed prior to 1980 didn't consider heat recovery from process streams that were below 350°F–400°F. Such process streams should include the flue gas from heaters and boilers that are part of the process heating system. Flash steam near atmospheric pressure that is vented also is a common case of low level heat rejection.

In recent years, low level heat recovery has gained more attention for three reasons: 
1. Higher energy prices could justify the additional heat recovery equipment; 
2. Improved heat recovery technologies have become commercially available; and
3. Innovative energy integration concepts could be developed into the processes.

At current high energy prices, process engineers should consider opportunities to recover low level heat. I’ll describe several.

If the flue gas from heaters and boilers is leaving the stack at temperatures above 300°F, then adding an air preheater to supply hot combustion air may make sense. In the past, conventional recuperators and regenerators were the only options for air preheaters. Now, heat pipes used to preheat the combustion air are gaining more interest.

Heat pipes use a low-boiling medium that results in better heat transfer efficiency and can operate at very low differential temperatures between the hot and cold streams. Heat pipes have already been successfully implemented in some industries for low level heat recovery.

For example, a pharmaceutical plant in North Carolina maintained a portion of its process area at a controlled humidity and temperature. The air circulated in that area was first dehumidified with chilled water and then reheated to the required temperature with steam. Because it was a critical process area, 100% fresh air was supplied. We recommended installing wrap-around heat pipes across the chilled water coil in the supply air duct. This heat pipe installation reduced the air temperature by 7°F before entering the chilled water coil, and increased the air temperature by 6°F before it entered the reheat coil. The plant engineers credited this as one of the best applications of low- level heat recovery.

At a Midwest refinery, product gasoline from a debutanizer column was cooled from 300°F to 50°F by an array of heat exchangers with cooling tower water. Very close to that location, a propane vaporizer was heated with steam to supply the fuel gas system. We proposed a new heat exchanger upstream of the existing propane vaporizer, in which the hot gasoline from the debutanizer passed through to vaporize the propane stream. This heat integration arrangement yielded both steam savings as well as a reduction in cooling tower load.

Low level heat recovery applications such as these can save energy and improve efficiency.

Another potential for low level heat recovery is the flash steam vented from the blowdown water of the process-waste-heat steam generators. Although blowdown heat recovery systems are commonly installed at utility boiler houses, they usually are omitted at the process-waste-heat steam generators. At a Midwest refinery, it was observed that 3,000 lb/hr of additional low pressure steam could be generated from the blowdown water drained at its process-waste-heat boilers.

More innovative concepts of low level heat recovery applications will be highlighted in part II.