Editor's note: In March 1980, Heating/Piping/Air Conditioning, as HPAC Engineering then was known, published "Heat Balance Review for Double Digit Savings" by Mike Bekedam, founder, and James F. Williams, vice president of engineering, of Industrial Steam. Finding the article still relevant, the magazine recently asked Industrial Steam to update it for a new generation of readers.

The days of passing on to customers the cost of now seemingly negligible fuel-price increases are but a distant memory. To remain competitive in this age of record-high fuel costs, energy users must take a hard look at their fuel consumption and find ways to optimize it.

Although economizers, heat exchangers, low-oxygen burners, insulation, and the like are excellent means of reducing fuel consumption and worthy of consideration, without a doubt, the most productive place to start looking for maximum fuel/energy savings--the one offering the greatest return on investment--is the boiler/steam/condensate cycle. Unless plant heat balance is correct, unreclaimed flash steam and hot condensate represent significant--in fact, the greatest--heat loss. Consider that fuel loss from a 15-psig open condensate system can amount to 6 percent of the fuel required to produce steam. In a 100-psig system, the fuel loss is 19 percent. With a well-designed closed condensate system, payback can be a matter of months, if not less.

This article will discuss how a review of the heat balance of the boiler/steam/condensate cycle can yield double-digit energy savings.

Energy Requirements

The easiest and most logical way to review plant heat balance is to observe plant operation. Go to the roof, and see if steam is being wasted to the atmosphere; if it is, determine the source. Check the process(es) for the loss of potentially
recoverable condensate.

While the observation of plant operation may reveal large "holes," it does not tell the entire story. For instance, consider a paper-corrugation plant in which steam is utilized at 175 psig by a machine operating in a closed cycle. Live steam is consumed for the plasticization of paper prior to corrugation, but the heat balance is such that considerable positive pressure is left over. It is not unusual to see a plant of this type blasting steam into the atmosphere to keep production processes in motion.

Another example is a canning plant, where food is prepared inside of large atmospheric hot-water cookers. Prior to this article's initial publication, water for the cookers was heated through the direct injection of steam. With the introduction of heat exchangers and the return of condensate in a trapless closed-loop system, a savings of 18 percent could be realized. When fuel cost 3.5 cents per therm, such a conversion was difficult to justify for a seasonal operation, but with impending fuel costs of 70 cents or more per therm, payback can be a matter of weeks.

Having failed to review their energy requirements, many industries now are realizing the fuel dollars they let be thrown away (condensate to drain) or go up in smoke (flash steam).

Actual Losses

Table 1 shows actual volume, total-heat, and fuel losses at various pressures. The "Total heat lost to flashing" column indicates heat loss attributable to the need to replace lost flash steam with makeup at 60 F. The real eye opener, though, is the fuel required to regenerate this lost heat in a boiler operating at 80-percent efficiency ("Fuel lost in replacement of lost heat" column). With a closed-loop system, these fuel-loss percentages are direct fuel savings.

TABLE 1. Volume, total-heat, and fuel losses at various pressures.

Table 1 is not entirely accurate because it fails to take into account such uncalculable losses as increased blowdown, increased chemical use, cost of makeup water, sewer cost, and water required by some municipalities to cool blowdown.

Heat-Balance Categories

There are two types of heat balance: one based on a plant's current operating condition, the other based on a plant's optimum energy use. Most plants fall into one of three basic categories:

• Category 1. All steam is used by the process and is unrecoverable because of direct injection, contamination, or impracticality. An aggregate plant, a steam vacuum jet, and a pressure cooker with potentially contaminated steam are good examples.

• Category 2. Most or all condensate is recoverable, and the average feedwater temperature is considerably higher than the feedwater-system operating pressure. Plywood plants, corrugators, rendering plants, and rubber plants are good examples.

• Category 3. Only a portion of the condensate is recoverable, and the heat balance is less than 212 F. Canneries, chemical plants, and paper mills are examples.

With Category 1 plants, it is important that the possibility of the conversion of unrecoverable cycles to recoverable ones be considered. For instance, a vacuum pump may pay for itself by eliminating a wasteful steam jet. Heat exchangers can be used in place of the direct injection of steam. Lost condensate attributable to impracticality suddenly may become cost-effective.

A Category 2 plant calls for a closed condensate system. This type of plant usually produces a large amount of flash steam that, unless utilized, results in a high degree of inefficiency. The utmost care is required not only to select components suitable for the pressures and temperatures involved, but to tailor the system to the plant. It is important that systems people be familiar with the process and production equipment involved. Sometimes, the increased production possible with a closed-loop system can exceed the fuel savings.

Although there appears to be no potential for savings with Category 3 plants, close scrutiny reveals hard-to-detect losses, such as condensate coolers and atmospheric trapped condensate systems, which can be closed for better heat utilization. Furthermore, a well-designed trapless closed-loop system can increase production significantly by enabling unrestricted drainage and continuous removal of non-condensable gases for maximum heat transfer.