More often than not, producers find winter-production capacity limited by their hot-water system. After working so hard over the summer and fall to win new customers, their increased volume over the previous year may require more hot water than a plant's system can provide. The producer faces a dilemma: trying to promptly service more customers, with a limited quantity of hot water, while minimizing capital and operating expenses on equipment used only three to four months per year. Over the years, we have modified many concrete plant distribution systems, after production demands had overpowered their heated water capacity. While each plant is unique, those producers were able to dramatically reduce their hot-water heating costs and increase capacity by combining their boilers with steam to water exchangers and modifying the piping to their storage tanks.
Heating water once: Many producers purchase a boiler and install it to provide "instantaneous" hot water. As unheated water passes through the boiler, producers can expect a nominal temperature increase of 80 to 100º F. For example, water entering the boiler's tube system at 40º F exits at 120º F. This setup works well for an operation with about six trucks, but the plant's production capacity is limited by the rate at which the boiler transfers heat to the water. As production demands increase, some producers increase hot water availability by adding a hot-water storage tank. Producers operate their boilers at full throttle to keep their storage tanks full of hot water. The tank's pre-loaded hot water then allows producers to batch trucks at a usage rate that exceed greater than the boiler's production rate. But in many situations we have found that a large, poorly designed surge tank actually limited the system's total hot-water production. One problem is that once the water is in the tank, producers must constantly heat it to maintain proper batching temperatures. Maintaining heat in a large tank with a small boiler is like using a candle to heat a large fireplace. Another common problem is that the hot water flows across the top of the tank from boiler input to batcher output, resulting in a lower water temperature in the storage tank.
Looping heat through the tank: Producers can eliminate these problems and increase their boiler's heat-exchanging capacity by looping it together with their storage tank. This is similar to the way a small boiler heats a large residential home. After heated water passes through the home's boiler, it expands, decreasing its specific gravity. In a closed piping system, the hot water's lower specific gravity causes it to "float" or rise in relation to the colder water. The hot water passes through the home's radiators, warming the room. Once cooled, the higher-specific gravity water sinks to the boiler, to be reheated and begin its journey again.By converting the boiler from instantaneous duty to recycling duty, producers can create this same radiant-heat loop inside their storage tank. The conversion requires producers to add a pipe path from the storage-tank's bottom to the boiler and back to a blending valve at the tank intake area. A steam-to-water heat exchanger is added to the boiler. The boiler tubes become a closed circuit system, which heats process water up to 210º F, just below steam level. The water from the storage tank passes through the exchanger, where it's heated by the boiler tubes.The key to the system's success is the manner in which water is added back into the storage tank. Adding super-hot water at the feed intake tempers new make-up water. As the tempered make-up water goes into the tank, the lighter hot water rises to the top. The colder tempered water continues through the tank back to the heat-exchange path. The efficient exchanger's heat transfer quickly provides hot water for batching. The entire system is controlled by a computer system that monitors temperatures at the fresh water intake, tank midpoints, blending valves and batching-water meter. The controller regulates boiler burner levels to control natural-gas consumption.
Actual case: The system can produce substantial savings for different plant layouts and climates. During a three-year period, one ready-mixed concrete plant added a heat exchanger to its boiler and created a closed-loop tank heating system. The results indicate that the plant reduced its natural-gas consumption costs by more than 75% while increasing heated concrete production by 50%. The producer has reduced its fuel-cost per yard from $1.98 in 1992-1993 to an estimated 1996 cost of $0.06. These dramatic fuel-cost savings, coupled with an increase in available heated water have given this producer a unique advantage in a very competitive market.
A flexible system: Producers may find the additional exchanger loop helpful in other ways. During the summer, the same computer controls can keep concrete cool by blending the tank's cooler water. Producers can also equip boilers with a specially designed heat exchanger that warms gray water with specific gravities of up to 1.20. Its advanced design uses a low-pressure vacuum steam system that allows for the inherent problems of scaling and abrasion when heating recycled gray water. Bruce Corliss is systems analyst for Boiler & Distribution Systems, Lynnwood, Wash.
KEYWORDS: heating aggregate, plant equipment