Effective Heat Recovery
Plate heat exchangers have been used for heating process streams with steam or hot water and for cooling product streams with tower, city or seawater. However, most installations are for heat recovery - with good reason.
The plate heat exchanger provides an excellent means of achieving high efficiency in heat recovery. Full countercurrent flows, as well as its ability to produce high heat transfer coefficients, enable it to produce a close end temperature difference. For example, some plate heat exchangers can achieve 97 percent heat recovery even at temperatures approaches as low at 2°F. This is highly important to plants with sources of low grade or poor quality heat.
There is nothing new about heat recovery. In the past, many processors regarded it as not worth the capital expenditure. With today’s high energy costs though, it is not merely considered a desirable - it is a necessity.
In many applications, an immediate savings can be accomplished with an existing system by adding a plate heat exchanger. Other processes can be made more economical by replacing a shell-and-tube unit with a plate heat exchanger. Of course, in all new process systems, provision for heat recovery should be considered from the start. The result is a system optimized for maximum economy savings.
Plate heat exchangers designed for heat recovery are specifically configured to meet the duty, taking into account the specific product characteristics. Hot and cold fluids are directed between thin metal plates, corrugated to induce turbulence. The hot fluid transfers the heat to the cold stream, which is heated to a point where it requires little additional energy expenditure after the heat exchanger to reach the desired final temperature. In many applications, the fluids are of the same product. Examples are raw and pasteurized milk, wet and dry crude oil, lean and rich amine. In other cases, one stream is product, and the secondary stream is process water being preheated to be used elsewhere in the plant or cooled to be sent down the drain. Heat recovery also is possible with two different products or with uneven flows. Capturing the heat before it is lost is the key to major energy savings.
Heat recovery can be accomplished with various types of exchangers. Experience has shown that plate heat exchangers can handle the requirements efficiently and economically while providing compact size and minimal weight. When the application is within the pressure and temperature limits of the design of a plate heat exchanger, it is a solid choice. Some real-life examples demonstrate its capabilities.
Power House Recovery Payback. In an operation in the Northeast, a leading chemical producer has installed two plate heat exchangers units to handle power house heat recovery duties. Running 300 gal/min of 224°F (106°C) condensate against 400 gal/min of process water, slightly more than 5.5 million BTUs of heat are recovered per hour as the process water temperature is increased to 209°F (98°C). It was estimated that the payback for this plate heat exchanger was achieved in 483 hours.
At the same facility, by passing the 187°F (86°C) condensate to a second plate heat exchanger against 700 gal/min of 65°F (18°C) feedwater, an additional 16.3 million BTU per hour of heat is recovered as the feed water is heated to nearly 112°F (44°C). For this use, payback was achieved in 162 hours.
Whiskey Processing. At an East Coast import whiskey plant, the spirit is first chilled to precipitate insolubles, then filtered and warmed to avoid bottling difficulties. It is processed at 2,500 gal per hour, seven hours a day, in a three-section plate heat exchanger that replaced batch processing in five large tanks.
By using 80 percent heat recovery, chilling time was cut by more than 75 percent, reducing required refrigeration loads by 84 ton/hr and boosting production by 70 percent.
As the 90°F (32°C) product enters the regeneration section of the plate unit, it exchanges heat with the outgoing cold stream, and the 90°F product is cooled to 42°F (5°C). After chilling to 30°F (-1.1°C) in the second section of the plate heat exchanger, it leaves the plate heat exchanger briefly to be filtered. Upon its return, the whiskey is passed back to the third section of the plate heat exchanger for recuperative heating section and final trimming. Using hot water, the whiskey is heated to room temperature for bottling.
All steps are achieved in a plate heat exchanger with little more than 10 ft2 of floor space. The only maintenance required is an annual opening and cleaning. The increase in productivity of approximately 7,000 gal/day is accomplished without additional increase in manpower.
Canning Cooling. A single plate heat exchanger in a food plant is making substantial savings by using incoming cold process water to reduce the temperature of used can-cooling water. Spent can water at 2,350 gal/min is cooled from 95 to 83°F (35 to 28°C) using 600 gal/min of cold process water at 60°F (15°C). The cold process water leaves the plate heat exchanger at 85°F (29°C), ready to use without requiring any further heating.
The heat saved by recuperation is 9 million BTU/hr, and the plant is in use 17 hours/day, 5 days a week and 48 weeks a year. The system paid for itself in 3 months. Other benefits include a reduced BTU requirement for the cooling fan and lower evaporative water losses.
Conserving Water as a Means of Energy Savings. Although known primarily as a means for heating and cooling process streams, the plate heat exchanger works equally well as a water conservation tool. The close temperature approach characteristic of the plate heat exchanger permits the use of cooling tower water for longer periods of the year, so the need to substitute cooler and most-costly city or chilled water is minimized.
A plate heat exchanger was used to reduce water consumption by a chemical acid producer. In the past, the company viewed the cost of process water as a minor part of the operation. Constantly increasing charges for city water and sewer services altered that perspective.
The process required 340 gal/min of 70°F (21°C) water, which was used to reduce the temperature of 625 gal/min of product from 190 to 180°F (88 to 82°C). The company purchased nearly a million gallons of water weekly, which equated to a significant expenditure.
To reduce the chemical acid producer’s water costs, engineers at the heat exchanger company calculated that a small plate heat exchanger could replace the tubular unit being used. Along with the new plate heat exchanger, the process was adjusted to cool a smaller volume of product to a lower temperature. In this case, the plate unit was used to cool about 20 percent of the product flow, cooling the small volume to 138°F (59°C) and removing about 3 million BTU/hr, instead of cooling the entire 625 gal/min to 180°F (82°C). After the plate heat exchanger, the 138°F slip stream was mixed with the remaining hot product to arrive at the desired overall temperature of 180°F. With this design, the cooling water requirement was reduced to 149 gal/min. This resulted in a 56 percent reduction in cooling water consumption. Payback of the exchanger was calculated in just a few hours of operation.
Geothermal System Recovery. In the upper Midwest, a plate heat exchanger is installed in a geothermal heating system for municipal buildings. Because geothermal well water is corrosive, its heat is transferred via the heat exchanger to the closed-circuit system for space heating. The geothermal water pipes are cast iron, so the potential for corrosion problems is confined to the heat exchanger. There, it is eliminated by the use of 316 stainless steel plates.
Before entering the plate exchanger, the geothermal water is boosted to a pressure of 60 psig. It enters the exchanger at 165°F (74°C). At its maximum flow rate of 500 gal/min, it heats 320 gal/min of closed-circuit water from 100 to 130°F (38 to 60°C). The outlet temperature of the closed-circuit water is maintained by a pneumatic valve in the geothermal loop, which regulates the amount of incoming hot water.
The plate heat exchanger’s response is nearly instant, so temperatures can be controlled to an accuracy of 2°F (1.1°C). Under normal conditions, geothermal energy provides all of the heat required - nearly 8 million BTU/hr. If necessary, a boiler can also be employed in extreme conditions. This system saved valuable and expensive fossil fuel and paid for itself in three years.
In conclusion, the installation of a heat-recovery system does not have to involve a large capital outlay. Simple heat recovery system can consist of very little piping and a plate heat exchanger. Of course, more elaborate heat recovery systems can be designed, depending upon the needs of the user.
Justification based in payback can easily be figured. Many plate heat exchanger companies have both product engineering and system processing engineering experience. Reputable heat exchanger manufacturers will help you carry out a feasibility study of your existing system, or help you in the design phase of a new process.