Wasted Superheat Converted to Hot, Sanitary Water
Technical editor of Engineered Systems
Summary: The following article is a part of National Board Classic Series and it was published in the National Board BULLETIN. (5 printed pages)
High efficiency, five pass heat exchangers wring the maximum heat out of the discharge vapor from a two-stage, -35o F refrigeration system. The hot water is practically free, the head temperature is reduced to 145 psig, and the payback is one and one-half to two years.
Alta-Dena Certified Dairy in City of Industry, California, processes an average of 120,000 gallons of milk a day, six days a week, under the most sanitary conditions. Some 80,000 gallons is sold as pasteurized milk. The remainder is processed as ice cream (two million gallons a year), yogurt (6,000 gallons a day), and other products such as sour cream, buttermilk, and cottage cheese.
All processing equipment is stainless steel, including the pipes and fittings, which must be washed down after every usage. This requires pure, hot water at temperatures up to 185o F.
More water is needed for chilling the milk and other products. In fact, both hot and chilled water are used in the pasteurization process. The dairy prefers high-temperature, short-time (HTST) pasteurization, which employs a multi-stage plate heat exchanger. In the first stage, cool raw milk is preheated by a heat exchange with the hot pasteurized milk. It then flows to the pasteurizing stage where it is heated to 171oF by hot water in adjoining plates. Although the milk must be held at this temperature (actually any temperature over 161oF) for 15 seconds, there is no holding capacity in a plate exchanger. Therefore, the hot milk is diverted through a broad loop before returning to preheat the incoming raw milk. It is finally cooled in two stages, one employing 34oF chilled water, the other 28oF glycol. The resulting temperature must be lower than 40o F before the milk flows to the filling machine.
The chilled water flows from an ice bank, where 200,000 pounds of ice are built during the off-peak period. This is a 25-year-old system and a conventional part of most dairy operations whether at the milking barns or the main plant.
Sterilizing the pasteurizer requires 1,280 gallons of 185o F water and is scheduled four times a day.
The three CIP (Clean-In-Place) systems consume a maximum of 17,000 gallons a day. These are used to clean the yogurt processors, automatic milk packaging lines, and ice cream mixers, freezers, and automatic filling and packaging machines. The seven huge tanks where the raw milk is stored and awaits pasteurization must also be washed. Of the seven tanks, five have a 40,000 gallon capacity, and two have capacities of 30,000 and 20,000 gallons. An eighth tank has 4,000 and 6,000 gallon compartments.
Hose water used for washing down the floor consumes another 5,000 gallons a day.
Alta-Dena maintains its own herd of 12,000 cows, milking 8,000 at any one time. Their own 50,000 gallons-a-day milk supply, plus 70,000 gallons from other producers, is transported from the milking barns by a fleet of four stainless steel tankers making a total of 17 trips a day. After the tanks are drained, they must be washed, first with cold water, then hot. This requires another 7,950 gallons a day.
Prior to 1987, all of this water was heated by steam produced by two boilers. Cold 60o F makeup water for the boilers is a maximum of 10,490 gallons a day.
Today, the boilers are fed 113oF to 116o F water that has been preheated by the superheat from the 1,262 ton refrigeration system.
Alta-Dena's is a two-stage, two-temperature ammonia system. The booster, or low side, stage operates at -35oF and serves the ice cream hardening tunnel. After hardening, the product is stored at -30o F.
The second stage, or high side, operates at +15oF and provides the refrigeration needed to chill the ice cream mix and to hold temperature in the storage rooms; 27 percent of the system operates at -35oF, and 73 percent operates at +15o F.
The booster system is comprised of five compressors. The base load is handled by a 130 ton screw compressor. Additional loads are distributed between four reciprocating compressors; three are rated at 52.6 tons, the fourth at 49.5 tons.
The high stage system has seven compressors; two screw compressors, each rated at 284 tons, are base loaded. Additional loads are distributed between three reciprocating compressors rated at 70 tons, and two others rated at 63 and 84 tons.
While only the low temperature refrigerant is handled by the booster compressors, the high stage compressor must handle all the refrigerant in the system.
Low temperature vapor is compressed by the boosters and discharged into an intercooler containing liquid ammonia. There the superheat is removed, reducing the 140oF discharge to +15o F.
This vapor is merged with the +15o F vapor from the high side system and compressed by high stage compressors. On a hot day, a discharge pressure of 170 psig was observed. Superheat from the high stage was dissipated by the condenser before condensation began.
A portion of this superheat is now being used to preheat potable water and to reduce gas consumption by the boilers.
Heat exchangers bear the ASME "U" stamp.
A heat recovery system consisting of four shell-and-tube heat exchangers has been installed. One is mounted between the booster discharge and the intercooler, the other three are in parallel between the high stage discharge and the condensers.
These are vented, double tube heat exchangers, the type required by most plumbing codes for use with potable water. In this design, there is a 0.003 inch passage between the inner tube containing the water, and the outer tube in contact with the ammonia. Any leakage, from either the water or ammonia side, will seep through this vent passage to the atmosphere and apprise the operator of the tube failure.
The heat exchangers, which might be called the "Cadillac of the double tubes," bear the ASME "U" stamp and The National Board of Boiler and Pressure Vessel Inspectors registration number. Design pressure for both the shell and tube sides is 400 psig test pressure is 600 psig.
The tubes are a patented design. The water tube is copper that is internally clad with 316 stainless steel. The ammonia tube is also stainless steel. They are joined by an embossment that creates a spiral on the inner tube, which produces water turbulence and maximizes heat transfer.
Each tube is vacuum tested for 15 minutes, then assembled with sealed bushings that allow the tubes to expand and contract. This bushing system makes it possible to replace the tubes in about two hours.
The booster heat exchanger is equipped with conventional transverse cross baffles. The three high stage exchangers are counterflow, with a unique five pass baffling system for the ammonia.
In the early fall, the heat recovery system heats 30,000 gallons of water a day. The heated water is stored in a 20,000 gallon tank and replenished as used. In the off hours, the stored water is recirculated through the heat recovery system.
A trace from a pen recorder shows how this recirculation system performs. With a steady drawdown, the water in the tank is between 113oF and 116oF. If the demand slackens just a bit, the temperature rises to 118o F.
With no demand, the tank fills to capacity and the system turns to the recirculation mode. Within six minutes the water is at 131oF, within 12 minutes it is between 134oF to 137o F.
Energy savings are a moving target. When the heat recovery system was installed, natural gas was 48 cents a therm. Now it is 34 cents a therm, but expected to rise with the increase in oil prices.
Southern California Edison, too, seems to be adjusting its strategy. In early 1986, demand charges were $5.05 on-peak, $.65 cents mid-peak, and no charge off-peak. In April 1986, the on-peak charge was reduced to $3.00, the mid-peak charge was increased to 85 cents, and an off-peak charge of $.50 cents was added.
On-peak kWh charges jumped dramatically. In early 1986, kWh rates were the same during the winter and summer at .08536 cents per kWh. Current charges are .13858 cents in the summer and .11673 cents in the winter. Charges during the other two periods are up and down and higher during winter than summer. The old mid-peak rate was .07136 cents year round. They are now .07196 cents in the summer and .07387 cents in the winter. Off-peak was .05966 cents. It is now .05954 cents in the summer and .06057 cents in the winter.
Payback for the system is estimated to be one and one-half to two years.
Low head pressure
One of the primary objectives was to reduce head pressure. At times the dairy was running at 170 psig. Not much could be done about it because the load depends on demand for the products, not the weather. Today the dairy is running at 145 psig and doesn't want it to go much lower than 140 psig because hot gas is needed to defrost the evaporators.
A second benefit is increased capacity on the high side. The booster used to dump 140oF vapor into the intercooler where it was cooled by flashing off liquid ammonia, which had to be recompressed by the high stage system. After it goes through the heat exchanger, that 140oF vapor has been cooled to 94o F before entering the intercooler. The same is true with the high side heat exchangers and it makes a difference in condenser capacity.
Copyright © 1987 Engineered Systems. Reprinted with permission from Engineered Systems , September/October 1987, pp 32-36. Reprinted text has been edited for clarity and space requirements.
Editor's note: Some ASME Boiler and Pressure Vessel Code requirements may have changed because of advances in material technology and/or actual experience. The reader is cautioned to refer to the latest edition of the ASME Boiler and Pressure Vessel Code for current requirements.