Info

15/15

0.0005/0.0005 4 tube side 3 shells in series $135,000 Mild steel ft2

lb/in2

This example demonstrates that quite high heat-transfer coefficients can be obtained from a PHE with only a moderate pressure loss.

Figure 7. Heat recovery—comparison between plate and tubular heat exchangers.

unnecessary penalty to pay for these features both in price and size.

For heat recovery duties in excess of 70%, the plate heat exchanger will become increasingly more economical than the tubular.

Typical Applications

One of the more common uses of regeneration is found in many of the nation's breweries, where it is necessary to cool huge amounts of hot wort before it is discharged to fermentation tanks. Typical in scope is an operation where

850 barrels per hour of wort (220,000 lb/h) are cooled from 200 to 50°F by means of 242,000 lb/h (R = 1.1:1) of water entering at 35°F and being heated to 165°F. As a result; approximately 33,000,000 Btu/h are saved, there is an excellent water balance for use throughout the brewery, and no water is discharged to the sewer.

For a dairy, regeneration usually involves the transfer of heat from pasteurized milk to cooler raw milk entering the system. After initially heating 100,000 lb/hr of milk from 40 to 170°F by high temperature, short time pasteurization, 90% regeneration permits cooling of the milk back down to 40°F with a savings of 11,700,000 Btu/h. Only 10%

of the total heat or cooling must be supplied, and no cooling medium such as city water is used and discarded.

Chemically, there are many and varied regenerative applications. For desalination, APV has supplied a number of Paraflows, which are virtually perpetual motion machines. These units achieve 95% regeneration in heating 87,000 lb/h (175 gpm) from 70 to 197°F while cooling a secondary stream flowing at 78,000 lb/h from 214 to 73°F. Savings in the Btu load in this case are 11,050,000 per hour.

While flow rates in hot oil applications are quite low in comparison, temperatures are very high. It is possible to cool 400 gph of vegetable oil from 446 to 226°F while heating 400 GPH of oil from 200 to 400°F with 80% regeneration.

In the production of caustic soda, where very corrosive product streams are encountered, Paraflows with nickel plates are being used to cool 10,000 lb/h of 72% NaOH from 292 to 210°F while heating 14,000 lb/h of 50% NaOH from 120 to 169°F.

Cost and Efficiency

To examine a hypothetical case of Paraflow regeneration from the viewpoint of efficiency and dollar savings, consider the following process duty:

Duty Heat 100 GPM of fluid 1 from 50°F to 200°F while cooling 100 gpm of fluid 2 from 200 to 50°F (see Tables

Under ordinary conditions, steam required to heat fluid 1 would be in the nature of 7500 lb/h with an equivalent cooling requirement for the second stream of 625 tons of refrigeration. Using 85% regeneration, however, the energy needs are drastically reduced.

At this point in the process, fluid 2 in a conventional system has been cooled to only 90°F by means of 85°F city water and will require supplemental refrigeration for final cooling to 50°F. At the same time, fluid 2 in an APV regenerative system has been cooled to 72.5°F by means of 85% regeneration and must be cooled further to 50°F.

Duty

Cool from 90 to 50°F

Supplemental 50,000 X 40 93.5 tons refrigeration 12,000

required = 166.5 tons

Assuming average 166.5 X 3.5 93.5 X 3.5

electrical cost = 583.8 kWh = 327.25 kWh of 40/kWh

supplemental 100 100

refrigeration x 365 = $204,571 X 365 = $114,669

cost

Definition

The number of heat-transfer units is defined as the temperature rise of fluid one divided by the mean temperature difference. A further term for the HTU is the temperature ratio (TR), ie,

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