Sanitary Plate and Frame
Heat Exchangers

Gasketed chevron for clean, water-like product; double-wall where the utility must never reach product; wide-gap where pulp or soft solids would lodge. Decide in that order. If the utility is inhibited glycol, tower water or plant condensate against a product with no kill step downstream, the double-wall plate pays for its 10 to 20 percent extra area. If the product carries pulp, trub or fiber, the wide-gap pressing keeps the pack open. One frame can combine them: wide-gap heating and regeneration sections on a juice pasteurizer feeding a double-wall glycol cooler at the end.

About nine-tenths of it: at 90 percent regeneration a 100 gpm milk heater buys roughly 600,000 Btu/h of steam instead of 6 million. The arithmetic, using the water rule of 500 Btu/h per gpm per F, which is close enough for milk, is 100 gpm times 500 times the 121 F rise from 40 to 161 F, which is 6,050,000 Btu/h of heating. Regeneration supplies 90 percent of that from the pasteurized product on its way to the cooler, leaving about 605,000 Btu/h for the hot water section, and the cooler shrinks by the same share. At roughly 900 Btu per pound of plant steam that is the difference between 6,700 and 670 lb/h. The saving is why a plate frame is chosen even when a shell and tube would do the thermal job.

The distance between the fixed and movable frame plates when the pack is correctly compressed, stated on the data sheet for each plate count. Closing past the minimum crushes the gaskets and can crack plates at the contact points; stopping short leaves gaskets unseated and the pack weeps at the first thermal cycle. The figure changes whenever plates are added or gaskets are replaced, so ask for both the nominal and minimum dimension on the data sheet and record the actual measurement after every closing.

Flows, inlet and outlet temperatures on every section, viscosity at the coldest point, largest particle, the regeneration percentage wanted, and the pump head and CIP flow available. The first four decide plate pattern and whether plates fit at all; the last three decide how many plates and whether the pack can be cleaned. On an HTST frame also state the hold temperature and the pressure differential the regulator requires across regeneration, since the booster pump and plate count are sized together. Leaving out the CIP flow is the common gap, and it produces a pack that meets the duty and fails the swab.

Pick the plate size from the flow, not the area; port velocity and channel count decide it. Plates run from 0.5 to 51 sq ft each, and for a given flow a larger plate puts the same gpm through fewer, longer channels at a larger port. Too small a plate means a 6 inch port on a tall stack of tiny channels and a frame as long as a car; too large means a dozen channels running slow, below the velocity that keeps them clean. The sizing software lands on a plate whose port carries the flow at a sensible velocity and whose channel count gives 1.5 times product velocity in CIP.

Because condensing steam puts the plate wall near steam temperature, which bakes protein and sugar onto the product side and shortens gasket life. Even 5 psig steam condenses at 227 F against milk being heated to 161 F, a wall more than 60 F above the product where the film burns on. A hot water set, a closed loop heated by steam through an injector or a small shell and tube, holds the water 3 to 5 F above the outlet temperature, so the wall is barely hotter than the product. The gaskets see about 165 F instead of 227 F, burn-on drops, and the hold temperature controls more smoothly because the water loop damps steam pressure swings.