Flow, Temperatures and Regeneration

Getting the Inputs Right Before the Size

Flow is misread more often than any other input. A line rated at 60 gpm is often started at 45 and pushed to 70 on a busy day, and the exchanger needs to hold outlet temperature across that whole range. State the maximum for sizing and the minimum for control, because a unit with a wide turndown needs a utility valve that can throttle far enough without hunting. On plates, low flow also means low channel velocity, which drops the coefficient and lets deposits settle; if the night shift runs at half rate, say so and the plate pattern can be chosen for it.

Temperatures are usually stated correctly but incompletely. The product inlet is the temperature at the exchanger, after the balance tank, not the silo set point. The outlet is the required temperature plus the tolerance: a pasteurizer that must never dip below 161 F and is set at 165 F has a 4 F margin, and the control loop and the surface area both have to respect it. On the utility side, supply temperature is one number and return temperature is another; if the plant's hot water set has a fixed return, the utility flow is fixed too, and that can cap the duty regardless of the exchanger.

Approach temperature is a cost decision dressed up as a thermal one. The closer the product outlet is allowed to come to the utility inlet, the less utility is wasted, and the more surface is bought to get there. On a gasketed plate a 2 to 3 F approach is routine and a 1 F approach is possible on clean fluids at some cost in plates. On a sanitary shell-and-tube the approach is 10 F or more, because the geometry is not fully counterflow and the coefficient is lower. A tube-in-tube in series is fully counterflow and can be pushed to a 5 F approach on viscous product, at a price in length.

A temperature cross, where the cold stream leaves warmer than the hot stream leaves, is only possible in true counterflow. It is what a regenerator does all day: raw milk leaves the regenerator at 152 F while pasteurized milk leaves it at 53 F. A shell-and-tube with two tube passes cannot do that in one shell, and asking it to is the commonest cause of an impossible sizing. If the duty needs a cross, it is plates, tube-in-tube, or shells in series.

Regeneration is decided by the utility bill and by the product. On HTST milk the answer is nearly always yes, at 85 to 95%. On a juice pasteurizer it is usually yes. On a small batch process that runs an hour a day the payback may not be there. On a viscous product the regenerator approach widens to 10 or 15 F and efficiency falls to 70 or 80%, which is still worth having. The design has to fix the efficiency first, because it sets the inlet temperature to the heater and the outlet from the cooler, and therefore the size of both.

The pressure balance across a regenerator is a hygiene requirement, not a thermal one. Pasteurized product must be at a higher pressure than raw product wherever they share a plate or a tube wall, so that any leak goes the safe way. That is arranged with a booster pump on the raw side after the regenerator, or a timing pump downstream, and the exchanger's pressure drops on each side have to be known to set it. It is one more reason to specify the pressure drop budget with the flow rather than accept whatever falls out.

Utility flow is the input most often left off the sheet. A 190 F hot water set that returns at 175 F delivers 15 F per pound of water, so a 1.16 MMBtu/h heater needs about 155 gpm of water; with regeneration at 90% it needs 16 gpm. A chilled water loop at 36 F with a 10 F rise on a 1.18 MMBtu/h cooler needs about 236 gpm; with regeneration, under 30 gpm. Give the utility flow the plant can actually spare and the size and the regeneration decision can be made together. Call and talk it through with an engineer: 1-805-484-2992