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Quotes - Engineering - SalesDairy line circuits
Why the pasteurizer circuit is the hardest-working caustic heater in a dairy
An HTST or UHT line is a chain of heat exchangers with product on one side and hot water or steam on the other, and the product side cooks a little every run. Whey protein denatures above about 160°F and binds calcium phosphate into a hard, chalky milkstone on the heating and holding sections, with a softer protein film on the regeneration section. On a UHT line the final heater sees product-side wall temperatures well above 250°F, so the deposit there is thin but very hard. This soil decides how hot and how strong the caustic must be.
The pasteurizer is cleaned as a line circuit: solution is pushed through the product path at roughly the production flow, often a little more, so the plates and tubes see the same velocity that product saw. A 3-inch line at 5 ft/s is about 70 gpm; a 4-inch UHT line is nearer 120 gpm. The circuit is usually fed from the CIP tank and returned to it, and the caustic must already be at 165–175°F when the first wash begins, not twenty minutes into it, because the cold product-side steel of a pasteurizer pulls the solution temperature down the moment it enters.
Our part of that circuit is the heater between the tank or supply pump and the line, and this page is about sizing and building it for the pasteurizer duty.
The line steals heat from the caustic before the first pass ends
A plate pasteurizer holds a lot of cold stainless when the wash starts. Caustic that leaves the tank at 175°F can come back at 140°F on the first circuit, and putting that 35°F back at 70 gpm is roughly 1.2 MBtu/h before any soil has been touched.
The circuit as a pasteurizer runs it
Caustic at 1–2% sodium hydroxide, run at 165–175°F for 30–45 minutes, is the main step; UHT plants tend toward the top of that range because baked-on protein will not lift at 150°F no matter how long the wash runs. The acid step is not optional here. Nitric or phosphoric at 0.5–1.5% and 120–140°F takes the calcium phosphate out of the milkstone, usually every day on a UHT line and several times a week on an HTST unit. Final rinse runs 120–140°F and many plants follow with a hot-water hold near 180°F before restart.
The first-pass temperature drop is the figure that catches people out. A return of 140°F against a 175°F supply at 70 gpm is a 35°F rise the heater must make up on the fly, about 1.2 MBtu/h, on top of holding the tank. Steam at the heater is commonly 50 psig (298°F saturation), so there is plenty of approach; what runs short is surface, and a heater sized only for the tank loss leaves the circuit cold for its first ten minutes.
The heater we build for it
A steam-to-caustic U-tube shell-and-tube with steam inside 3/4-inch tubes and the caustic in a 316L shell, rated 150 psig at 250°F on the solution side. Steam in the tubes means the bundle expands freely as the steam valve opens and closes every cycle, and the shell side, which is the side that fouls, is the side you can get at. The bundle pulls for inspection when carbonate scale or carried-over protein film builds on the outside of the tubes; the steam side stays clean.
Size it for the first-pass recovery, not the steady state, and carry a fouling allowance of 0.001–0.002 on the shell side. Where the plant runs the circuit once-through from a makeup loop rather than a tank, the same unit is sized for the full rise at 70 or 120 gpm, and the steam valve must turn down to the holding load without hunting. Tri-clamp to 4 inches where the circuit is treated as sanitary; most pasteurizer CIP heaters are mill-finish utility equipment.
What to send us for a pasteurizer or UHT line circuit
The duty is set by the line, so tell us about the line as much as the tank.
- HTST or UHT, and the product-side line size (2, 3 or 4-inch)
- Circuit flow in gpm, or the production rate we can derive it from
- Caustic concentration and the target supply temperature, 165–175°F
- Observed or expected first-pass return temperature from the cold line
- Steam pressure at the heater inlet, with the control valve and trap arrangement
- Acid step chemistry and frequency, so the head gasket is specified right
- Tank recirculation or once-through, and tri-clamp or flanged connections
Call and talk it through with an engineer: 1-805-484-2992
UHT line questions
It can be done, but it rarely works well. The hot-water set is sized to hold product at 161 or 280°F with a tight approach, not to lift 70 gpm of caustic by 35°F on a cold start, and routing caustic through a loop designed for water puts 1–2% sodium hydroxide where it was never expected. A dedicated steam-to-caustic shell-and-tube between the CIP tank and the line keeps the two apart.
Probably, or it was sized for the wrong condition. A heater picked to hold a 2,000-gallon tank at temperature between cycles has nowhere near the surface needed to recover a 35°F first-pass drop at full circuit flow. Check the steam pressure actually arriving at the heater as well; 15 psig at the inlet instead of the 50 psig on the drawing cuts the approach and the capacity hard.
It changes the elastomer and the inspection habit, not the metal. 316L shell, tubes and heads handle nitric or phosphoric at 0.5–1.5% and 140°F without trouble. The head or bonnet gasket is the only soft part on the solution side, and daily nitric is the case where we specify FKM instead of EPDM. Order the heater for the acid duty from the start.
Yes, if the two lines are never washed at the same time and the heater is sized for the larger circuit, normally the 4-inch UHT line at about 120 gpm. The steam control valve then has to turn down cleanly to the 70 gpm HTST duty without cycling. If both lines must wash together, two heaters sized to their own circuits are simpler to control then one large unit.
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