1-805-484-2992
Quotes - Engineering - SalesThe legal hold
Where the 15 seconds actually happen
The heat exchanger gets the milk to 161 F. The holding tube keeps it there for 15 seconds, and the flow diversion valve at the end of the tube decides whether the milk goes forward to the cooler or back to the balance tank. Those two pieces, plus the recorder that watches them, are what an inspector times, seals and signs off.
Getting the tube right is arithmetic plus a few physical rules. Getting the valve right is buying the correct valve and wiring it to the correct signal. A pasteurizer sized perfectly and fitted with a tube 8 feet short is not a pasteurizer.
Sizing the tube, choosing the valve, wiring the recorder
Tube length from velocity and hold time
Length equals fastest-particle velocity times hold time. For 100 gpm in a 2.5 inch sanitary tube the mean velocity is about 7.3 ft/s; turbulent flow puts the fastest particle at 1.2 times mean, so 8.7 ft/s times 15 seconds is 131 feet of tube. Laminar flow, which cream and mix can reach, puts it at 2.0 times mean and doubles the length. The PMO does not let you assume turbulence: the tube is timed with a salt injection at the sealed pump setting and the measured hold governs.
The tube also has to hold temperature. Milk at 161 F loses heat to the room and the outlet sensor may read no more than 1 degree below the inlet, so long tubes in cold rooms get light insulation. It cannot be heated, because a heated tube is an exchanger.
- Upward slope of 1/4 inch per foot so it drains and cannot trap air
- No valves, reducers or instruments that change the cross-section
- Outlet sensor within 18 inches of the diversion valve
The flow diversion valve
The valve at the tube outlet is a three-way sanitary valve: forward to the regenerator and cooler, or divert to the balance tank, failing to divert on loss of air or power. It must be leak-protect, with two seats and a vented cavity between them in divert, so a worn seat cannot let sub-temperature milk creep forward. Single-seat valves are not accepted.
Response is under one second, and the signal comes from the safety thermal limit recorder, not a PLC in the middle. When the valve diverts, the forward line loses pressure and the booster pump interlock trips at the same instant so the regenerator pressure rule holds. HeatX includes the divert leg in the pressure drop calculation so the timing pump cannot run faster in divert and shorten the hold.
The safety thermal limit recorder
The STLR reads the sensor at the tube outlet, pens the temperature on a chart, and closes a contact above the cut-in setpoint. The valve goes forward only on that contact. The inspector tests it by lowering the hot water until the pen crosses the line and watching the valve move; charts are kept for months.
What is not optional is the sensor location: after the tube, before the valve, in the product stream, not in a deep well that lags. A well can read 161 F while the milk it measures has been at 158 F for five seconds.
The common mistake
A hold that changes with the product. A plant sizes the tube for milk at 100 gpm and turbulent flow, then runs 40 percent cream through it. Slow the pump for cream and the flow can go laminar, the fastest particle doubles its speed, and the tube that held milk 15 seconds holds cream 9. HeatX times the tube at the lowest Reynolds number on the product list; where a plant runs both, a second sealed pump setting with its own timed hold is the honest answer.
Send the products, the flow, the tube size or the room you have for one, and whether the recorder is new or existing. Call and talk it through with an engineer: 1-805-484-2992
Holding tube and flow diversion process conditions
The rows list the tube, valve and recorder build we size to, along with the velocity and hold assumptions behind the length. Check them against your product and flow, then send your figures with the quote request.
| Condition | Detail |
| Product | Milk, cream, mix or juice leaving the heating section at setpoint |
| Flow range | Fixed by the sealed timing pump; 20 to 400 gpm |
| Temperatures in / out | 161 F minimum at inlet; at most 1 degree lost to the outlet sensor |
| Hold | 15 seconds at 161 F for milk; 25 seconds at 175 F for mix; at the fastest particle |
| Utility | None; the tube cannot be heated |
| Velocity | 3 to 6 ft/s mean; fastest particle 1.2 to 2.0 times mean by flow regime |
| Construction | Sanitary tube 1.5 to 4 inch, 316L, sloped up 1/4 inch per foot, no restrictions |
| Finish / class | 3-A, 32 Ra, tri-clamp or welded runs |
| Valve | Three-way leak-protect diversion valve, diverts in under 1 second |
| Records | Safety thermal limit recorder, sensor at tube outlet, chart kept |
Common FAQs
About 130 feet in 2.5 inch tube for turbulent flow, about 215 feet if the product runs laminar. It is confirmed by timing a salt injection at the sealed pump setting, and the timed hold governs.
Yes, a helical coil that rises continuously is common on tubular and small plate lines. Same rules: no restrictions, continuous upward slope, sensor and valve at the top.
A single seat wears, and during a divert a worn seat lets sub-temperature milk into the forward line. Two seats with a vented cavity leak to the floor drain instead, where it is visible and safe.
Not under the PMO. The STLR is the legal device; a PLC may duplicate the record and add alarms, but the divert signal comes from the recorder's own contact and the chart is the record.
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