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Quotes - Engineering - SalesWhy the pasteurized side has to win
A milk-to-milk regenerator puts raw milk on one face of a plate and pasteurized milk on the other, with well under a millimeter of stainless and a gasket between them. Plates pit, crack at port corners and lose gaskets. The PMO cannot stop that from happening, so it decides which way a leak must travel: pasteurized milk is held at least 1 psi above raw milk whenever the regenerator holds product. A pinhole then pushes finished milk into the raw stream, where it is pasteurized a second time, and never raw milk into the cooler and filler.
The rule reads simply and is easy to break by accident. Pressure on each side depends on where the pumps sit, the drop through each pass, the balance tank level and how high the pasteurized line climbs after the press. A layout that shows a healthy margin at full flow can lose it at startup, in diversion or the moment a pump trips. That is why the inspector looks at the whole circuit, not one gauge.
For us the differential is a sizing input. Plate pattern, passes and flow set the pressure drop on each side of the regenerator, and we state both figures at design flow so the controls engineer can check the margin before the frame is bolted up.
Three conditions the inspector checks on the regenerator
- Pasteurized side at least 1 psi above raw at all times
- Booster pump stops the moment the margin is lost
- Raw milk drains freely back to the balance tank
Where each side gets its pressure
The two sides of the regenerator are pushed by different pumps and lose pressure in opposite directions, so the margin has to be judged at the worst pair of points, not the average.
Raw side of the regenerator
- Feed: Balance tank by gravity, or a booster pump
- Highest pressure: At the raw inlet of the regenerator
- At shutdown: Drains back toward the balance tank
- Balance tank overflow: Below the lowest raw milk in the regenerator
Pasteurized side of the regenerator
- Driven by: Timing pump, or homogenizer used as one
- Lowest pressure: At the pasteurized outlet of the regenerator
- After the press: Rises above the raw milk level and vents
- Watched by: Differential controls tied to the diversion valve
Booster pumps and differential controls in outline
Systems without a booster pump
In the simplest HTST the timing pump draws raw milk through the regenerator on its suction side and pushes heated milk back through the pasteurized side on its discharge. Raw milk is under suction, pasteurized milk is under pump pressure, and the differential falls the right way without any controls. The limit is suction: a long, high-regeneration pack has enough raw-side drop to starve the pump, so this layout suits smaller units and moderate regeneration.
Adding a booster pump
Once regeneration climbs toward 90% or the flow grows, a centrifugal booster is added ahead of the regenerator to push raw milk through it. Now the raw side is pressurized, and the PMO expects that pump to be fenced in by interlocks.
- Runs only while the flow-diversion device is in forward flow
- Runs only while the timing pump is running
- Runs only while the differential controller shows the pasteurized side ahead
- Stops at once when any of those drops out
The differential recorder-controller
Two sanitary pressure sensors feed the controller: one where raw pressure peaks, at the raw inlet, and one where pasteurized pressure is lowest, at the pasteurized outlet. The controller records the margin and drops the booster when it falls below the setting. The regulatory agency checks the sensors and the cut-out at installation and on its routine schedule. The holding tube, flow-diversion device and recorder this controller talks to are set out on our sanitary process page on holding tubes and flow diversion; we size and build the regenerator that sits inside that loop.
Design choices that make the margin easier to hold
- Give the pasteurized side enough pass drop to stay well above raw
- Keep raw-side drop low so the booster can be small
- Use a back-pressure valve after the pasteurized outlet where needed
- Check the margin at CIP flow as well as product flow
Pressure differential questions
Only if the readings come from the right places and hold in every state the system runs in. A gauge at the raw outlet will read lower than the raw inlet, which flatters the margin. Read raw where it is highest, pasteurized where it is lowest, and watch both through startup, diversion and a pump trip.
In most layouts the diversion valve sends product from the holding tube back to the balance tank instead of through the pasteurized side, so that side loses pressure. That is exactly why the booster must stop in diversion: with no booster pushing, the raw side falls back under suction and the margin stays the right way around.
Do not plan on it. The differential rule is written for milk-to-milk regenerators, and most plants that buy double-wall plates keep the controls as well and treat the second wall as extra warning. If you want to argue for a different arrangement, take the drawing to your regulatory agency before the order is placed, not after.
Higher regeneration means more plates and a longer path on both sides, so raw-side drop grows and the booster becomes almost unavoidable. We report the pressure drop of each regenerator side at design flow with the proposal, so you can see the margin before you choose between 85% and 94% regeneration.
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