With the utility loop correctly pressurized, water can remain liquid above 100°C and deliver controlled heat to the UHT pasteurizer. The design must maintain pressure above the local saturation pressure at every operating point, contain thermal expansion, and safely relieve credible overpressure conditions.
Design basis and heat-transfer fluid
The term pressurized hot water here means liquid water maintained above its atmospheric boiling temperature by system pressure. Pressure suppresses boiling; it does not create the heat. Loss of pressure can make the liquid flash into steam, producing a rapid volume increase and a severe burn hazard.
Start with the pasteurizer duty rather than an arbitrary utility temperature. Record the required product inlet and outlet temperatures, product flow, allowable utility temperature, heat-exchanger pressure drop, startup condition, and maximum credible temperature. Calculate the steady heat duty as:
Q̇ = ṁp × cp,p × (Tp,out − Tp,in)
The required utility flow follows from:
ṁu = Q̇ / [cp,u × (Tu,in − Tu,out)]
Use fluid properties at the actual operating temperature. If a water–propylene glycol mixture is considered, obtain its concentration-dependent heat capacity, density, viscosity, boiling behavior, and compatibility data from the fluid supplier. A pasteurizer may require an HT1 food-grade fluid where incidental contact is possible. Glycol changes pump duty and heat-exchanger performance, so it is not a drop-in substitute for water.
Check 1: Expect the documented utility heat duty to equal the product-side duty after accounting for stated heat losses, with both calculations expressed in consistent units.
Minimum operating pressure
The controlling value is absolute pressure, not gauge pressure. At every location containing hot liquid, the local absolute pressure must exceed the saturation pressure corresponding to the local fluid temperature:
Pabs,local > Psat(Tlocal)
For design work, express the minimum required pressure as:
Pabs,min = Psat(Tmax) + ΔPoperating
ΔPoperating is the engineering allowance for elevation, piping and exchanger losses, pump transients, control variation, instrument uncertainty, and other credible disturbances. Select it through the pressure-system design review; no universal margin can be inferred from temperature alone.
Evaluate the lowest-pressure point, not just the pressure transmitter near the heater. A high elevation, pump suction, restrictive heat exchanger, or partially closed valve can reach saturation pressure while another gauge still shows acceptable pressure. For a 105°C operating example, read Psat(105°C) from an approved water-property source and add the project-specific operating allowance. Select all components using their pressure ratings at design temperature rather than their ambient ratings.
Check 2: Expect the calculated absolute pressure at the hottest, lowest-pressure location to remain above local saturation pressure during steady operation and the worst defined transient.
Closed-loop pressure and expansion equipment
A closed loop needs a controlled means of establishing pressure and accepting thermal expansion. Heating a liquid-filled, blocked-in volume without expansion capacity can raise pressure rapidly because liquid water is only slightly compressible.
| Element | Design function | Commissioning evidence |
|---|---|---|
| Expansion vessel or equivalent | Accepts fluid expansion while maintaining the operating pressure band | Pressure changes smoothly from cold to hot conditions |
| Pressurization connection | Establishes cold fill pressure and replaces controlled losses | Stable pressure with no continuous unexplained makeup |
| Air removal | Removes trapped gas that disrupts circulation and heat transfer | No erratic flow, pump noise, or unstable temperature |
| Pressure-relief path | Limits pressure during credible heating, isolation, and equipment faults | Set pressure and discharge destination match the approved design |
| Low-pressure trip | Stops heat input before boiling or flashing develops | Simulated low pressure removes heat and places the loop in its defined safe state |
Locate pressure sensing so it represents the condition that protects the heater and loop. A relief device does not replace normal expansion control, and a control valve does not replace an independent protective trip. Route relief discharge for the possible release of hot liquid and steam. A qualified pressure-system engineer must approve containment, relief capacity, isolation arrangements, and the applicable pressure-equipment and food-processing requirements.
Check 3: Expect cold fill pressure, expansion capacity, relief settings, equipment ratings, and protective trip settings to form one documented pressure envelope with no blocked-in heated segment left unprotected.
Heat exchanger and control connections
Separate the pressurized utility loop from the product with a heat exchanger selected for the calculated duty. The first-pass exchanger relationship is:
Q̇ = U × A × ΔTlm
Obtain U, allowable pressure drop, fouling allowance, and correction factors from the exchanger design. Maintain a positive temperature approach throughout the heating profile; a high utility supply temperature alone does not prove that the exchanger can transfer the required duty.
Control the heat source or utility flow from the process temperature objective. Add high-temperature protection independently of the normal controller. Interlock heat input with proven circulation and adequate pressure. The protective logic must remove heat when pressure falls, flow is lost, or the high-temperature limit operates. Define valve and heater states for loss of control power.
Instrumentation should expose utility supply temperature, utility return temperature, product temperature, loop pressure, and circulation status. Place sensors where they measure the controlled process and protective boundary rather than convenient but thermally isolated pipe sections.
Check 4: Expect stable circulation, a utility temperature drop matching transferred heat, and immediate removal of heat input when any protective permissive is withdrawn.
Commissioning sequence
- Inspect component pressure-temperature ratings, flow direction, isolation positions, relief paths, sensor locations, and expansion connections against the approved design.
- Fill the loop with the specified fluid and concentration. Remove trapped air from high points and equipment cavities.
- Establish the approved cold pressure. Record pressure and temperature together so gauge readings can be converted to absolute pressure correctly.
- Start circulation without heat. Confirm pump behavior, flow indication, valve travel, and differential pressure across the heat exchanger.
- Introduce heat in controlled increments. At each plateau, record supply and return temperatures, pressure at the limiting location, flow, and expansion-vessel response.
- Challenge the low-pressure, low-flow, and high-temperature protective functions using the approved test method. Confirm that heat input stops and requires the specified reset action.
- Run at the maximum commissioned temperature and process duty. Compare measured product-side and utility-side heat balances.
Do not commission above 100°C by defeating a trip, throttling an isolation valve to create pressure, or relying on pump discharge pressure alone. Those practices can leave part of the circuit at or below saturation pressure.
Check 5: Expect monotonic, controlled pressure change during warmup, no boiling indicators, no unexplained makeup demand, and successful operation of every independent trip.
End-to-end performance verification
-
Pressure check: Expect the minimum calculated absolute pressure anywhere in the hot circuit to exceed
Psat(Tlocal)by the approved operating allowance. -
Energy check: Expect
ṁ × cp × ΔTon the utility side to agree with the product-side heat duty within the project’s measurement tolerance. - Control check: Expect the product outlet temperature to hold its approved setpoint without sustained oscillation or operation against a protective limit.
- Containment check: Expect no leakage, relief discharge, pressure drift, or recurring need for makeup fluid during the rated run.
- Trip check: Expect loss of pressure or circulation to remove heat before the local pressure-temperature condition reaches the boiling boundary.
Frequently asked questions
What happens if pressure is lost above 100°C?
Part of the liquid can flash into steam as pressure falls below the saturation pressure for its temperature. The resulting expansion can discharge hot liquid and steam, so the low-pressure trip must remove heat and the relief path must terminate at an approved location.
What happens if the expansion vessel is undersized?
Pressure can rise sharply during warmup, cycle against the relief device, or exceed a component’s allowable pressure. Compare the vessel’s usable acceptance volume with calculated fluid expansion across the full cold-to-hot temperature range.
What happens if propylene glycol replaces water?
Boiling behavior changes, but heat capacity, viscosity, density, pump duty, and exchanger performance also change. Use supplier properties for the selected concentration and specify an HT1 food-grade fluid where incidental product contact is possible.
What happens if only the heater pressure gauge is checked?
A remote high point or pump suction can reach saturation pressure first. Final verification step: calculate or measure the limiting location’s absolute pressure at maximum temperature and expect it to remain above Psat(Tlocal) by the approved operating allowance.