A dairy in Ireland is replacing a 20-year-old milk pasteuriser. The supplier offers a plate heat exchanger with about 33 m² of plates for 10,000 L/h, 90% regeneration, a booster pump after the holding tube and a flow diversion valve. The production manager wants to know whether the area is sensible, what 90% regeneration saves and why the booster pump is not optional. A few equations and one food safety rule answer all three.
In short
- A plate heat exchanger (PHE) is a pack of thin, corrugated steel plates whose gaskets route two liquids through alternate channels, usually in opposite directions.
- Heat duty comes from Q = m·cp·ΔT; the area needed comes from Q = U·A·LMTD, where LMTD is the log mean temperature difference.
- An HTST milk pasteuriser has regeneration, heating, holding tube and cooling sections. At 90% regeneration, heating and cooling duties fall to a tenth.
- In the regenerator, pasteurised milk must be at higher pressure than raw milk, so any leak flows from pasteurised to raw.
- Fouling lowers the U-value; a creeping hot-water temperature is the usual early sign.
What is a plate heat exchanger?
A plate heat exchanger is a stack of pressed stainless steel plates, each sealed by a gasket and clamped in a frame. The gaskets direct the hot and cold liquids into alternate channels, so every plate has product on one side and a heating or cooling medium on the other. Plates are usually under 1 mm thick, with channels a few millimetres wide.
The pressed pattern, often a chevron, makes the flow turbulent (constantly eddying and mixing) at modest velocities. Turbulence thins the slow liquid film next to each plate, which speeds heat transfer, so PHEs are compact and standard for milk, juice, beer, wort and other thin liquids. Limits: narrow channels block with fibres and particles, gaskets need replacing, and plates can crack or develop pinholes. Pulpy products go to tubular or scraped-surface exchangers instead.
How does a plate heat exchanger pasteuriser work?
In a high-temperature short-time (HTST) pasteuriser, milk passes through a fixed sequence of sections, plus a holding tube. The Codex Code of Hygienic Practice for Milk and Milk Products (CXC 57-2004) gives 72 °C for 15 s, applied to every particle, as the reference continuous-flow treatment. Many plants run a little higher for margin; this example uses 74 °C.
- Balance tank. Raw milk at about 4 °C enters; diverted milk returns here.
- Regeneration, raw side. Raw milk is warmed by hot pasteurised milk flowing the other way on the far side of the same plates.
- Heating. Hot water, heated by steam, takes the milk to the pasteurisation temperature.
- Holding tube. An insulated, upward-sloping tube sized so the fastest-moving milk stays inside for at least the required time.
- Flow diversion valve. A sensor at the end of the holding tube lets milk forward only at or above the set temperature; otherwise it returns to the balance tank.
- Regeneration, pasteurised side. Pasteurised milk gives most of its heat to raw milk.
- Cooling. Chilled water or glycol brings the milk to 4 °C or below.
A timing pump (a positive displacement pump set to a fixed maximum rate), or a flow meter with speed control, caps the flow, because more flow means less holding time. Holding tubes are sized on the fastest particle, which moves at roughly 1.2 times the mean velocity in turbulent flow and twice the mean in laminar (smooth, layered) flow.
How do you calculate the heat duty with Q = m·cp·ΔT?
Heat duty is the rate of heat transfer, in kW. For a liquid that changes temperature without changing phase:
Q = m × cp × ΔT
m is the mass flow (kg/s), cp the specific heat capacity, the heat needed to warm 1 kg by 1 K (kJ/(kg·K)), and ΔT the stream’s temperature change (K). The heat one stream gains equals the heat the other loses, ignoring small losses.
Worked example
Basis: 10,000 L/h of whole milk at about 1.03 kg/L, so 10,300 kg/h or 2.861 kg/s; cp = 3.9 kJ/(kg·K); raw milk at 4 °C; pasteurisation at 74 °C; 90% regeneration.
Total rise needed = 74 − 4 = 70 K. Regeneration supplies 0.90 × 70 = 63 K, so raw milk leaves regeneration at 67 °C.
Regeneration duty = 2.861 × 3.9 × 63 = 703 kW.
Heating duty = 2.861 × 3.9 × (74 − 67) = 78.1 kW.
The pasteurised milk, with the same flow and cp, falls by the same 63 K to 74 − 63 = 11 °C. Cooling to 4 °C takes 2.861 × 3.9 × 7 = 78.1 kW.
Without regeneration: 2.861 × 3.9 × 70 = 781 kW of heating and another 781 kW of cooling.
How do you calculate the LMTD for counter-current flow?
The log mean temperature difference (LMTD) is the correct average temperature difference between two streams whose gap changes along the exchanger. For counter-current flow, with the liquids moving in opposite directions:
LMTD = (ΔT₁ − ΔT₂) ÷ ln(ΔT₁ ÷ ΔT₂)
ΔT₁ = T_hot,in − T_cold,out, at the end where the hot stream enters, and ΔT₂ = T_hot,out − T_cold,in, at the other end. The area then follows from the design equation:
Q = U × A × LMTD
Worked example
Heating section: hot water enters at 77 °C at 3.0 kg/s (cp = 4.19 kJ/(kg·K)) and heats the milk from 67 °C to 74 °C, so Q = 78.1 kW. Take U = 4.0 kW/(m²·K) for clean plates (illustrative).
Hot-water outlet = 77 − 78.1 ÷ (3.0 × 4.19) = 77 − 6.21 = 70.79 °C.
ΔT₁ = 77 − 74 = 3.00 K. ΔT₂ = 70.79 − 67 = 3.79 K.
LMTD = (3.79 − 3.00) ÷ ln(3.79 ÷ 3.00) = 0.79 ÷ 0.234 = 3.38 K.
Area = 78.1 ÷ (4.0 × 3.38) = 5.8 m².
Here the end differences are similar, so the LMTD is close to their average. When they differ widely the average misleads: with 40 K and 5 K, the LMTD is 35 ÷ ln 8 = 16.8 K against an arithmetic mean of 22.5 K, and sizing on the mean would leave the exchanger about 25% short of area.
Counter-current flow gives a larger LMTD than co-current flow, where both streams move the same way, and lets the cold stream leave hotter than the hot stream leaves. In a balanced regenerator, ΔT₁ = ΔT₂ and the mean difference is that constant gap. Section by section, the method sizes the whole pack:
| Section | Milk (°C) | Duty (kW) | Mean ΔT (K) | U (kW/(m²·K)) | Area (m²) |
|---|---|---|---|---|---|
| Regeneration | Raw 4 → 67; pasteurised 74 → 11 | 703 | 7.0 | 4.5 | 22.3 |
| Heating (water 77 → 70.8 °C) | 67 → 74 | 78.1 | 3.38 | 4.0 | 5.8 |
| Cooling (water 1 → 5.7 °C at 4.0 kg/s) | 11 → 4 | 78.1 | 4.06 | 4.0 | 4.8 |
| Total | About 33 |
The U-values are illustrative clean figures, and the total matches the Irish quotation.
What is a U-value and what lowers it?
The overall heat transfer coefficient, or U-value, is the heat flow per square metre of plate per kelvin of temperature difference, in W/(m²·K). It adds up the resistances in series: two liquid films, the plate and any fouling:
1/U = 1/h_hot + x/k + 1/h_cold + R_f
h is each film coefficient, the convective heat transfer coefficient between a liquid and the plate (W/(m²·K)); x is the plate thickness (m), k its thermal conductivity (about 16 W/(m·K) for stainless steel) and R_f the fouling resistance (m²·K/W).
Worked example
Hot-water side h = 10,000 W/(m²·K), milk side h = 7,000 W/(m²·K), plate 0.5 mm thick, k = 16 W/(m·K). All values are illustrative.
1/U = 1/10,000 + 0.0005/16 + 1/7,000 = 0.000100 + 0.000031 + 0.000143 = 0.000274 m²·K/W, so U ≈ 3,650 W/(m²·K).
The steel contributes only about 11% of the resistance; the two liquid films contribute the rest.
Add a fouling resistance of 0.0001 m²·K/W and U falls to about 2,670 W/(m²·K), a loss of about 27%.
Fouling is the build-up of deposits on heat transfer surfaces; in milk heaters, denatured whey proteins and calcium phosphate coat the hottest plates. As U falls, the controller raises the hot-water temperature to hold the milk at set point, which speeds further fouling. Cleaning in place (CIP), cleaning without dismantling, removes deposits with alkaline and acid washes. The Introduction to Food Process Engineering course builds U from these resistances step by step.
What is regeneration efficiency, and how much is enough?
Regeneration efficiency is the share of the total temperature rise supplied by regeneration:
RE = (T_after regeneration − T_raw) ÷ (T_pasteurisation − T_raw)
In the example, RE = (67 − 4) ÷ (74 − 4) = 63 ÷ 70 = 0.90. Because both sides carry the same flow with almost the same cp, the temperature gap along the regenerator is constant and equal to (1 − RE) × (T_pasteurisation − T_raw). That gap shrinks as RE rises, so the area climbs steeply.
| RE | Raw milk leaves regeneration (°C) | Heating duty (kW) | Temperature gap (K) | Regeneration area at U = 4.5 kW/(m²·K) (m²) |
|---|---|---|---|---|
| 0.80 | 60.0 | 156 | 14.0 | 9.9 |
| 0.85 | 63.5 | 117 | 10.5 | 14.1 |
| 0.90 | 67.0 | 78 | 7.0 | 22.3 |
| 0.95 | 70.5 | 39 | 3.5 | 47.1 |
Basis: 2.861 kg/s of milk, cp 3.9 kJ/(kg·K), 4 °C to 74 °C; cooling duty falls in step with heating. Moving from 80% to 90% halves both duties for a little over twice the area; moving from 90% to 95% halves them again but more than doubles the area.
Why must pasteurised milk be at a higher pressure than raw milk?
Because a leak flows from high pressure to low pressure. In the regenerator, raw and pasteurised milk are separated only by a thin plate. If a plate cracks while the raw side is at the higher pressure, raw milk leaks into pasteurised milk, and nothing downstream will detect it.
Good design keeps the pasteurised side at the higher pressure at all times, including start-up and low flow. The pump and valve layout does this, for example a booster pump that pressurises the pasteurised milk after the holding tube, or a back-pressure valve on the pasteurised outlet. A differential pressure transmitter watches the gap and triggers an alarm or diversion if it is lost. Many dairy codes require this and some set a minimum difference, so check your national legislation.
Pressure control does not replace plate checks: regeneration plates are leak tested periodically, for example by dye penetrant or pressure-hold tests. The holding tube, flow control, diversion valve and pressure differential protect the pasteurisation critical control point (CCP), a step where control is essential to prevent or eliminate a hazard, so changes to them need revalidation. For how pasteurisation compares with other preservation methods, see Introduction to Food Technology.
Frequently asked questions
What is the difference between a plate and a tubular heat exchanger?
A plate heat exchanger uses thin corrugated plates with narrow channels, giving high heat transfer coefficients in a compact frame, but it blocks with fibres and particles. A tubular heat exchanger has wider passages, fewer gaskets and higher pressure limits, so it suits pulpy juices, tomato products and UHT duties, at the cost of more area.
Why is counter-current flow used in plate heat exchangers?
Counter-current flow gives the largest log mean temperature difference for given inlet and outlet temperatures, so it needs the least area. It also lets the cold stream leave hotter than the hot stream leaves. With co-current flow and equal flows, both streams can only approach their mean temperature, so regeneration could never exceed 50%.
What is a typical regeneration efficiency for a milk pasteuriser?
Modern milk HTST pasteurisers are commonly designed for regeneration efficiencies of 90% or more. Each step up needs much more plate area, because the temperature gap in the regenerator equals (1 − RE) times the total temperature rise. At 90% with milk heated from 4 °C to 74 °C, the gap is only 7 K, so regeneration is usually the largest section in the frame.
What does the flow diversion valve do?
The flow diversion valve is an automatic valve that sends milk forward only when the temperature at the end of the holding tube is at or above the set pasteurisation temperature. If the temperature falls, or another permissive such as maximum flow or the pressure differential is lost, it diverts milk back to the balance tank. It fails safe, moving to divert on loss of air or power.
How do you know when a plate heat exchanger needs cleaning?
Watch the heating-medium temperature and the pressure drop. As deposits build up, U falls, so the controller raises the hot-water temperature to hold the product at set point, and pressure drop rises as channels narrow. Plants set run lengths from validation studies and treat these trends as early warnings, after checking that flows and inlet temperatures have not changed.
Next step. The Introduction to Food Process Engineering course explains the overall coefficient, LMTD, regeneration and fouling, and how to choose plate, tubular or scraped-surface heat exchangers. It ends with a proctored final assessment and an ASC certificate. You can also see all eleven food science and technology courses.
Sources. R. Paul Singh, Dennis R. Heldman and Ferruh Erdogdu, Introduction to Food Engineering, 6th edn (Academic Press, 2024). P. J. Fellows, Food Processing Technology: Principles and Practice, 5th edn (Woodhead Publishing, 2022). Romeo T. Toledo, Rakesh K. Singh and Fanbin Kong, Fundamentals of Food Process Engineering, 4th edn (Springer, 2018). Codex Alimentarius Commission, Code of Hygienic Practice for Milk and Milk Products (CXC 57-2004), available from the Codex Alimentarius website.
This article is general guidance on heat exchanger and pasteuriser principles and is not a substitute for the applicable standard, your national legislation or the advice of a qualified engineer.