Sizing a plate heat exchanger for your brewhouse wort chiller

Start with the heat load

The heat you must remove is mass flow times specific heat times temperature drop. Take a 10 hl brewhouse knocking out 1,000 litres of 12 °P wort (specific gravity about 1.048) in 50 minutes, from 98 °C down to 12 °C for a lager pitch.

• Flow: 1,000 L in 50 min is 20 L/min, or about 0.35 kg/s.
• Wort specific heat: about 4.0 kJ/kg K.
• Temperature drop: 86 K.

Heat load is 0.35 x 4.0 x 86, roughly 120 kW. That is the number the plates, the coolant supply and the pumps all have to meet. Shorter knockouts raise it. Ale pitches at 18 to 20 °C lower it.

Do the sum for your own worst case: the biggest batch, the strongest wort, the coldest pitch and the shortest knockout you plan to run in the next five years. Brewhouses grow into their chillers faster than owners expect.

How much coolant you need

Most small brewhouses use a single-stage chiller fed by chilled water from an ice water tank at about 2 to 4 °C. The water leaves hot and goes to the hot liquor tank, so the heat is reused.

If the water warms from 3 °C to 80 °C, each kg absorbs about 4.18 x 77, or 322 kJ. At 120 kW you need roughly 0.37 kg/s, about 22 L/min, or around 1,100 litres of chilled water per brew. That is a little more than the batch volume, which is why the ice water tank is usually sized at 1.2 to 1.5 times the cast-out volume. Check this against your hot liquor needs too. Excess hot water you cannot use is wasted energy.

Plate area and approach temperature

Plate area comes from heat load divided by the overall heat transfer coefficient (U) and the log mean temperature difference (LMTD). In our example the hot end gap is 98 minus 80, or 18 K. The cold end gap is 12 minus 3, or 9 K. LMTD is (18 - 9) / ln 2, about 13 K.

With a U of around 3,500 W/m2 K, a reasonable figure for clean wort and water plates, area is 120,000 / (3,500 x 13), roughly 2.6 m2. Suppliers will add margin for fouling and for your worst day. The cold end gap, the approach, is the number to watch. Push it below 3 or 4 K and the plate count rises steeply.

What Indian water does to the design

Here is where many designs go wrong. Borewell and municipal water in much of India sits around 25 to 32 °C for much of the year. You cannot cool wort to 12 °C with 30 °C water in a single stage. A two-stage chiller uses tower or mains water first, taking wort down to the mid 30s °C, then glycol or chilled water for the last stretch. The second stage carries a smaller share of the load but needs the colder coolant.

Size the refrigeration as well. Recharging 1,100 litres of water from 30 °C to 3 °C takes about 124,000 kJ, around 34 kWh of cooling. Spread over eight hours, that is over 4 kW of refrigeration before losses. Our guide to glycol cooling systems covers the plant side.

The floor test: watch the last ten minutes

The classic symptom of an undersized system is wort out temperature creeping up near the end of the cast. The plates are fine. The ice water tank has warmed, so the approach collapses. Log wort out and coolant in temperatures every five minutes on a few brews. If both rise together in the last quarter, the tank or the refrigeration is too small, not the exchanger.

Fouling, cleaning and commissioning

Hot break and hop matter foul the wort side. U falls as plates foul. Clean in place after every brew with hot caustic, flowing in the reverse direction to wort. Open the plate pack on a schedule to inspect gaskets. During brewhouse commissioning, run a water test at full flow and record pressure drop and temperatures. That baseline tells you later when plates are fouling.

Fit thermometers on wort out and coolant out, plus a flow meter on the wort line, if the supplier did not. Without them you are sizing and troubleshooting blind. Add an oxygen injection point after the chiller as well, because cold wort is where aeration belongs, not hot wort upstream.