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The milk gets thicker at every exchanger
Making evaporated, condensed or sweetened condensed milk means taking milk from 12 percent solids to 26, 40 or 72 percent, and the exchangers along that path see a different fluid at each step. The forewarmer sees milk. The evaporator preheaters see milk at rising temperature. The concentrate cooler sees something between cream and honey, and the sweetened condensed cooler sees a supersaturated lactose syrup that has to crystallize, on purpose, to a size the tongue cannot feel.
Concentrates are where viscosity and fouling both go off the scale, and this page is about the exchangers built for them.
Forewarming, preheating, concentrate cooling and crystallization
Forewarming for heat stability
Evaporated milk is sterilized in the can or by UHT after concentration, and milk that has not been forewarmed gels or separates in that step. Forewarming heats the milk to 200 to 240 F and holds it 1 to 10 minutes, which denatures whey protein onto the casein and shifts the calcium balance so the concentrate stays fluid through sterilization and six months on the shelf.
The forewarmer is a tubular heater on pressurized hot water with a sloped hold coil, and it fouls the way any milk heater above 200 F fouls: multi-tube at 5 to 7 ft/s with a 5 to 8 degree approach runs a shift. The hot water set is a U-tube steam-to-water exchanger.
Evaporator feed preheating
A multi-effect falling film evaporator takes milk in at the first effect temperature and out of the last effect at 105 to 120 F and 40 to 50 percent solids. The feed is preheated in stages on condensate and bled vapor, so the approach at each preheater is fixed by the evaporator. These are shell and tube preheaters, milk in the tubes at 4 to 6 ft/s, with removable heads so they can be rodded.
They scale because the milk is at its hottest before it is concentrated. HeatX quotes them straight-tube with a pitch a rod passes, in 316L, with 15 to 20 percent area margin so a week of scale does not starve the first effect. The whole condensed milk line, from standardization to filling, is on our condensed milk processing page.
- Two to four preheaters in series on successively hotter condensate
- Straight tubes, removable heads, pitch open enough to rod
Cooling the concentrate
Concentrate leaves the last effect at 110 to 120 F and 40 to 50 percent solids, at 30 to 80 cP and climbing as it cools. Evaporated milk at 26 percent is 10 to 20 cP and a wide-gap plate handles it. Condensed milk at 40 percent is tube-in-tube: 1.5 to 2 inch bore, glycol in the annulus, 2 to 3 ft/s, pressure drop checked at the outlet viscosity.
Sweetened condensed milk is a separate problem. At 72 percent solids the lactose is supersaturated below about 100 F and will crystallize; the cooler's job is to make it crystallize small. Cool quickly to 85 to 90 F, seed with fine lactose, and cool with agitation to 65 to 70 F so the crystals stay under 10 microns, in a jacketed vessel or scraped-surface unit after a tube-in-tube precooler.
The common mistake
Sizing the concentrate cooler on the inlet viscosity. At 120 F a 40 percent condensed milk is 30 cP and looks like cream. At 45 F it is 300 to 500 cP and the last section, where the glycol is coldest and the product thickest, delivers a tenth of the front section's coefficient. A cooler sized at the average lands the product at 55 F and the plant adds a second cooler. Size it at the outlet, at a pressure drop the positive displacement pump can make.
Send the product and solids, the evaporator outlet temperature and flow, the storage or canning temperature and the utilities. Call and talk it through with an engineer: 1-805-484-2992
Condensed and evaporated milk process conditions
Each row follows the milk from forewarmer to concentrate cooler, with viscosity climbing along the way. Check your solids and temperatures against the rows, then send the evaporator figures so the sizing starts from them.
| Condition | Detail |
| Product | Milk 12 percent solids in; evaporated 26 percent; condensed 40 percent; sweetened condensed 72 percent |
| Flow range | Feed 20,000 to 150,000 lb/h; concentrate 5,000 to 40,000 lb/h |
| Temperatures in / out | Forewarm 40 F to 200 to 240 F; feed to first effect 160 F; concentrate 120 F to 40 to 45 F |
| Hold | Forewarming 1 to 10 minutes at 200 to 240 F for heat stability |
| Utility | Steam via U-tube hot water set; evaporator vapor and condensate; chilled water and glycol |
| Approach | 5 to 8 degrees forewarming; set by the effect on preheat; 8 to 12 on concentrate cooling |
| Viscosity | 2 cP feed; 30 to 80 cP at 40 percent and 120 F; 2,000 to 5,000 cP sweetened condensed |
| Construction | Multi-tube forewarmer; brushable straight tube preheaters; tube-in-tube concentrate cooler |
| Finish / class | 3-A, 316L on warm duties, 25 to 32 Ra; welded tube-in-tube on concentrate |
| CIP | Caustic and acid every cycle; preheaters rodded weekly |
Common FAQs
Forewarming heats milk to 200 to 240 F and holds it 1 to 10 minutes before evaporation so the concentrate is heat-stable through in-can sterilization. Without it the evaporated milk gels or separates on the shelf.
Tube-in-tube with a 1.5 to 2 inch bore, glycol in the annulus, 2 to 3 ft/s, sized at the outlet viscosity of 300 to 500 cP. Plates are used only on evaporated milk at 26 percent, and then wide-gap.
They see hot milk before it is concentrated, on a wall heated by evaporator vapor the exchanger cannot lower. Daily acid CIP and weekly rodding are the routine, so they are built straight-tube with removable heads.
HeatX supplies the tube-in-tube precooler to 85 to 90 F and the jacketed crystallizing vessel or scraped-surface unit, sized to the lactose crystal size the plant specifies.
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