Three streams, one ambiguous capacity figure
Evaporator quotations often use a single capacity figure. It may describe the feed entering the plant, the concentrate leaving it or the water removed between them. Those are different mass flows, and a proposal that does not say which one it means leaves the buyer comparing unlike duties.
The distinction is practical rather than academic. Feed flow sets the size of the feed system and the amount of material handled. Concentrate flow sets the discharge, cooling and storage requirement. Evaporated water sets the vapour, condenser, vacuum and heat-transfer duty. A quotation that gives only one number cannot show how any of those duties was derived.
Ask for a stream table at normal, maximum and minimum load. It should show feed, concentrate and evaporated water separately, with units and operating hours attached. If the proposal uses a range, state what changes at each end. This is the discipline described in our guide to specifying falling-film capacity, applied to the commercial document.
Fix the solids basis before comparing prices
The solids basis decides what the stream table means. Total non-volatile solids, soluble solids and Brix are not interchangeable for every food liquid. A Brix reading is an approximate proxy whose relationship to mass solids depends on the product and the measurement method.
Ask for the measurement temperature, instrument or laboratory method, and whether the value is a mass fraction or a refractive reading. For a pulpy or cloudy feed, state how the sample was prepared. The FAO juice guidance lists solids and acidity among the quality criteria that affect processing and payment.
The Tetra Pak handbook gives typical final solids for dairy products to illustrate its own examples. Those values are not targets for juice, extract or another food liquid, and they should not be copied into a fruit-line specification. The relevant basis is the one measured on the actual feed and intended product. Review the source's scope.
Balance water removal and name every stream
For a steady process with conserved non-volatile solids, the solids entering in the feed equal the solids leaving in the concentrate, apart from any additions, losses or accumulation. That relationship links feed, concentrate and water removal. It does not by itself establish a machine rating, and it becomes incomplete when the process adds ingredients, removes a side stream or loses solids to the vapour path.
Ask the quotation to name its boundary: preheating, aroma recovery, recycle or transfer flush, and condensate return. A balance that excludes one of those streams will not reconcile with the utility and equipment schedule.
Our mass-balance guide sets out the calculation assumptions and a worked example. The quotation review should not repeat that arithmetic with hidden assumptions; it should show the same three streams and identify the measurement basis behind each one.
Put utility and temperature assumptions beside the duty
Every capacity statement depends on utilities and temperatures. Ask for the steam pressure, temperature, quality and available flow at the machine under load, not only the boiler's nameplate rating. For a TVR option, the motive-steam conditions and the thermocompressor operating map belong with the duty. For MVR, request the compressor lift, vapour flow, electrical demand and operating range.
Cooling-water inlet temperature, allowable rise, available flow and seasonal variation affect the condenser and vacuum package. So do non-condensable extraction and ambient conditions. Start-up heating, supplementary heating and the electrical load of pumps and controls should appear in the same schedule as the normal case.
The handbook reports steam consumption figures for dairy falling-film evaporators in its own examples. Those figures explain why effect count and recompression are evaluated, but they are not a guarantee for a fruit line or a different utility boundary. Do not accept a quoted steam economy that is not tied to the proposed heat balance and operating hours.
Describe the fouling and cleaning case with the offer
Fouling changes the heat-transfer surface required and the cleaning interval that the plant can live with. The handbook notes that viscosity and heat stability limit how far concentration can be forced. The FAO guidance describes sanitation and preparation as critical to juice quality, and it warns that pilot-scale success does not automatically settle commercial operation.
A quotation should state which product properties were assumed, how deposits were assessed and what cleaning procedure is proposed. Ask for the product-contact materials, elastomer selection, surface finish, drainability and CIP boundary. Confirm whether the cleaning supply unit is included, shared with existing equipment or supplied by others.
Where fouling behaviour is uncertain, propose a representative trial rather than converting an assumption into a guarantee. The trial should include the final concentrate, not only the feed, because the most concentrated condition is often limiting. Define how the cleaning endpoint will be checked and who accepts it.
A quotation review checklist
| Item | What the quotation should state | Why it matters |
|---|---|---|
| Stream basis | Feed, concentrate and evaporated water at named operating cases | Prevents a feed rate from being read as evaporation capacity |
| Solids basis | Mass solids or Brix, method, temperature and sample preparation | Makes the concentration target comparable between offers |
| Heat balance | Effect arrangement, heat recovery, preheating and utility quantities | Connects the duty to the proposed equipment and operating cost |
| Operating range | Normal, maximum and minimum load with control response | Shows whether the plant can run the required schedule |
| Product evidence | Viscosity, particles, fouling observations and trial plan | Tests whether the film and cleaning concept fit the actual liquid |
| Scope boundary | Inclusions, exclusions, interfaces, materials and acceptance method | Allows a like-for-like commercial comparison |
Use the checklist alongside the technical review, not instead of it. A quotation can be internally consistent and still describe a duty the plant cannot operate. The next step is to connect the stream table to the product trial and the utility schedule. Discuss a quotation review