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Quotes - Engineering - SalesEvaporator Condensers
Surplus Vapor Condensing for Multiple-Effect, TVR and MVR Plant
On an evaporator the condenser sets the vacuum, the vacuum sets the boiling temperature, and the boiling temperature sets product quality. We size surplus vapor condensers for multiple-effect, TVR and MVR plant with that chain in mind.
Falling film evaporators concentrate liquids by spreading a thin film down the inside of heated tubes under vacuum. Residence time is short — often only a few minutes — and boiling temperature is low, which is what makes the technique suitable for heat-sensitive products: milk, whey, fruit and vegetable juices, coffee and tea extracts, plant proteins, pharmaceutical intermediates, stillage and process effluents. The condenser is what maintains the vacuum that makes low-temperature boiling possible, which puts it directly in the product quality path even though it never touches concentrate.
Multiple effect. Vapor from one effect becomes the heating medium for the next, which operates at lower pressure. Each additional effect improves steam economy and reduces boiler demand. Only vapor leaving the final effect needs condensing, so the surplus vapor condenser is sized against that last effect's load, at the lowest pressure in the train — where vapor-side pressure drop matters most.
The condenser on the final effect of an evaporator. Only vapor leaving that effect needs condensing, so the unit is sized against the last effect’s load at the lowest pressure in the train, where vapor-side pressure drop matters most.
Thermal vapor recompression (TVR). A steam jet thermocompressor entrains part of the vapor from an effect and lifts it to a higher pressure for reuse as heating medium. Steam consumption falls to a fraction of total evaporation capacity. The condenser then handles only the portion the thermocompressor does not recycle.
Mechanical vapor recompression (MVR). A high-pressure fan or compressor draws vapor out of the evaporator, compresses it, and returns it to the heating side. Once running, an MVR system needs little or no live steam, which is why it is now common on dairy, plant-based, beverage and pharmaceutical duty and why all-electric evaporators are built this way. Because most of the vapor is recycled, there is comparatively little left to condense — but the condenser that handles the excess is still what holds the vacuum, and MVR plant is often more sensitive to losing it than a multiple-effect train, because compressor operating point moves with suction pressure.
Condensate quality and aroma. Vapor leaving an effect carries volatiles and sometimes entrained product. Separators and demisters upstream limit carryover and protect both condenser and compressor, and cyclone-type separators generally give cleaner condensate than integrated designs. Where the volatile fraction carries aroma — as it does on juice, coffee and tea — a dedicated aroma recovery condenser captures that fraction and returns it to the product instead of losing it to drain.
Why performance drifts. Fouling on the vapor side from carryover, noncondensable accumulation from air in-leakage or gas released from the feed, and cooling water warming through the season all raise condenser pressure. That lifts boiling temperature throughout the train, which increases thermal damage to a heat-sensitive product and reduces capacity. When concentrate quality drifts across a season with no change in recipe, the condenser is a good place to look.
Related pages: vacuum condensers, sanitary condensers for product-contact duty, and dairy condensers.
Surplus vapor condensing
Through the vaccum, and the effect is large enough to measure on a thermometer. A juice evaporator whose final effect runs at 2 in Hg absolute boils at about 101 F, and a condenser that slips to 3 in Hg lets the same effect boil at 115 F. Every effect upstream shifts with it, because each one condenses its vapor in the next, so the first effect may climb 20 F for a 1 in Hg loss at the condenser.
The loss shows as darker concentrate, cooked notes and in some products a drop in vitamin content, and because the recipe and the feed have not changed, the evaporator operator is usually blamed before the condenser is. A pressure gauge on the condenser inlet, trended against cooling water temperature, is the cheapest quality instrument on the line.
The difference you will feel is in the utility bill. A TVR evaporator still uses roughly 1 lb of motive steam for every 2 to 3 lb of water evaporated, while an MVR unit uses around 10 to 20 kWh per 1,000 lb evaporated and almost no steam. At typical utility prices MVR costs a fraction as much to run, which is why it is chosen for large continuous dairy and juice plants. TVR wins on smaller capacity, on products that foul the heating surface and need frequent CIP, and in plants that already have excess steam.
For the condenser, the two differ in what they leave over: a TVR system sends the thermocompressor's motive steam plus the surplus vapor to the condenser, so its load is larger and steadier, while an MVR condenser handles only a small balance flow but has to hold suction pressure for a compressor that does not tolerate drift.
Because the compressor cannot recycle every pound, and the small surplus it leaves still has to go somewhere at the pressure the compressor needs on its suction. The balance vapor on an MVR evaporator is typically 5 to 10 percent of total evaporation, coming from the heat of compression, from feed arriving hotter than it needs to be and from flash in the separator, and the condenser handles that plus the air that leaks in.
The condenser also carries the whole evaporation load at startup, before the compressor is up to speed and the vacuum is established, so it is sized for more than the balance flow. In operation, a loss of condenser performence raises suction pressure, the compressor's volumetric flow falls, evaporation drops and the plant can walk itself toward surge, which is why MVR operators watch condenser pressure so closely.
The steam economy gains roughly 0.85 to 0.9 lb of water evaporated per pound of steam for each effect added, so a triple effect approaches 2.6 and a five-effect about 4.3. The gain per effect falls as the count rises. The limit is temperature. The total driving force available between the first-effect steam and the condenser is fixed, perhaps 60 to 70 F for a dairy plant, and every effect takes a slice of it, so seven effects leave each with less than 10 F and the surface per effect grows.
Products that boil with elevation from dissolved solids, such as sugar or whey concentrate, lose part of each slice to that elevation and run out of effects sooner. A quick check is the simple payback on the next effect against its steam saving, which tends to run past five years beyond the fifth effect.
Most of the aroma leaves with the first 10 to 15 percent of water evaporated. That is why an aroma recovery condenser is placed on the first effect or on a small stripping column ahead of it, rather than on the surplus vapor at the end of the train. That stream is a small fraction of total evaporation but it carries the esters and aldehydes that define the product, and condensing it means cooling the vapor to 40 F or lower in stages so the volatile fraction is captured as a concentrated essence rather than diluted in the main condensate.
For an orange juice line the essence may be a few hundredths of the feed volume and still be worth more than the concentrate it came from. Whether it pays depends less on the condenser cost than on whether the plant can sell or use the essence.
Design for the warmest cooling water the condenser will see at full rate, not the annual average, because an evaporator loses vacuum in August and the quality lab will notice. Tower water that runs 70 F in spring can reach 88 F in a humid summer, and with a final effect condensing at 100 F that moves the available temperature difference from 30 F to 12 F, which is a condenser two and a half times larger if the rate is to hold.
The alternatives are to accept a seasonal rate cut, to bring in chilled water for the summer months or to design the condenser for the summer and run it with throttled water the rest of the year. Each is a legitimate choice, but it has to be made at the datasheet stage, and stating the cooling water inlet as a range with the rate expected at each end is what lets us size it properly.
