Forced-Circulation Evaporator

Overview

A forced-circulation evaporator concentrates liquid foods by boiling off water under vacuum while a pump keeps the product moving fast through the heating tubes. The forced flow is the defining feature. In a natural-circulation or falling-film unit the product moves on its own, but viscous, fouling, or crystallizing liquids do not move fast enough that way and they scorch on the tube wall. The Forced-Circulation Pump solves this by pushing product through the Calandria (Tube Bundle) at 2 to 4 m/s, far faster than gravity alone would carry it.

Because the product flow is forced, this evaporator handles materials that defeat other designs: tomato paste at 30 percent solids, glucose and sugar syrups that crystallize, fruit purees with pulp, and gelatin or starch slurries. It is the workhorse of high-concentration food and process duty.

The unit runs under vacuum so the product boils at 40 to 70 deg C rather than 100 deg C. Lower boiling temperature protects flavor, color, and heat-sensitive vitamins, and it lets low-pressure steam do the heating.

How it works

Feed enters the Product Piping and joins the recirculating stream. The Forced-Circulation Pump drives this combined stream up through the Product Tube Bundle inside the Calandria (Tube Bundle). Steam condenses on the outside of the Heat-Transfer Tube tubes and transfers its latent heat into the product.

Here is the key trick: the product inside the tubes is kept under enough static pressure that it does not boil while it is in the heating zone. It only superheats. The hot, pressurized stream then shoots into the Separator Vessel, where the pressure suddenly drops to the vessel vacuum. The superheated liquid flashes, meaning a fraction of it boils instantly into vapor.

The Tangential Inlet Elbow spins the incoming flow so liquid is thrown to the wall and vapor rises up the center. A Demister Pad near the top catches entrained droplets so they do not carry over into the vapor line. Concentrated liquid collects in the bottom of the vessel, where most of it is drawn back into the circulation loop for another pass, and a controlled bleed of finished concentrate is withdrawn.

Vapor and vacuum system

The vapor leaving the separator still carries large latent heat. The Vapor & Vacuum System recovers and disposes of it. A Thermocompressor, which is a steam-driven jet ejector, recompresses part of the vapor and feeds it back to the steam side of the Calandria (Tube Bundle), cutting fresh steam use. The rest of the vapor goes to the Surface Condenser.

The vacuum itself is held by a Liquid-Ring Vacuum Pump of the liquid-ring type. Its Liquid-Ring Rotor spins eccentrically inside a partly water-filled Vacuum Pump Body, forming moving liquid pockets that pull non-condensable gases out of the system. Vacuum level sets the boiling temperature directly, so the control system trims the vacuum pump to hold the target.

Condenser and condensate

The Surface Condenser is a surface or direct-contact unit that turns spent vapor back into water using cooling water through the Condenser Tube Bundle. The recovered condensate, together with the steam condensate from the heating side, is pumped away by the Condensate Pump and a Steam Trap. In a multiple-effect arrangement, the vapor from one effect becomes the heating steam for the next, which is what raises steam economy from roughly 0.9 up toward 6 kg of water removed per kg of fresh steam.

Materials and hygiene

Every product-contact surface is stainless steel 304 or 316L with a polished, crevice-free finish so the unit can be cleaned in place. The Heat-Transfer Tube tubes are expanded into the Tube Sheet plates with a hygienic joint that leaves no pockets for product to lodge. The Mechanical Seal on the circulation pump is a double seal with a clean flush so the bearing oil never reaches the product.

The Clean-in-Place System runs automated caustic and acid wash cycles through Rotary Spray Ball heads and the full recirculation loop. Because the product side fouls, especially with milk proteins and sugars, regular CIP keeps the Product Tube Bundle heat transfer from degrading between production runs.

Instrumentation and control

The Control System coordinates the whole process. An Inline Density Meter of the Coriolis type measures concentration in real time and trims the concentrate bleed valve to hold the target total solids. A Level Probe keeps the separator pool steady, Pressure Sensor units watch the vacuum, and RTD Temperature Sensor RTDs track boiling temperature so the steam valve can hold it within a degree. The operator runs everything from the Operator HMI Panel, driven by the PLC Module.

Variants and use

Single-effect units suit small or batch food plants. Two to five effects in series are standard for large continuous duty such as tomato paste and dairy, trading capital cost for steam economy. Mechanical vapor recompression replaces the Thermocompressor and condenser with an electric vapor blower where electricity is cheaper than steam. A finishing effect with extra residence time is added when the product crystallizes, as in sugar and glucose lines. Whatever the configuration, the Forced-Circulation Pump and the flash-in-the-separator principle stay the same, which is what lets this machine push food up to 70 percent solids without burning it.