FOOD ENGINEERING

Module 4. Freeze dehydration

Lesson 37
INDUSTRIAL FREEZE DRYERS

37.1 Introduction

The general commercial freeze dryer process is comprises of (1) Preparation of the material; (2) freezing; (3) conditioning of the frozen mass; (4) drying, that is, sublimation and desorption; and (5) conditioning the product. .

37.2 Batch Freeze Driers

The main parts of a plant for batch freeze-drying are: A freezing cabinet, a freeze-drying cabinet, an ice condenser. Auxiliary equipment necessary includes plant for the production of heat, cold and vacuum.

The products are frozen separately and are subsequently put into trays and placed on the heating plates in the freeze drying cabinet. The drying cabinet and the condenser are then evacuated as quickly as possible and the heating is switched on. The vapor is condensed into ice in the ice condenser, which consists of tubes or plates whose temperature is mostly about 15 to 20 ⁰C lower than the sublimation temperature. The condenser which actually is evaporator coils of refrigeration system therefore acts as a vacuum pump with a very great suction capacity for the condensable gases.

The pumps, which are connected after it, only have the function of maintaining the vacuum in the plant and of removing the non-condensable gases which are liberated during drying or which have gained access through leakage.

The layers of ice, which form on the condensers, are removed from time to time by defrosting. For continuous operation several condensers are used. They are alternately separated from the drying cabinet by vacuum valves, air is admitted, they are warmed for defrosting, cooled again after discharge of the water, evacuated and connected again to the cabinet by opening the valve.

37.3 Continuous Freeze Driers

Continuous freeze dryers are designed like cabinet drier which is still much in use. It consists of stacks of heating plates whose area may be up to 40 m² per cabinet. Between these plates the trays containing the product are placed either by hand or, all at once, by a mobile tray bearing trolley. The mobile cable trolley shortens the loading time and thus minimizes the danger of thawing.

Some of the industrial freeze-drying plants are based on the principle of the cable trolley operating as the product tray carrier. The cars operate automatically and move intermittently through the plant, which is composed of a number of cabinets, is separated from the rest of the plant by inlet and outlet locks with gates and chambers. Differences in temperature between the cabinets are possible.

In these cabinet driers the product remains undisturbed during the whole of the drying operation. The driers are very flexible in use. They can be easily adjusted to changes in throughput as well as the size and shape of the product.

37.4 Industrial Freeze Drying

Construction Material

Chamber, door, hinges, shelf modules, trays and vapour condenser are mostly constructed from AISI 304 stainless steel.

Chamber

Shelf module and vapour condenser are contained inside chamber.

Viewing ports are provided in the door, allowing observation of both the vapour condenser and product trays during the drying cycle.

Chamber is fitted with stainless steel pneumatically operated valves that isolate the vacuum line connection, drain, water defrost and vacuum release.

Modular Shelf Heating Plates

Plates are usually fabricated from type 304 stainless steel. Shelf module is removable from the chamber for maintenance or cleaning, and is done by rolling onto a trolley which is provided.

Trays

Trays have two sets of Type 304 stainless steel trays are provided as standard. Tray trolleys are optional.

Heating System

Heating system has thyristor controlled electric boiler, connected in series with the heat plates.

Heating fluid is glycol based and is circulated by a centrifugal pump, allowing heated or cooled fluid to be circulated through the plates on demand of the electronic load controller. A cooling heat exchanger is provided in the circuit for reducing the temperature of the plates.

Vacuum System

Roots blower is provided for developing vacuum. Roots blower and backing pump are connected to the chamber by heavy duty PVC lines and pneumatic isolating valve. Pump exhaust is vented to the exterior of the building housing the freeze dryer.

Refrigeration

Refrigeration condensing units are purpose-built specifically for each freeze drying system with capacity control to allow economical use of the low-temperature R507 refrigerant. It includes water cooled condenser.

Vapour Condenser

Vapour condenser has type 304 stainless steel tube in parallel circuits to form a direct expansion refrigerated coil.

Defrosting ice after a product cycle is by water. Hot water defrost optional using heat recovery from the refrigeration system.

Low Temperature Option

The equipment has an option of operating in low temperature if required, giving –55°C vapour condenser temperature, and –35°C/+70°C shelf freezing/heating.

Control System

Automated systems will reduce energy to govern sublimation pressure to pre-set parameters.

Fig. 37.1 Industrial freeze dryer