Monday, 31 October 2011

Tray Dryers


The Practical Action tray dryers aim to fill the gap in the equipment market between the many solar (and assisted solar) drier designs (described in the Practical Action Technical brief on solar drying) and the large industrial equipment.
Semi continuous tray dryer
The range of industrial technologies used for food drying includes; tunnel dryers, spray, roller freeze and tray dryers. Tray dryers are the most suitable in terms of cost and output, for small and medium sized enterprises.
The term tray drying is normally refers to small industrial systems with some form of air heater and a fan to pass air over the product being dried although sun drying on trays or in solar dryers can also be called tray drying.
While small tray dryers are available from Europe and the USA, where they are used in pilot plants and Universities, their cost makes them unaffordable and uneconomic for producers in developing countries.
Practical Action (previously know as ITDG) recognised the need for small, controllable, powered tray dryers that could be constructed by engineers in developing countries mainly from locally available materials that were capable of producing high quality products. This has resulted in a range of tray dryers that can be manufactured at a lower price and there are now tray dryers, based on these designs in many countries with the greatest up-take of the technology in Latin America.
The key point to bear in mind when considering the local construction of such a dryer is to understand the basic principles involved and adapt them to local conditions such as the dimensions of local plywood sheet, common stock steel sizes, social conditions and fuel availability.

Tray dryer principles

Basic airflow pattern

The dryers are made of trays held in a cabinet which is connected to a source of air heated by gas, diesel or bio-mass such as rice husk.  The air temperature is usually controlled by a thermostat which is normally set between 50 and 700C. The air enters the bottom of the chamber below the trays and then rises, through the trays of food being dried, and exits from an opening in the top of the chamber. In the Practical Action systems the trays are designed to force the air to follow a longer zigzag route which increases the air/food contact time and thus improve its efficiency. This system also reduces back pressure which means that cheaper, smaller fans can be used.  The airflow in a typical system is shown in Figure 2. There are three basic types of tray dryer cabinets; batch, semi-continuous and cross flow dryers.  Practical Action has worked with the first two systems.
Practical Action is able to supply information to allow the construction of four dryers. In some cases full drawings and manuals are available together with a case study book and a short video.

Batch Tray Dryer

Batch tray dryer
Batch cabinets are the simplest and cheapest to construct.  The cabinet is a simple large wooden box fitted with internal runners to support the trays of food being processed.  The trays are loaded into the chamber, the doors closed and heated air is blown through the stack of trays until the entire product is dry.  As the hot air enters below the bottom tray, this tray will dry first.  The last tray to dry is the one at the top of the chamber.  The advantages and disadvantages of this system are:
* simple, low cost chamber
* low labour costs – simply load and then unload
* a tendency to over-dry the lower trays
* low efficiency, in terms of furl consumption, in the later stages of drying when most of the trays are dry.

The Practical Action Batch Tray Dryer

This, the smallest dryer in the range, is designed to allow trials to be carried out, the production of samples to demonstrate to buyers and for basic product costings to be calculated.  Due to its low capacity the tea chest dryer is not intended for commercial production but does allow product development at very low cost.
Construction:  plywood walls over wood frame
Dimensions:  0.5 m3
Capacity:  6 trays
Heat source: 2kw electric fan heater wired with external thermostat so that fan runs continuously
Capacity:  approx. 1kg/day


The Semi-Continuous Tray Dryer

Semi-continuous cabinets were developed by Practical Action in order to overcome some of the disadvantages of the batch system.  In a semi-continuous cabinet a lifting mechanism allows all of the trays except the bottom tray to be lifted. It is thus possible to remove the lowest tray as soon as the product is dry. The mechanism then allows all the trays to be lowered (now tray 2 is at the bottom of the stack). This leaves a space at the top of the stack to load a tray of fresh material.
Two types of lifting mechanism are available both of which activate four movable fingers that lift the second tray upwards. One design is operated by a handle which is pulled downwards. The other design, developed in Sri Lanka, has been found more suitable for use by women and here the lifting mechanism is a car screw jack which, on winding up, lifts the four fingers.
Tray lifting mechanism
 The advantages/disadvantages of this system are:
* over-drying is avoided
* product quality is higher
* fuel efficiency is considerably increased
* a higher daily throughput is possible
* the cabinet is however more expensive to construct
* labour costs are higher due to loading and unloading trays at regular intervals
* in order to maximise output 24 hour working is recommended
Typical semi-continuous dryers are shown in Figure 3, which shows the lifting mechanism and the gap above the lowest tray that is ready for removal.
Detail of lifting fingers
1 External lifting mechanism
2 Main lifting lever pivot support
3 Vertical lifting rods
4 Lifting fingers
5 Trays with angle iron edge bearers
6 Inner wall of drying cabinet
7 Lifting finger pivot support
Basic airflow pattern

The Small Practical Action Semi-continuous Tray Dryer

Construction:  Plywood wall over wooden frame
Dimensions:  Cabinet 1.08 x 0.98 x 0.83m
Overall height:  1.9m
Capacity:  13 trays, useful area per tray 0.53m2
Heat source:  Direct gas heater/blower (max output 25kw and 470m3 air/hr
Fuel consumption: 0.5lbs per hour at 50°C
Throughput:  120 kg of herbs per 24 hour cycle
Labour:   3 workers on shift

The Large Practical Action Semi-continuous Tray Dryer

Construction:  plywood walls over angle iron frame
Dimensions:  cabinet 1.4 x 1.2 x 0.9m
Overall height:  2.2m
Capacity:  15 standard trays, each 0.8m2,/sup>
Heat source:  kerosene/diesel indirect heater-blower. Max heat output
   60kw. Air output 2800m3
Fuel consumption: 0.8 gals/hr at 50°C
Throughput:  360 kg/ 24 hrs for herbs
Labour:   3 workers over 24 hrs