Solar Drying Series HELIOBOLT ®

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Solar Drying
Series HELIOBOLT ®

Description Solar Drying Series HELIOBOLT ®

1.- FUNCIONING PRINCIPLE

1.1.- General principle of solar drying

Solar drying consists in putting in contact, under a greenhouse, a regularly renovated air and sludge distributed over a concrete floor and mechanically removed. Between the entrance and exit of the greenhouse, the air is charged with water vapor, at the expense of the sludge that is drying. A ventilation system insures the renovation of the air with the finality of evacuating the water vapor proceeding from the sludge.
The amount of water evaporated depends on the characteristics of the air (temperature, humidity), and of the sludge (temperature, humidity, mechanical properties).

1.2.- The HELIOBOLT ® procedure

The HELIOBOLT ® procedure Works on an annual cycle: it is designed to admit, store and dry all the sludge produced during one year.

The solar drying combines the functions of drying and storing: schematically it is considered that the storing last for seven months, and that the drying is only effective during five months ( from may to September), because it is during this time of the year that the air is hotter and dryer.


1.3.- Functioning description
The dehydrated sludge feed to the system is made by means of a helicoidall screw pump. The sludge is distributed in one of the extremes of the greenhouse, in the storage area, that occupies the main part of the installation. The storage area is delimited by two semi walls of 80 cm height, parallel to the walls of the greenhouse, leaving an access way free.


The overturning and the mixing of the sludge are guaranteed by a tumbler, equipped with a rotor specially designed for such effect. The sludge is overturned once a day as an average, with the purpose of homogenizing it, of breaking the dry layer formed in the surface and of facilitating the inter exchange with the air.

The tumbler has also as function to move the sludge towards the exit of the greenhouse, some centimeters at each pass.
During the process, the humidity container in the sludge is gotten by the air in the greenhouse, being evacuated afterwards to the outside by jeans of ventilators specific central controls the ventilators.

The discharge of the greenhouse, and thus, the evacuation of the sludge is made once a year after the summer period to profit the most from the solar energy.
To accomplish this, a suitable carrier, dimensioned with the maneuvering zone under the greenhouse, is used to load the sludge.


2.- ADVANTAGES OF THE SOLAR DRYING AND THE HELIOBOLT ® PROCEDURE
The main advantages of solar drying are:
Reduction of the volume of sludge: savings in transport and cost of eliminating ths sludge.
Automatic system: simple and economical management
Lime conditioning elimination
Final product of quality, easy to manipulate: high dryness, granulated product and not dusty, absence of odor nuisances.
A sustainable solution: multiple possibilities (incineration, land fill , agriculture.)
Low operation costs.
Besides, HELIOBOLT ® has some strong advantages against other similar solar drying systems:
The using of a tumbler designed specifically for this application, strong, reliable, and safe in the long term operation Greenhouse atmosphere controlled and optimized
Lateral accesses that ease the interventions without contact with the sludge.


3.- INSTRUMENTACIÓN DESCRIPTION
3.1.- The greenhouse
The greenhouse does the following functions:
Protection against the rainwater
Transmission of the incident radiation
Heat accumulation under the greenhouse

The sizing accounts for the Standard designs and the material is variable according to the local requirements. The greenhouse is equipped with access door
that allows the incoming of a carrier for the yearly evacuation of the sludge.

These high and fixed greenhouses are horticulture standard: their capital cost is reduced and its resistance tested.

3.2.- Sludge overturning.
Sludge overturning complies with several functions:
Texture:
The “blades” of the rotor tear the sludge and give it a granular structure. These grains have a great surface of exchange with the air, which favors

evaporation and at the same time avoids that the sludge goes into anaerobic conditions.

Homogenization
The tumbler breaks the dry layer that forms in the surface and homogenizes the bed by finely mixing the sludge. The migration of the humidity in the lower part of the of the storage walkway is the avoided: evaporation is eased.

Moving
The tumbler moves the sludge bed to the exit of the greenhouse alter each pass. This way the front sludge advances regularly in the storing walkway. It reaches the end of the greenhouse (the opposing one to the sludge feed entry), at the end of the annual cycle.
The frequency of the overturn is as a yearly average, once each working day.

It overturns:
In winter once a day as maximum (mediocre drying conditions, the overturning of the sludge is a must to avoid the anaerobiosis and in consequence, the smells)
In summer once or twice a day (optimum drying conditions: it is a must to break the layer and mix the sludge).
The tumbler developed for the HELIOBOLT ® is based on framing technology, and has determining characteristics for the ease of operation:
Lifting rotor
Easy and permanent access to the sludge bed and engine
Reliable guiding system
A moving deflector avoids any sludge projection

4.- CONTROL AND COMAND OF THE INSISDE ATMÓSPHERE
The drying is more intense as more hot and dry the air is in the greenhouse
The control system perfected for the HELIOBOLT ® is based in the regulation of the renovation of the air in the greenhouse, according to the continuous measurement of the differences in temperature and absolute humidity between the inside and outside of the system.
Schematically, since the automat detects that the evaporation capacity of the air in the exterior of the greenhouse is higher than the one in the inside, the renovation of the atmosphere starts.
Thus , the intensity of the drying is optimized during the year’s length.
The renovation is assured by motor centrifugal ventilators.
The motor ventilators are also started:
In case of condensation risk
In the presence of personnel in the greenhouse, for security reasons (at the maximum of 6 minutes working time the air in the greenhouse is completely renovated)


As an average during the year, the working time of the motor ventilators is of 11 hrs. per day.
On the other side, it is imperative a ventilation that eliminates the stratification. Its function is to oppose the natural upwards flow of the hot and dry air towards the high part of the greenhouse, which damages the evaporation. Fixed under the greenhouse roof, the desestratification ventilators remove the upper air downwards.


5.- SIZING
The sizing of the HELIOBOLT ® procedure is realized by a software specifically designed for this purpose, whose conception is based on the realization of models based on the physical phenomena that intervene al trough the solar drying of sludge.

The phenomena considered by the model are:
Thermal transfers:
Sludge heating by the solar flow transmitted trough the greenhouse.
Convective exchange between the sludge and the air
Exchange by evaporation between the sludge and the air
Radioactive exchange between the sludge and the greenhouse
Convective exchange between the greenhouse and the air
Conductive exchange between the sludge and the sun

Water evaporation in the surface or in the inside of the sludge
Water vapor transfer in the air to the sludge surface
The writing of the equations of the vapor and heat balances lead to a system of differential equations whose resolutions allows to access the parameters that characterizes the procedure: temperatures, air and sludge , evaporated water flow, drying time, etc.

The meteorological data of the site and the characteristics of the greenhouse’s cover (thermal and luminous transmission) allow to establish the value of the coefficients that appear in the differential equations