1. General information
Operating philosophy of a hot water system with tobacco curing barns.
Hot water installations for tobacco curing barns are designed so the heat to the tobacco barns is supplied by hot water produced in solid fuel(biomass) water boilers or liquid or gas fuel fired water boiler(s) and distributed to the barns through insulated pipelines.
Contrary to the usual way of curing barns operation where the heat source is on each barn individually, when firing solid fuels (biomass) it is not efficient and safe to fire it independently on each air furnace ( with a modern pellet burner) for several reasons (biomass firing cannot be precisely controlled as liquid or gas fuel is , many exhaust stalks close to each other risking to supply fumes to the next barn’s air intake resulting high nitrosamines, more risk of combustion chamber’s leaks resulting exhaust fumes escaping in the tobacco, much more servicing -cleaning ashes work on each furnace etc….).
The use of hot water for curing is highly recommended when firing biomass since the combustion of the biomass is away from barn’s air intakes and the supply of hot water can be precisely controlled for the smoothest possible curing.
Hot water tobacco curing barns operation depends on the proper and careful design of a central hot water installation since the heat source is not any longer on each barn individually but on a central location where the heat produced is distributed to usually more than one barns so if installation is not working perfectly all connected barns will be influenced.
Each hot water installation as well as the barn’s heating coils are designed specifically for each installation where they are supposed to run ,after taking in consideration certain factors .i.e. the size and number of curing barns attached to each installation, different ways of water distribution ,passages and size of pipelines, kind of fuel fired, operation with higher or lower water temperature, operation with small or bigger barn fans etc.
Tobacco curing with hot water barns is ideal . It is the best possible quality tobacco curing.
Curing performance with hot water heat exchangers is higher than with any other heat exchanger with diesel or gas burner fuel ,due to the progressive (analog) heat supply to each barn and the lower temperature of the heating element.
Specific and experienced installation’s design ensures the minimum possible fuel consumption, reliable and safe installation’s and barn’s operation with excellent curing results .
Poor installation’s design leads to high fuel consumption , unreliable and unsafe operation with impossible control of the curing process on all the attached barns .
How the Hot water installation is designed.
The solid fuel hot water boilers are designed so they can efficiently burn wood or coal or other solid fuels either manually fed or automatically fed by mechanisms designed for automatic supply of the combustible material when this is in form of chips or pellets ,even semi automatic mechanisms for supply of wood in logs.
The hot water produced in the boiler is recirculating in one or more insulated piping system(s) through one or more water recirculating pump(s).
On the hot water piping system there are independently connected one or more barns equipped with hot water heating coils (radiators) with servocontrolled water valves .
The most efficient installations (both on installation’s and running cost)are those that have the barns grouped in lots of 5-7 ,where the principle of “load one barn a day” ensures a continuous & stable heat supply throughout the whole curing cycle resulting the highest combustion efficiency.
Curing lasts 6-7 days. During those days ~half of the barns will be asking for little heat (few btus) for~ 3 days (coloring & end of stem drying) and the oter half for maximum heat ( wilting-leaf drying & beginning of stem drying).
Following the “load one barn a day” principle on each group of 5-7 barns , heat demand is kept stable during the whole curing cycle , the size of required boiler can be much smaller , it’s combustion capacity almost stable during the week and it’s efficiency maximized .
Usually the set of boiler and it’s hydraulic parts are mounted in a “boiler room” .
One (or more) hot water boilers ,working parallel or separately may be installed in the boiler room .
One (or more) group of tobacco curing barns can be connected to the hot water boiler room as long as the size of water boiler(s) is enough to supply the required heat to the group(s) of tobacco barns.
One (or more) big hot water inertia (storage) tank is part of this piping system when the hot water boiler has small water volume content , in order to give a big inertia to the system.
One (or more) set of hydraulic parts i.e. recirculating pump , expansion tank , auto refilling valve ,magnesium electrolytic protection ,regulating valves, safety valves ,air purge valves , thermostats and other mechanisms on the boiler ensure proper water circulation in the piping , reliable and safe operation of the hot water system and the attached on it curing barns.
When a water inertia tank needs to be added it can be mounted in or close to the boiler room.
A piping system is connecting the boiler with it’s parts to the tobacco barns .Loop is designed according to each installation and may be individual with a recirculating pump for each barn or group of barns or a main loop with one recirculating pump for all barns.
A ) Solid fuel Hot water boiler.
The boiler is made in heavy gauge steel with a heavy cast iron grid or “wet” grid ( grid made from heavy water pipes) where on the solid fuel (wood or coal) is burning either manually fed by the fireman or automatically fed in order to keep the water hot.
When solid fuel is in form of pellets or smaller than ~3 cms pcs the boilers are equipped with automatic pellet feeding mechanism of the“screw” type with inverter auto -adjust feeder or automatic pellet pressure burner of the latest technology.
Boiler is made by experienced manufacturer designed for solid fuel with large combustion chamber so it can hold a lot of wood or coal and burn efficiently biomass as well as big volume of water content to give as big as possible inertia to the system.
The necessary oxygen for efficient combustion is :
-either naturally sucked ( small boilers) from a fresh air intake damper driven by a mechanical thermostat with a lever arm which is dipped in the hot water on boiler’s top increasing the fresh air intake opening when water temperature drops below a set point or decreasing it when the water temperature exceeds the set point ,or
-supplied by fan(s) who’s air supply is automatically controlled through frequency converter according to the solid fuel’s supply and desired water temperature.
Boiler must be equipped with at least the basic necessary devices to ensure safe operation
Boilers (mainly on big sizes) may be equipped with automatic ash removing mechanisms and dust collectors.
Size & type of boiler & installation’s design depend on fuel to be fired , barn’s size & type and installation’s total heating capacity.
The chimney must be insulated according to the boiler’s manufacturer recommendations with enough height to ensure the proper exhaust fumes suction , ending at least 5 meters higher than the barn’s roof to avoid any fume suction from barn’s fresh air intake dampers.
A regulating damper ( either manual or automatic on the bigger boilers) on boiler’s exhaust permits the regulation of exhaust gases suction to increase combustion’s efficiency.
Dust collectors, flue cleaning mechanisms either manual or automatic with air pressure may be added on the boilers to ensure continuous high combustion efficiency and keep environment clean .
The type ,size and height of the chimney depend on the boiler’s design & capacity.
C) Hot water inertia (storage) tank
This tank is a big insulated tank that is placed next or close to the boiler room and is connected on the pipeline loop.
It may be avoided in a few cases where the water content of the hot water boiler boiler is sufficient .
This tank is increasing the water volume of the installation giving the required inertia to the installation to minimize the water temperature fluctuations and helps slow down water boiling in case of malfunction of other safety devices .
It usually has an inlet and an outlet port a couple of auxiliary ports , a thermometer and a safety relief valve.
The size of the inertia tank depends on each installation’s specs.
D) Recirculating pump and hydraulic parts and mechanisms for the boiler room.
The recirculating pump(s) ensure the proper water recirculation in the pipelines and some of them or all ( according to installation’s design) are always running when boilers are firing to ensure continuous water recirculation in the system.
Ventobacco’s Smart curing controls may change the water’s temperature , delivery & pump’s pressure according to the heat demand of the barns in order to ensure progressive curing .
Location of the pump(s) depend on installation’s design in order to avoid cavitation problems etc.
The water circulation is variable according to installation’s heat demand .
Other attached hydraulic parts :
The water expansion tank can be open or closed pressure tank depending on the installation’s design .
The volume and pressure of the expansion tank is according installation’s design and the operating temp and pressure .
Pressure relief valves , safety valves, water auto refilling valve, auto bleeding valves , manometer, thermometer are on boiler’s supply and return lines in order to complete the” boiler room”.
Other hydraulic parts according to the installation’s specs.
E) Tobacco curing Barns
The tobacco curing barns in such installations are equipped with heating coils( radiators) instead of the classical furnace – burner.
Each curing barn has a heating coil usually made of copper tubes and aluminum fins or galvanized or stainless steel material according to installation’s design.
Size of coil, number of tubes ,tube diameter and fin spacing vary according to installation’s water temperature and water supply volume , the size of barns and their fan’s capacity & pressure .
Each heating coil is equipped with either a water secondary pump or a 3 wayservovalve that according to the barn’s Dry bulb control’s order either directs the hot water from the piping to enter the heating coil or bypasses it back to the piping without driving it in barn’s heating coil .
The design of the heating coil is according to installation’s design , each barn’s capacity and fan characteristics.
F) Hot water piping (recirculation)
The insulated pipelines of each installation connect the hot water boiler ( with inertia tank etc parts) to the curing barn’s heating coils.
Piping is made either in copper (small installations) or steel tubes following each installation’s hot water piping diagram taking care to avoid air ”trap’’ points with auto air bleed valves on the higher points.
It is well insulated to minimize thermal losses.
Size of piping according to installation’s design.
G) Fresh water piping. (supply)
The fresh water pipelines are arriving:
-to the boiler room in order to ensure permanent fresh water supply to boiler’s auto refilling valve so boiler stays full of water in case of water loss from the hot water system.
-to each barn in order to feed it’s sprayers for tobacco conditioning after stem drying.
CAUTION: Fresh water supply mainly to the boiler room is absolutely necessary to ensure installation’s safety. If water pressure is lower than 2-3 bar ,the installation of a pressure pump is necessary .
Size of fresh water supply piping to the boiler room is according to the boiler’s size .
On each barn pipe is ½” to run at ~4 bar pressure for the sprayers.
H) Generator set
A diesel Generator set is used as a back up source of energy to ensure :
– That the hot water recirculating pump will keep running when city power goes off to ensure boiler’s cooling .
-That the barns which are on critical curing stage ( wilting-leaf drying) will keep running .
REMEMBER: Wood or coal firing does not instantly stop when electrical power goes off and water may start boiling in the boiler if water circulation is restricted .
Of course there are safety devices built in the system to slow down combustion in such a case (boiler’s feeding mechanism , combustion’s air damper , inertia tank , relief valves) but the operation of the recirculating pump whenever boiler is firing solid fuel is obligatory.
Size of Genset according to the barn’s total hp and installation’s electrical loads.
I) Electrical Installation
The electrical installation consists of a main electrical panel 3 phase + Neutral + Ground
with properly sized fuses according to each installation’s loads.
An alternate switch (either automatic or manual) is usually installed close to the panel to divert power from city or genset to the installation .
From the main electrical panel , electrical supply must arrive to the boiler room and to each barn .
Electrical design and installation to be made by local electrical engineer following the local regulations according to each installation’s electrical specs.
G) Barn Monitoring (Ventobacco® 12 software)
Barn’s & curing site’s operation can be monitored on a PC wirelessly or by running a 2 wire cable from each barn to a hub on the PC.
Operator can follow curing site’s operation on his smart phone or tablet from any place of the world and he can be alerted with SMS messages or emails for any curing alert or barn malfunction.
Copyrigh 2012 Vencon Varsos
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