Curing efficiency with Heat Exchangers
Fuel and Electrical consumption
and the factors that directly affect both when Curing with
Heat Exchangers.
Fuel consumption during curing depends on
Air Furnace's efficiency i.e.
· Amount of Air supplied by the fan &
· Heat Exchanger's efficiency .
Amount of Air supplied to the tobacco by the fan:
Measured in cfm (after the Heat exchanger has been installed).
It is the main factor that greatly affects fuel consumption
during curing (up to even 25% consumption increase when
air supply is insufficient).
It also greatly affects dry leaf weight and quality.
The air supply from the Air furnace to the tobacco changes
according to Heat Exchanger's and it's casing's design.(restriction
to the air flow causing air pressure drop)
Heat Exchanger's air pressure drop:
Measured in inches of water column determines the restriction
that the Heat Exchanger causes to the air flow as the air
moves around it inside the air furnace box.
This inevitable restriction (smaller or greater according
to Heat Exchanger's &casing's design) decreases the amount
of air supplied by the fan to the tobacco in the barn.
Small air restriction (small air pressure drop) means more
air supplied to the tobacco
greater restriction means less air to the tobacco , since
smaller or greater percentage of power is absorbed from fan's
motor just to overcome the resistance of the Heat Exchanger.
Heat Exchanger's efficiency :
Usually called Combustion Efficiency it is measured from the
combustion gases and expressed in % (Net or Gross)
In the U.S.A we measure Gross Efficiency i.e. we take in account
the efficiency of the fuel too.
It is the percentage (i.e. 85%) of the heat transfer from
the combustion to the air circulating around the Heat Exchanger.
The rest (i.e. 15%) is lost through the chimney.
To measure properly Heat Exchanger's Efficiency it is necessary
to first adjust the burner according to manufacturer's specification
with the following sequence:
-Set the burner at recommended by manufacturer btus according
to the size of barn's fan motor.(i.e. according to the amount
of air supplied to the barn)
-Adjust burner's air intake damper according to specification
in order to get in the exhaust gases O2 =2,5%-3% ( excess
air 15%-20%).
-Let the burner fire without stop for at least 10 minutes
so Heat Exchanger reaches operating temperature.
Perform the measurement.
Note : Low exhaust gases temperature mean high efficiency
Recommended exhaust gas. temp :400 - 500 F.
Avoid temperature above 550 F to save fuel.
Avoid temperature below 380 F to save the Heat Exchanger from
early deterioration due to condensation when firing fuel oil
or natural gas.
Note: Low O2 (excess air) in the exhaust gases means
higher efficiency.
Avoid lowering O2 below 2,5% not to get smoke deposits in
your Heat exchanger.
Avoid increasing O2 above 3% to save fuel.
Note: Heat Exchanger's Efficiency increases by setting
lower the Btus.
Avoid setting too low Btus because they will not be enough
to cure.
Avoid setting them too high because the burner is going to
run with many short operating cycles instead of few long ones.
To save fuel and get better & high texture tobacco, the
burner must be adjusted to that many Btus so it fires for
as long as possible during leaf drying stage.
Heat Exchanger's efficiency increases by adding more heated
surface and more passages to the hot fumes, by delaying their
exit through the chimney & by increasing the air velocity
around the Heat Exchanger so the air moving faster cools the
exiting gases more.
i.e. A Heat Exchanger 85% efficient is consuming 5% less fuel
than a 80% efficient for the same btus supplied.
This is true but it will most probably not consume 5% less
fuel when it will be mounted on the Air Furnace of a tobacco
curing barn. It may consume up to 25% more fuel than the 80%
efficient Heat Exchanger when it's design has not seriously
taken in account the air restriction (pressure drop) it will
cause to the air flow.
Heat Exchanger's efficiency is not Air Furnace's Efficiency
.
Air Furnace's efficiency takes also in account the air supply
to the barn after the inevitable restriction that the Heat
Exchanger caused to the air flow.
Air Furnace's efficiency :
It is usually called Curing Efficiency.
Takes in account both Heat Exchanger's efficiency & Fan's
air supply to the barn.
Measures Heat Exchanger's combustion efficiency, Heat Exchanger's
air pressure drop & the amount of air supply to the barn.
The efficiency of the Air furnace complete (Heat Exchanger's
+ Fan's efficiency) determines how much fuel the barn will
consume to cure the tobacco.
This is what we care for since this is the efficiency that
will lower considerably the fuel consumption during curing.
Therefore our aim is to design Heat Exchangers with a descent
efficiency of 80%-85% Gross (~ 89% - 94% Net) having the
minimum possible resistance to the air flow (air pressure
drop in the furnace) so we can supply the barn and the
tobacco with as much air as possible.
Fuel consumption :
Usually measured in pounds of fuel per pound of cured (dry)
tobacco shows accurately the amount (weight) of fuel consumed
to cure a certain amount (weight) of dry tobacco.
Fuel consumption when curing with VCU® (our Indirect fired
complete air furnace) varies between 0,35 -O,45 pounds
of fuel per pound of cured (dry) tobacco (average of all
leaf positions)
Leaf position & maturity ,the ambient humidity, the curing
schedule ,some small differences of fuel's thermal output
(Btus per pound)and the barn's design & thermal losses
are resulting above variation. The lower consumption is when
curing in dry climate, ripe middle stalk position leaves,
in a good barn with appropriate air supply.
Fuel is usually sold by volume ( i.e. gallons, liters , etc.)
not by weight.
Multiplying the volume (gallons or liters)* the specific gravity
of each fuel you know how many gallons (or liters) are in
a pound of fuel.
Electrical consumption:
Usually measured in kwh/pound of dry tobacco shows the amount
of electricity consumed to cure a certain amount of dry tobacco.
The main factor that affects electrical consumption is the
amount of air supplied to the tobacco.
Insufficient air supply will delay the cure and will increase
dry leaf weight losses both resulting high electrical consumption
in kwh per pound of dry tobacco.
The amount of air supplied to the tobacco depends mainly on
:
-fan motor installed,
-fan's efficiency,
-heat exchanger's air pressure drop .
Electrical consumption when curing with VCU® is on average
0,27 -0,37 kwh per pound of dry tobacco ( depending on
leaf maturity, climatic conditions & leaf position ).
[HeXe
FAQ] [Octopus] [PL300]
[Bull
][Stick
Kiln ]
[Hot water installations]
[Efficiency-emmisions
]
|
