Curing efficiency with Heat Exchanger Retrofits

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 ).