OH 1

Airborne Dryer

Humidity Control

Applications

Flakt Dryers

OH 2

Airborne Dryer

Humidity Control - Pulp and Paper

Humidity control can help to optimize the performance of the hot air /

direct contact drying processes used in the pulp and paper industry.

These processes include Yankee Hoods and Through Air Dryers

used in tissue manufacturing and Air Float dryers and Flash Dryers

used to dry pulp. Humidity control has the potential to improve their

thermal efficiency, sheet moisture control, and can assist in process

diagnostics.

In the past, a fundamental impediment to success has been sensor

accuracy and reliability in high temperature/particulate exhaust

streams. Our recent experiences with the Dewcon Humidity sensor

have been very positive.

OH 3

Airborne Dryer

Dewcon Sensor

Standard Unit

Accurate and

reliable in high

temperature,

dirty exhaust

streams

OH 4

Airborne Dryer

Dewcon Sensor – Principle of Operation

Dipole Measurement

OH 5

Airborne Dryer

Humidity Control

Impact on Thermal Efficiency

The exhaust humidity is one of the most important factors determining dryer

thermal efficiency. Increasing the exhaust humidity reduces the dry air

flowrate into/out of the dryer. The energy required to heat up dry air to the

exhaust temperature is reduced proportionately.

In the majority of dryers, the dry air feed-rate is maintained relatively

constant over the entire operating range. The Makeup Air/ Exhaust Air

damper positions are rarely adjusted. This approach means that the exhaust

humidity will vary with the evaporation rate. At high production rates the

evaporation rate is high (achieved by increasing the drying air temperature)

and the exhaust humidity will also be high. The reverse is also true. If the

production rate is constant but the incoming sheet moisture content

decreases, the evaporation rate will decrease and the exhaust humidity will

decrease. In most mills, the makeup and exhaust damper positions are set

conservatively and the average humidity is substantially lower than

necessary.

OH 6

Airborne Dryer

Energy Consumption vs Humidity

Hot Air drying processes

1000

1500

2000

2500

3000

3500

0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5

BTU /lb water evap

Absolute Humidity, lbs water per lb dry air

Energy Consumption vs Exhaust Humidity, Temp

Exh Temp700 Deg F600 Deg F500 Deg F400 Deg F300 Deg F

Heat Capacity, Dry Air, BTU/lb-DegF 0.24

Heat Capacity WV, BTU/lb-DegF 0.46

Heat Capacity Water, BTU/lb-DegF 1

WV Latent Heat Vaporization, BTU/lb 970.3

5 lb dry air per lb

water evaporated 3.33 lb dry air per lb

water evaporated

OH 7

Airborne Dryer

Flakt Dryer – Typical Air

Balance

1000 ADTPD Production

Machine room

Dryer

Cooler

A/A

Heat recovery

Building heat

2 - Supply Fan

3 - Exhaust Fan

Ventilation

270 Deg F

200 Deg F

920 TPD Water

4610 TPD Dry Air

0.2 lb water/lb dry air

50 Deg F

1000 TPD (10% Moisture)

220 Deg F

200 Deg F

Incoming Sheet

at 53% Moisture

180 Deg F

0.008 lb water/lb dry air

The dryer should be in balance:

slight infiltration at the bottom of

the box and slight pressure at the

top.

Fans 2 and 3 should be in balance,

and set to give a 160 Deg F wet bulb

temperature at the dryer outlet.

Hum

VFD

Steam 1.3 lb steam/lb water

evaporated, 100000 lb/hr

PC

OH 8

Airborne Dryer

• Humidity controller to adjust Supply Fan Damper or VFD

• Discharge Pressure controller to adjust Exhaust Fan

Damper

• A primary disturbance to exhaust Humidity is incoming

sheet moisture. An increase in the sheet moisture content

will increase dryer exhaust humidity. The Humidity

controller will increase Supply Fan Flowrate and the

Discharge pressure controller will increase Exhaust Air

Flowrate.

• The Humidity sensor can be used as a feedforward signal

to adjust energy input (steam pressure) to the dryer.

Flakt Dryer – Humidity Control

OH 9

Airborne Dryer

Flakt Dryer

Multiple Pass Heating

Successive Reheating of Air

0

0.05

100

150

200

250

1 2 3 4 5 6

Passes of Air through Heating Coils

Moisture Content of Air

Wet Bulb Temp. oF

Operating

Target

Top of Dryer Humidity lb water/

lb dry air

Bottom of Dryer

OH 10

Airborne Dryer

Energy Cost

0.2 lb water/lb da in exhaust

0.20, 1116583

0.20, 6679522

200

300

400

500

600

700

800

0

1000000

2000000

3000000

4000000

5000000

6000000

7000000

8000000

0.15 0.2 0.25 0.3 0.35 0.4 0.45

Gas Flow

SCFMDrying Cost

EvaporationDA Heating

$/yr

Exhaust Humidity lb water/lb dry air

Drying Cost vs Exhaust Humidity

Assume

$8/MMBTU Energy Input required to heat Dry Air Q= m*Cp*ΔT=7300 lb/min*0.24BTU/lb-DegF*(200-50) DegF

=262800 BTU/MIN = 378 MMBTU/Day = $1.1 M/yr

OH 11

Airborne Dryer

0.30, 734107

0.30, 6587261

200

300

400

500

600

700

800

0

1000000

2000000

3000000

4000000

5000000

6000000

7000000

0.15 0.2 0.25 0.3 0.35 0.4 0.45

Gas Flow

SCFMDrying Cost

EvaporationDA Heating

$/yr

Exhaust Humidity lb water/lb dry air

Drying Cost vs Exhaust Humidity

Energy Cost

0.3 lb water/lb da in exhaust

Energy Input required to heat Dry Air Q=

m*Cp*ΔT=4880 lb/min*0.24BTU/lb-DegF*(200-50) DegF

=175600 BTU/MIN = 252 MMBTU/Day = $0.73 M/yr

OH 12

Airborne Dryer

• The Dewcon humidity sensor can help to optimize Flakt

Dryers

• The potential energy savings is significant and can easily

justify the cost of the sensor (approx $25000)

• The exhaust humidity is an indication of the evaporation

load, and can be used to signal changes in the sheet

moisture content at the dryer inlet.

• The humidity sensor can be an important diagnostic tool

for operators and engineers.

Flakt Dryer – Humidity Control