Lab #10Spray Drying

Outline

• Goals of lab• Spray dryer• Hygrometer• Psychrometer• Heating ambient air and drying a product

– Depiction on psychrometric chart– Performing mass balance and energy balance during the

process

2

Goals of Lab

• Familiarization with the use of the psychrometricchart

• Perform mass and energy balances to determine the mass flow rate of air and the energy required for spray drying

3

Spray Dryer (Industry)

A

B

Moist Product(Atomized)

Ambient air

Hot Dry Air

Warm Moist Air + Dry Product

Heater

Spray dryer calculations involve: 1. Energy balance equation for air between

points ‘A’ & ‘B’(heater adds energy to air at point ‘A’)

2. Water balance equation for air between points ‘B’ & ‘C’(product adds moisture to air at point ‘B’)

4

Blower

Dry Product

Air + Tiny Particles

of Product

CycloneSeparator

“Atomization” involves breaking up a liquid product into tiny droplets by forcing the product and compressed air into an atomizer (disc with multiple slots at periphery that spins at a high rpm) at the TOP. This increases the surface area of the product, thereby increasing the rate of heat transfer and thus the rate of evaporation.

Atomizer

Wet Product

C

Spray Dryer (Lab)

A

B

C

Moist product (atomized)Ambient air

Hot Dry Air

Warm Moist Air + Dry Product

Heater

5

Wet Product

Compressed Air

Blower

Dry Product

Air + Tiny Particles

of Product

CycloneSeparator

“Atomization” involves breaking up a liquid product into tiny droplets by forcing the product and compressed air into an atomizer (disc with multiple openings at periphery that spins at a high rpm) at the TOP. This increases the surface area of the product, thereby increasing the rate of heat transfer and thus the rate of evaporation. In this lab, we are using a nozzle at the CENTER instead of a true atomizer at the TOP.Nozzle

Spray dryer calculations involve: 1. Energy balance equation for air between

points ‘A’ & ‘B’(heater adds energy to air at point ‘A’)

2. Water balance equation for air between points ‘B’ & ‘C’(product adds moisture to air at point ‘B’)

Hygrometers to Measure RH (and Twb, Tdp)• Psychrometer

– Two thermometers; one has moist wick (sling or mounted with fan)• Mechanical

– Metal-paper coil (paper strip attached to metal coil; coil changes shape with moisture; dial rotates similar to that in a bimetallic thermometer; inexpensive & not very accurate)

– Hair tension (human hair attached to spring & dial; hair swells as RH inc.)

• Electronic– Change in electrical resistance of LiCl or semiconductor

• Chilled mirror– Optoelectronic mechanism; very accurate

• Capacitive, resistive, thermal conductivity, gravimetric• Other (Change in weight, volume or transparency of a material)High RH: Sweating; promotes growth of mold during storage of foodsLow RH: Static electricity

6

wet wick

wet bulb temp.

dry bulb temp. handle is rotated

Sling psychrometer

Hygrometers (contd.)

7

Psychrometer

8

Wet bulb

Dry bulb

Dry bulb

Fan

Ambient Air (Point “A” on Low Temp. Chart)

Moisture Co

nten

t (g/kg dry air)

A

Dry bulb Temperature (°C)

. .

9

18

Example:Tdb = 18 °CTwb = 15 °C

From A, go horizontally across to read moisture content (8 g/kg dry air)

9

Tdb: -10 to 55 ° C(Page 819 of textbook)

Tdb: 20 to 120 ° C(Page 820 of textbook)

Moisture Co

nten

t (g/kg dry air)

Ambient Air (Point “A”) and Hot Air (Point “B”) on High Temp. Chart)

10

. .A

9

Example:Tdb = 18 °CTwb = 15 °CThough, point “A” can not be located on this chart, heating of ambient air at point “A” can be depicted on this chart as a horizontal line drawn from the point corresponding to a moisture content of 8 g/kg dry air on the y-axis. If Tdb = 110 °C for ambient air that is heated (point “B” on chart), point “B” can be determined by intersecting the horizontal line with a vertical line upwards from Tdb = 110 °C.

B

Moisture Co

nten

t (g/kg dry air)

Heating of Air (Constant Moisture Content or Humidity Ratio)

11)H(mQ)H(m:BalanceEnergy BaAa

'VVm:Note a .

.

. .

*A > *B

Q: Why do we feel dry in a heatedroom?

Moisture Co

nten

t (g/kg dry air)

Drying of Product (Constant Enthalpy & Wet Bulb Temp.)

12

B

'VVm:Note a .

.

C*

)W(m)moisture(%m)W(m:BalanceWater CapBa . ..

*

Moisture Co

nten

t (g/kg dry air)

Heating Ambient Air & Drying a Product

13

B

C*

.

*A > *Energy Balance

A: Ambient airB: Heated airC: Exit air (after heating product)