DROPWISE AND FILMWISE CONDENSATION APPARATUS

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Dropwise and filmwise condensation apparatus

AIM:

To determine the inside and outside heat transfer coefficient of Filmwise and Dropwise condenser.

To study the Dropwise and Filmwise condensation phenomena

APPARATUS:

Filmwise and Dropwise Condensers enclosed in a Borosilicate Glass Tube with flow control valves, Steam generator with heating elements, Digital Temperature Indicator with selector switch, Rotameter.

THEORY:The processes involving phase change, like boiling and condensation, are also very important in engineering practice. Boilers and condensers are essential components of modern thermal power plants; so are the evaporators and condensers in the vapor compression refrigeration cycles. The phenomenon of boiling heat transfer has also been successfully used in the cooling of nuclear reactors and rocket motors where the energy dissipation rates are extremely large. In order to be able to design these components effectively the phenomena of phase change processes must be well understood.

The phenomena associated with boiling and condensation is much more complex than the convection process. This is due to the fact that latent heat effects, surface tension, surface characteristics and other properties of two phase systems are very significant. High heat transfer rates associated with small temperature differences are usually involved in boiling and condensation.

Condensation Heat Transfer:

The process of condensation is the reverse of boiling. Whenever a saturated vapor comes in contact with a surface at a lower temperature, condensation occurs. There are two modes of condensation; filmwise, in which the condensate wets the surface forming a continuous film which covers the entire surface and dropwise in which the vapor

condenses into small liquid droplets of various sizes which fall down the surface in a random fashion.

Filmwise condensation generally occurs on clean uncontaminated surfaces. In this type of condensation the film covering the entire surface grows in thickness as it moves down the surface by gravity. There exists a thermal gradient in the film and so it acts as a resistance to heat transfer. In dropwise condensation a large portion of the area of the plate is directly exposed to the vapor, making heat transfer rates much larger (5 to10 times) than those in filmwise.condensation. Although dropwise condensation would be preferred to filmwise condensation yet it is extremely difficult to achieve or maintain. This is because most surfaces become wetted after being exposed to condensing vapors over a period of time. Dropwise condensation can be obtained under controlled conditions with the help of certain additives to the condensate and various surface coatings but its commercial viability has not yet been proved. For this reason the condensing equipment in use is designed on the basis of filmwise condensation.

APPARATUS DESCRIPTION:The apparatus consist of:

Steam Generator: (8 liter capacity) equipped with 2 kW heater and Pressure Gauge, Manual Release Valve, Feed Line and Steam Line.

Dropwise Condenser:

MOC: Copper with chrome plating

Dimensions: ID (di) 16 mm

OD (do) 19 mm

Length (L)170 mm

Filmwise Condenser:

MOC: Copper with Natural finish

Dimensions: ID (di)

16 mm

OD (do)19 mm

Length (L)170 mm

Temperature Indicator with Selector Switch measures the Temperature of :

T1Steam Chamber Temp.

T2Cooling Water In

T3Filmwise Condenser Surface Temp

T4Dropwise Condenser Surface Temp

T5Cooling water outlet from film wise condensation surfaceT6Cooling water outlet from drop wise condensation surface

Rotameter (0.3-3.0 LPM) has been provided on the cooling water line to measure the rate of water flowing through the condenser.

Ball valves have been provided to change the direction of cooling water flow between either of the condensers.

EXPERIMENTAL PROCEDURE:

Fill the steam generator with about 10 15 liter of water (preferably soft).

Connect supply socket to the mains and switch on the heater.

Switch on the heater while keeping the steam line and feed line valves in closed position.

Adjust the temperature of the steam near to the 105 110 OC.

Allow the steam generation to take place.

This may take 30 40 minutes depending on the initial temperature of the feed water.

The pressure of the generated steam will be indicated on the pressure gauge. Note down the Pressure reading of the steam inlet.

Now select the condenser to be tested first and open the ball valve of the same for cooling water supply.

Now start the supply of cooling water in the selected condenser (Dropwise or Filmwise)

Depending upon the type of condenser under test Dropwise or Filmwise condensation can be visualized.

If water flow rate is low than steam pressure in chamber will rise and pressure gauge will read the pressure.

If the water flow rate is matched than condensation will occur at more or less at atmospheric pressure.

Process of Dropwise and Filmwise condensation can be easily viewed through the front glass window of main unit.

Note down the inlet temperature of the cooling water, Outlet Cooling water Temperature as indicated by the DTI.

Slowly open the steam line valve and allow the steam to enter the steam chamber.

Observe the condensation phenomena and also note down the condenser temperature, steam inlet temperature.

Measure and note the cooling water flow rate through Rotameter provided.

Repeat the above procedure for the second type of condenser.

Use the wiper provided if the fog/ mist restricts the visualization of the glass vessel.

Precautions:

Do not start heater supply unless water is filled in the test unit.

Operate gently the selector switch of temperature indicator to read various temperatures.

Increase the temperature gradually of the heater during initial set-up experimentation.

Never use the heater at full wattage for longer period of time. Use the proper range of Rotameter.

Operate the change over switch of temperature indicator gently from one position to other, i.e. from 1 to 4 positions.

OBSERVATIONS: Filmwise Condensation:

Cooling Water Flow Rate (mw):

LPM

Temperature:

TSteam Chamber Temp:

TCooling Water In:

TFilmwise Condenser Surface Temp:

TCooling water outlet from film wise condensation surface

Dropwise Condensation:

Cooling Water Flow Rate (mw):

LPM

Temperature:

TSteam Chamber Temp:

TCooling Water In:

TDrop wise Condenser Surface Temp:

TCooling water outlet from drop wise condensation surface CALCULATIONS:Normally steam will not be pressurized, but the pressure gauge reads some pressure than properties of steam should be taken at that pressure or otherwise atmospheric pressure will be taken.

FIRST CALCULATE THE HEAT TRANSFER COEFFICIENT INSIDE THE CONDENSER UNDER TEST.

For this properties of water are taken at bulk mean temperature of water

i.e. (Twi + Two) / 2.

Following properties are required:

Density of water ( kg / m3 Kinematic Viscosity ( m2/ sec

Thermal Conductivity k kcal / hr m C

Prandtl Number Pr

Reynolds Number NRe = ( v di / ( Where

v (=mw*4/ ( * di2) = (Flowrate/Area)di = Inner Diameter of Condenser

If this value of NRe > 2100 then flow is turbulent, below this value flow is laminar.Normally flow will be turbulent in the tube.

Nusselt Number NuD = 0.023 (ReD)0.4 (Pr)0.4 Inside heat transfer coefficient ( hi ) = NuD * k/ di kcal / hr m2 (C

CALCULATE HEAT TRANSFER COEFFICIENT ON OUTER SURFACE OF THE CONDENSER HO:

For this properties of water are taken at bulk mean temperature of condensate

(Ts + Tw) / 2.

Density of water ( kg / m3 Kinematic Viscosity ( m2/ sec

Thermal Conductivity k kcal / hr m C

Prandtl Number Pr

Reynolds Number NRe = ( v di / (Where, Ts Temperature of steam,

Tw Temperature of condenser wall

do outside diameter of condenser

ho = 0.725 * 2 * g * k3 / ( Ts - Tw) * do

From these values overall heat transfer coefficient (U) can be calculated,

1/ U = 1 / hi + (di / do) (1 / ho)

U =

kcal / hr m2 (C

The same procedure can be repeated for another condenser.

Except for some exceptional cases overall heat transfer coefficient for dropwise condensation will be higher than that of filmwise condensation. Results may vary from theory to some degree due to unavoidable heat losses from the glass tube walls.

RESULT:

sample Calculation:

First calculate the heat transfer coefficient inside the condenser under test. For this properties of water are taken at bulk mean temperature of water

i.e. (Twi + Two) / 2. Following properties are required:

Physical Properties at Water Mean Temperature

Physical PropertiesDropwise CondenserFilmwise Condenser

Steam Pressure

Kg/cm2P1.51.5

Steam Temperature OCTs99.399.4

Mean Temperature oCTm3535

Density of Water kg/m3(993.95993.95

Viscosity

Kg/ m s0.000730.00073

Kinematics Viscosity m2/ s(0.732*10-60.732*10-6

Cooling Water Flowrate LPMQ22

Vol. Flowrate

m3/sm0.0000330.000033

Velocity

m/sV0.1640.164

Thermal Conductivity

kcal / hr m CK0.5370.537

Specific Heat

CP0.9970.997

Reynolds Number

ReD = ( v di / (

= 993.95*0.165*0.016/0.00073

=3594

where

v = Velocity of Cooling Water Inside Condenser, m/s

= mw*4 / ( * di2where, mw

= vol. Flowrate, m3/s

= Q/ 60000

=2/60000

=0.00003333

Q= Cooling Water Flowrate, LPM

di = Inner Diameter of Condenser, m

(= Density of Water at mean Temperature, kg/m3 = Viscosity of Water at mean Temperature, kg/ms

If this value of ReD > 2100 then flow is turbulent, below this value flow is laminar.

Normally flow will be turbulent in the tube.

Prandtl Number

NPr=Cp * / k

=0.997*0.00073/0.537

=4.878

Where

Cp= Specific Heat of Water,

= Viscosity of Water, Kg/ m s

k= Thermal Conductivity of Condenser Material, kcal/ hr m OC

Nusselt Number

NuD = 0.023 (ReD)0.4 (Pr)0.4

=0.023 * 35940.4 * 4.8780.4

=1.1467

Thus Inside Heat Transfer Coefficient, kcal / hr m2 (C

hi = NuD * k/ di

=1.1467*0.537/0.016

=38.48kcal / hr m2 (C

Normally steam will not be pressurised, but the pressure gauge reads some pressure than properties of steam should be taken at that pressure or otherwise atmospheric pressure will be taken. Now calculate the heat transfer coefficient outside the condenser under test. For this properties of condensate are taken at bulk mean temperature of Condenser wall and the Steam Inlet Temperature

i.e. (Tw + Ts) / 2. Following properties are required:

Physical Properties at Condensate Mean Temperature

Physical PropertiesDropwise CondenserFilmwise Condenser

Steam Pressure

Kg/cm2P1.51.5

Steam Temperature OCTs99.399.4

Mean Tamp

0CTm78.4582.2

Density of Water kg/m3(971.8971.8

Viscosity

Kg/ m s0.0003550.000355

Kinematics Viscosity m2/ s(0.365*10-60.365*10-6

Cooling Water Flowrate LPMQ2

Vol. Flowrate

m3/sm0.000033

Velocity

m/sV0.164

Thermal Conductivity

kcal / hr m CK0.579

Specific Heat

CP0.997

Calculate heat transfer coefficient on outer surface of the condenser ho.

Where, Ts Temperature of steam, Tw Temperature of condenser wall

ho = 0.725 * 2 * g * k3 / ( Ts - Tw) * do

=0.725*2*9.81*0.579 3 /(99.3-78)*0.019

=6.605kcal / hr m2 (C

From these values overall heat transfer coefficient (U) can be calculated,

1/ U = 1 / hi + (di / do) (1 / ho)

= 1/38.48 +(16/19)(1/6.605)

=0.1534

Thus,

U =6.5188

kcal / hr m2 (C

The same procedure can be repeated for another condenser.

Except for some exceptional cases overall heat transfer coefficient for Dropwise condensation will be higher than that of Filmwise condensation. Results may vary from theory to some degree due to unavoidable heat losses from the glass tube walls.

SR.NOITEM NAMESIZEMATERIAL USEDQTY.

1. Nipple GI3

2. ( ( 9 GI4

3. ( ( 24 GI1

4. ( ( 2 GI3

5. ( ( 3 GI1

6. ( ( 6 GI2

7. ( ( 9 Copper1

8. Elbow 3

9. Union 1

10. Ball valve 7

11. Gate valve 1

12. Coupling 3

13. Elbow (L-type) 2

14. Glass tube1

15. Glass (Distance piece )200 ( 150 ( (6 )

16. Metal flange6 2

STEAM INLET

DROPWISE FILMWISE CONDENSATION APPARATUS

T1

WATER INLET

T2

T3

T4

DROPWISE CONDENSOR

MOC: COPPER WITH Cr.PLATING

DI = 160 mm

DO = 190 mm

L = 170 mm

FILMWISE CONDENSOR

MOC: COPPER WITH NATURAL FINISH

DI = 160 mm

DO = 190 mm

L = 170 mm

DROPWISE AND FILMWISE CONDENSATION