Department of Chemical Engineering, Aligarh Muslim University, Aligarh
CHC 3940 : HEAT TRANSFER OPERATIONS LAB
General Instruction:
1. Experiments will be conducted in groups of 3 to 4 students. The same group will continue through out the course.
2. For every session of experiment, students are required to come to the laboratory class prepared with the experimental procedure, the theory, the basic equations, the observation tables, and the calculations etc. The associated teachers will occasionally test the preparation and knowledge of the students.
3. Feel free to ask any doubts and questions to the teacher. For difficulties in operation of the equipment ask the teachers and laboratory technical assistants.
4. Make rough sketch of the experimental setup and note down on it all the needed dimensions and information.
5. All observations and sketches are to be made only on A4 size sheets separately by each student. Get the signature of anyone of the associated teachers on the observation and sketch at the end of the class and attach them with the final report. Observations should be complete with all required data, and information.
6. Each group should note at least two experimental runs duly verified by the teacher during performance of the experiment. Failing which the observation sheet will be considered as incomplete.
7. Report of the performed experiment is to be submitted positively on the next practical turn by each student separately. Without submitting the previous reports, students will not be allowed to perform any new experiment and shall be marked absent on that turn.
8. At the end of semester, each student is required to compile all of his corrected and updated reports in one file with proper indexing and submit at the time of his viva-voce examination.
HEAT TRANSFER LAB (CHC 3940)
i. Shell and tube heat exchanger ii. Double pipe heat exchanger iii. Natural convection
iv. Open pan evaporator
v. Film wise and dropwise condensation vi. Boiling heat transfer
vii. Heat Pipe
viii. Usteady heat transfer
SHELL AND TUBE HEAT EXCHANGER
Object
(a) To study the Shell and Tube Heat Exchanger available in Heat Transfer Laboratory And prepare a detailed dimensioned sketch.
(b) To determine experimentally the overall heat transfer coefficients at various cold water flow rates and compare these values to that obtained using standard equations.
Theory
A heat exchanger is a device in which two fluid streams ,one hot and another cold are brought into thermal contact in order to effect transfer of heat from the hot fluid stream to the cold . A Shell and tube heat exchanger is the most widely used heat exchange equipment .In this type of heat exchanger ,large heat transfer surface can be achieved economically and practically by placing tubes in a bundle; the ends of the tubes are mounted in a tube sheet. The resultant tube bundle is enclosed by a cylindrical casing (the shell),though which the second fluid flows around and through the tube bundle. To allow provisions for easy removal of the tube bundle for cleaning and to allow for expansion, a floating- head exchanger is used.
Experimental Setup
The Floating Head 1-2 Shell and Tube heat exchanger consists of a tube bundle mounted horizontally. It has 9 S.S. tubes with 19 mm OD (16 BWG) arranged in a triangular pitch of 31.75 mm. Effective length of each tube is 915 mm .The Tube bundle is housed in a shell made of mild steel .Shell contains 6 segmented baffles, having 25%
cut. Baffle spacing is 120mm.Hot water flows through the tubes and cold water passes through the shell side . A tank is provided with a steam coil to prepare the hot water at the required temperature (e.g. 60, 65, 70,75oC) and separate pumps are installed to circulate hot
and cold water in the system . Provisions are made to measure inlet and outlet temperatures , and flow rates of hot and cold fluid . Outer shell is made of M.S. with 7”
ID.
Procedure
1. Fill the tank with water to a predetermined level from the main feeder.
2. Close the valve leading to the exchanger and open the bye –pass valve.
3. Start the pump and let the water circulate in the tank for few minutes.
4. Open the valve leading to exchanger slowly, and simultaneously close the bye- pass valve to adjust the hot and cold water flow rates.
5. Circulate the hot water at constant flow rate and vary the cold water flow rate corresponding to each cold water flow rate record the inlet and outlet temperatures and flow rates of hot and cold fluids, after observing steady state of 2 minutes.
6. Again set different hot water flow rate and repeat the procedure.
Observation Table
Hot water flow rate =--- LPM.
S.No. Cold Water flow rate,LPM
Temperatures
Hot Water Side Cold Water Side
T
hi oC T
ho oC T
ci oC T
co oC
1.
2.
3.
. .
Results and Discussion
1. Apply the heat balance and estimate the heat duty (heat load) for both the fluids for each run.
2. Calculate the experimental values of overall heat transfer coefficient,U
3. Compute the values of overall heat transfer coefficient using a suitable correlation and compare it with that obtained experimentally.
4. Draw the Wilson’s plot(1/U vs 1/v0.8 ) and estimate the value of inside film coefficient,hi , assuming tubes to be clean.
5. Discuss the results and mention the sources of errors and discrepancies.
6. Mention the experimental difficulties and give suggestions for the improvement in the experimental facility.
Calculations
1. Heat transfer rate(or heat load) is calculated as Qh =mh Cph (Thi-Tho)
Qc =mc Cpc (Tco-Tci) Q = (Qh+Qc)/2
2. LMTD (logarithmic mean temperature difference) LMTD = ( Δ T LN) = (ΔT1 – ΔT2)/ ln (ΔT1/ ΔT2) For counter flow (CF) arrangement
ΔT1 = Thi - Tco ΔT2 =Tho - Tci
(Δ T LN)CF = [(Thi - Tco) – (Tho - Tci )]/ ln [ (Thi - Tco)/ (Tho - Tci ) ] Δ T LN = (Δ T LN)CF * Ft
3. Overall heat transfer coefficient can be calculated by using Q = UA (Δ T LN )
U = Q/ A (Δ T LN )
Nomenclature
Qh = Heat lost by hot fluid, W Qc = Heat gained by cold water, W Q = Average heat transfer,W
mh , mc = Mass flow rates of hot and cold fluid resp., Kg/s Thi = Hot water inlet temperature, oC
Tho =Hot water outlet temperature, oC Tci = Cold water inlet temperature, oC Tco = Cold water outlet temperature,oC
Cph , Cpc = Specific heats of hot and cold fluids resp, J/Kg oC A = Area of heat transfer, m2
U= Overall heat transfer coefficient, W/m2 oC
Ft = Temperature correction factor. It is a function of the shell and tube fluid
Temperatures and the number of tube and shell passes.
DOUBLE PIPE OR HAIR PIN HEAT EXCHANGER
Object
(a) To study the double pipe hair- pin heat exchanger installed in the laboratory and prepares the detailed dimensioned sketch.
(b) To determine experimentally the overall heat transfer coefficients at various cold water flow rates and compare these values to that obtained using standard equations.
(c) Draw the Wilson’s Plot and estimate inside and outside film heat transfer coefficients.
Theory
The simplest type of heat exchanger is the double pipe heat exchanger .It consists of essentially two concentric pipes with one fluid flowing through the center pipe while the other fluid moves cocurrently or countercurrently in the annular space. The heat exchange takes place through the inner pipe wall separating the two fluids. These exchangers are used when flow rate of fluids and the heat duty are small(less than 500kW).
Experimental Setup
The heat exchanger consists of three sections connected in series, each consisting of stainless steel tube (16 SWG, OD=31.65mm) kept concentrically in mild steel pipe of 63.5mm nominal bore. The effective length of each section is 1829mm.Water is passed through the exchanger with the help of a pump.
The hot water flows through the annulus while the cold water passes through the tube. There are provisions to measure water inlet and outlet temperatures and water flow rates.
Procedure
1. Fill the tank with water to a predetermined level from the main feeder.
2. Close the valve leading to the exchanger and open the bye –pass valve.
3. Start the pump and let the water circulate in the tank for few minutes.
4. Open the valve leading to exchanger slowly, and simultaneously close the bye- pass valve to adjust the hot and cold water flow rates.
5. Circulate the hot water at constant flow rate and vary the cold water flow rate covering the complete range of Reynolds number. Corresponding to each cold water flow rate record the inlet and outlet temperatures and flow rates of hot and cold fluids, after observing steady state of 2 minutes.
6 Again set different hot water flow rate and repeat the procedure.
Observation Table
Hot water flow rate =--- LPM.
S.No. Cold Water flow rate,LPM
Temperatures
Hot Water Side Cold Water Side
T
hi oC T
ho oC T
ci oC T
co oC
1.
2.
3.
Results and Discussion
1. Apply the heat balance and estimate the heat duty (heat load) for both the fluids for each run.
2. Calculate the experimental values of overall heat transfer coefficient,U
3. Compute the values of overall heat transfer coefficient using a suitable correlation and compare it with that obtained experimentally.
4. Draw the Wilson’s plot (1/U vs. 1/v0.8) and estimate the value of inside and outside film heat transfer coefficients,hi andho resp, assuming tubes to be clean.
5. Discuss the results and mention the sources of errors and discrepancies.
6. Mention the experimental difficulties and give suggestions for the improvement in the experimental facility.
Calculations
1. Heat transfer rate(or heat load) is calculated as Qh =mh Cph (Thi-Tho)
Qc =mc Cpc (Tco-Tci) Q = (Qh+Qc)/2
2. LMTD (logarithmic mean temperature difference) LMTD = (Δ T LN) = (ΔT1 – ΔT2)/ ln (ΔT1/ ΔT2) For counter flow arrangement
ΔT1 = Thi - Tco ΔT2 =Tho - Tci
For co-current flow arrangement ΔT1 = Thi - Tci
ΔT2= Tho -Tco
3. Overall heat transfer coefficient can be calculated by using Q = UA (Δ T LN )
U = Q/ A (Δ T LN ) Nomenclature
Qh = Heat lost by hot fluid, W Qc = Heat gained by cold water, W Q = Average heat transfer,W
mh , mc = Mass flow rates of hot and cold fluid resp., Kg/s Thi = Hot water inlet temperature, oC
Tho =Hot water outlet temperature, oC Tci = Cold water inlet temperature, oC Tco = Cold water outlet temperature,oC
Cph , Cpc = Specific heats of hot and cold fluids resp, J/Kg oC A = Area of heat transfer, m2
U= Overall heat transfer coefficient, W/m2 oC
Ft = Temperature correction factor. It is a function of the shell and tube fluid
temperatures and the number of tube and shell passes.
HEAT TRANSFER IN AN OPEN PAN EVAPORATOR
Object: To study the convective heat transfer coefficient in an open pan evaporator under laminar and turbulent flow conditions.
Theory: The value of heat transfer coefficient in natural convection is given by hL/k = C ( Pr *Gr)m
where,
Pr (Prandlt number) = Cp µ /k
Gr (Grashoff number) = (L3 ρ2 g ß ΔT)/ µ2
h = heat transfer coefficient , W / m2 K
L = vertical height of cylinder, m k = thermal conductivity in W/m K
C , m =constants depends on physical geometry and flow condition.
Cp = heat capacity ,J/Kg K µ =viscosity in Kg/ms
ΔT =temperature difference in oC ρ =density, Kg/m3
ß =volumetric coefficient of expansion of fluid , 1/K ß=1/v * (dv/dt) =1/T
g = acceleration due to gravity, m/s2
All the physical properties are calculated at mean temperature.
1. For laminar flow over cylinders , For Gr*Pr ~ 104 – 108
hL/k = 0.59 (Pr*Gr)1/4
2. For turbulent flow over cylinders For Gr*Pr ~ 109 – 1012
In forced convection heat transfer process, heat transfer coefficient is given by :
1. For laminar flow
hL/k = 0.664 (Re)1/2 (Pr)1/3 2. For turbulent flow
hL/k = 0.037 (Re)0.8 (Pr)1/3
where,
Re = Reynolds number = (Dv ρ)/µ
The heat balance between hot and cold fluids is given by:
Q= mCp ΔT(for hot stream) = mCp ΔT(for cold stream) A = heat transfer area of the pan
m= flow rate , Kg/s Q= UA(ΔT)
U= overall heat transfer coefficient, KJ/m2 s oC Procedure:
After drawing the schematic line diagram following procedure is adopted:
1. Record the water temperature of hot water and water inside the open pan at ambient conditions (without starting the pump).
2. Heat the water through immersion heater /steam to some temperature say 50 or 60oC. Do not run the agitator.
3. Start the pump and open the valve to circulate the hot water to heat the cold water in the open pan . Allow outlet hot water to go in the tank.
4. Record the inlet and outlet temperatures of hot and cold streams.
5. Take set of readings till inlet temperature of hot stream is very close to the outlet temperature of the open pan.
6. Repeat the experiment after starting the motor to agitate water of the open pan evaporator.
Observations:
Time,t(min) Hot water inlet temp. oC
Hot water outlet temp. oC
Pan water temp.
oC 0
5 10 15 20 25 30 35 40 45 50 55 60 65
Results and discussion:
S.No. Time ,min Grashoff no.,Gr
Gr*Pr h(J/sm2oC) q = Q/A (KJ/ m2 s)
U(KJ /sm2oC)
Plot U vs. time
HEAT TRANSFER IN NATURAL CONVECTION
Object:
To determine surface heat transfer coefficient for heated vertical cylinder in Natural Convection.
Theory:
When a hot body is kept in still air, it looses heat to the surroundings by the process of natural convection. The layer of fluid adjacent to the hot body gets heated up as a result of which its density decreases. Due to the buoyancy force created because of the density difference between the heated fluid and the surrounding cooler fluid, a convective motion is initiated with a continuous motion of hot fluid upwards along the plate and replacement of this fluid by the cooler fluid. The heat transfer coefficient is defined by :
h = q / [As (Ts –Ta )] --- (1)
q = rate of heat transfer, K cal/hr As = area of heat transferring surface, m2
Ts = mean surface temperature of hot body, oC
Ta = Temperature of undisturbed fluid i.e ambient or adjacent air temperature, oC
h = average heat transfer coefficient, K cal / hr m2 oC
For a vertical cylinder transferring heat to the surroundings, the following empirical relations hold:
a) For Laminar flow, 104 ≤GrL * Pr ≤ 108 Nuav = 0.59 (GrL * Pr)1/ 4
b) For Turbulent flow, 109 ≤ GrL * Pr ≤ 1012 Nuav = 0.13 (GrL * Pr) 1/ 3
Above equations hold good when D/L > 35/( GrL) 1/ 4 Where, Nuav = average Nusselt number
= hL/k
L = height of vertical cylinder, m
k = thermal conductivity of fluid, K cal/hr m oC GrL =Grashoff number based on length
= (L3 g ß ΔT)/ ν 2
ß = coefficient of volumetric expansion expansion, 1/ oK = 1/ (Tf + 273)
ΔT = temperature difference, oC
= (mean surface temperature – ambient fluid temperature) ν = kinematic viscosity, m2/s
Pr = Prandlt number, µ Cp/ k
h = average surface heat transfer coefficient, K cal / hr m2 oC D = diameter of cylinder,m
The fluid properties; k,ß,ν,µ, Cp are all evaluated at the film temperature Tf defined by:
Tf = (Ts + Ta)/2 , oC
Ts = mean surface temperature, oC Ta = ambient fluid temperature, oC
Description of the Apparatus :
The experimental apparatus consists of a vertical S.S. tube enclosed in a rectangular duct open at both top and bottom. The duct is of sufficient dimension as not to interfere with the convection process while at the same time preventing external disturbances affecting the data. One side of the duct is made of transparent section to facilitate visual observation. An electric heating element embedded in the copper tube acts as the heat source. The surface temperature of the tube is measured at different heights using thermocouples. The surface of the tubes is polished to minimize radiation losses. A Voltmeter and an Ammeter enable the determination of wattage dissipated by the heater and hence the heat input to the system.
Procedure:
1. Switch ON the mains.
2. Keep the input power to the lowest range, at this position band switch should be at zero position.
3. Allow the unit to stabilize (caution: input power should be stabilized).
4. Switch ON the band switch to No.1 and see that the readings should not change.
This indicates that the unit is in stabilized condition. Note down the readings by operating the Band switch, 1,2,3,4,5,6,7, temperature of the surface. 8 is the local chamber temperature.
Repeat the experiment for different inputs and tabulate the readings for calculations
Observations and calculations:
S.No. Voltage V(volts)
Current A(amps)
Heat input
V.A*0.86 K cal/hr
Surface temp.
T1 T2 T3 T4 T5 T6 T7
Ambient Temp.Ta
Average surface temp.Ts
= (T1+T2+T3+T4+T5+T6+T7)/ 7
Experimental value of heat transfer coeff., hexp
Predicted value of
heat transfer coeff., hth (using correlation)
Results and Discussion:
Plot hexp and hth vs GrL ,compare the experimental and theoretical value of h.
BOILING HEAT TRANSFER
Object
To study the boiling heat transfer phenomenon for pool boiling of water.
Theory
Boiling heat transfer is a mode of heat transfer that occurs because of vaporization.
Vaporization is a process in which a substance is changed from liquid to the vapor state.
Pool boiling takes place when a liquid is confined in a container and a heater is submerged in the liquid.
Consider that the rate of convective heat transfer q for a system is expressed by the Newton’s equation:
q = hA (ΔT) where,
h = heat transfer coefficient A = area involved in heat transfer ΔT = temperature difference
An analogous equation used for boiling heat transfer is:
q” = hb (ΔTs) where,
q” = q/A is called the heat flux hb = boiling heat transfer coefficient
ΔTs = wall superheat, defined as the difference between the wall temperature of the heating surface,Tw and the saturation temperature of the liquid Tsat
ΔTs = Tw - Tsat
The value of hb for a boiling system changes as the system passes trough different regimes. These regimes can be shown on a boiling curve. A plot of q”vs ΔTs on logarithmic coordinates will give rise to a boiling curve as shown in figure below
Fig. 1. Boiling curves showing regimes of boiling
log{Tw -Tsat}
lo g (q /A )
In the Pool Boiling experiment q”, the heat flux, and ΔTs , the difference between the temperature of the heater surface, Tw, and that of the boiling liquid,Tsat ,are measured.
At low temperature drops heat is transferred to the liquid by natural convection, this region is shown by a straight line 1-2 on a log-log plot(Figure1 shown above) .The liquid must be superheated to some degree before bubbles form on the heater surface.
At point 2 the liquid superheat has increased to a point where vapor bubbles begin to form on the heater surface at few preferred locations; this point on the curve is referred to as Onset of Nucleate Boiling(ONB).As the bubbles detach from the heated surface ,they form a column of bubbles moving towards the free surface of the liquid .Line 2-3 is the knee of the boiling curve and represents a region of changing heat transfer coefficient. When point 3 is reached, a sufficient number of nucleation sites have been activated to establish fully developed nucleate boiling. The line 3-4 representing this region will once again be straight but its slope will be much greater than in the convection region. This means that very large heat transfer rates are possible with relatively small temperature driving forces. The characteristically high heat transfer coefficient hb of nucleate boiling is obtained.
As heat flux is increased further, more nucleation sites are activated and a large amount of vapor is formed on the heater surface .This causes the heat transfer coefficient to decrease from point 4 to 5 .Point 4 is known as the departure from nucleate boiling(DNB).
At some critical value of heat flux at point 5 a blanket of vapor is formed over the entire surface of the heater and heat transfer rate is severely reduced .The vapor film so formed is unstable; it forms, collapses and reforms repeatedly. Therefore region 5-6 is referred to as unstable film boiling. Point 6 is referred to as Leidenfrost point ,the hot surface becomes covered with quiescent film of vapor, through which heat is transferred by radiation due to very high temperature drops, resulting in increased values of heat transfer coefficients. The region after point 6 is therefore one of Stable film boiling.
Experimental Apparatus
The Apparatus( Figure2) consists of vertical glass cylinder,G,in which water boils .Inside the glass cylinder is placed the copper condensing coil C.At the bottom of G is a copper bowl B heated electrically. Cooling water is circulated through the condenser coil by means of a pump P. Waterflow control is achieved through valve V1.Rotameter R1 gives an indication of water flow rate.
Thermocouple T1 measures the temperature in the heating pad T2 and T3
measures the liquid and vapor temperatures.T4 and T5 are thermocouples for measuring cooling water inlet and outlet temperatures resp.Voltmeter V and ammeter A measures the heater input voltage and current respectively.
Experimental Procedure
1. Fill the sample holding sump with sample of about 200ml (approx) through the feed valve V2 provided on the top of the column(ensure that the drain valve provided at the bottom is closed ).Close the feed valve after filling .
2. Ensure that the dimmer is OFF, thermocouple selector switch is at ZERO position and the pump toggle switch is OFF.
3. Fill the sump S with water and connect external water supply to sump.
4. Open the by-pass valve V4 completely.
5. Open the valve V1 slightly to pass minimum flow through Rotameter R1. 6. Switch ON the toggle switch for pump and adjust valve V1 for water flow rate.
7. With slowly increasing the voltmeter and ammeter, record temperatures T1,T2,
T3,T4 , and T5 until the liquid boils.
Observations S.no. Heating
pad
temperature
Liquid temperature
Vapor temperature
Cooling water temperature
I Amps
V volts
Ta
Amb- ient Temp.
inlet outlet
T1 T2 T3 T4 T5
Calculations
Heat input , q = V * I* 0.86 Kcal/hr ( 1Watt = 0.86 Kcal/hr) Heat Transfer area, A = Π D2 /4
Where,
D = Diameter of copper bowl = 100mm Heat flux, q”= q/A Kcal/hr m2
ΔT = Difference between heating surface temperature and liquid temperature ΔT= T1 - T2
Results and Discussion
1. Plot the graph of q”vs ΔT on the logarithmic coordinates.
2. Plot the graph of hb vs ΔT on the logarithmic coordinates.
3. Comment on the nature of graph obtained.
FILM AND DROPWISE CONDENSATION
Object: To determine the surface heat transfer coefficient for Dropwise and Filmwise condensation experimentally and compare the values to that obtained from standard equations.
Theory : Condensation is the change in phase from the vapour state to the liquid state .It can be considered as taking place either within the bulk material or on a cooled surface and is accompanied by a simultaneous heat and mass transfer.
Condensation plays a significant role in the heat rejection parts of the Rankine Power Cycle and the vapour compression refrigeration cycle, which generally involve pure
substances .Dehumidification in air conditioning and the production of liquefied petroleum gases, liquid nitrogen and liquid oxygen are examples in which condensation in a mixture takes place. Condensation on a cooled surface occurs in one of two ways: Film or dropwise condensation.
In Film Condensation the liquid condensate forms a continuous film which covers the surface and takes place when the liquid wets the surface. This film flows over the surface under the action of gravity or other body forces (surface tension or shear stresses due to vapour flow). Heat transfer to the solid surface takes place through the film which forms the greatest part of thermal resistance.
In Dropwise Condensation the vapour impinges on the cool wall, decreasing its energy and thereby liquefying and forming drops which grow by direct condensation of vapour on the drops and by coalescence with neighbouring drops until the drops are swept off the surface by the action of gravity or other body forces (surface tension or shear stresses due to vapour flow). As drops move they coalesce with other droplets in their path, sweeping a portion of the surface clean so that condensation can begin anew. The details of dropwise condensation are not completely understood but it is known to take place under
circumstances where the liquid does not wet the surface. Dropwise condensation of steam has heat transfer coefficient 2 to 10 times as large as film condensation. However it has been difficult to to sustain dropwise condensation commercially for long periods of time.
Prediction of heat transfer coefficient:
a)Dropwise condensation of steam on a vertical surface Nu= 1.46*10-6 (Re)-1.63 (Pr)1/2 (πk )1.16 --- (1) (Peterson and Westwater’s correlation)
Where,
Nu = (2σ Tv h) / (hfg ρl kl ∆Tw )
Pr = (µl Cpl) / kl
Re = (kl ∆Tw ) /(µl hfg ) πk= [2 σ (dσ/dT) Tv] / (hfg µl)
∆Tw =( Tv – Tw )
All the physical properties are evaluated at the saturation temperature. Equation (1) covers the range:
1.65 ≤ Pr ≤ 23.6
7.8*10-4 ≤ πk ≤ 2.65*10-2
Simpler correlation for water on copper vertical surface h = 2600 + 200 * (Ts *1.8 +32) *0.2048
for 180 C < Ts <100oC
h = 45000 * 0.2048 for Ts ≥100o C where, h is in Kcal /hr m20C
b)Filmwise Condensation of steam on a vertical surface (hm / kl)*( ʋl2/g)1/3 = 1.47* Rel-1/3 ---(2)
Where, Rel = (4*WL ) / µl , WL is the condensate flow rate at length L . From Nusselts theory hm can be directly evaluated as:
hm = 0.943*{[kl3 ρl (ρl – ρv) g hfg] / [(Ts – Tw) µl L] }1/4 ---(3) All the properties evaluated at saturation temperature.
Nomenclature:
σ = Surface tension
h = heat transfer coefficient
hm = Mean heat transfer coefficient over length of surface.
hfg = Latent heat of vaporization ρ = density
Cp = Specific heat ʋ = Kinematic viscosity
kl = Liquid thermal conductivity L = Length of vertical wall µ = Dynamic viscosity Tv = Vapour temperature Tw = Wall temperature Ts = Saturation temperature Nu = Nusselt number Pr = Prandlt number
l = Subscript indicating liquid v = Subscript indicating vapour Experimental Apparatus:
A schematic diagram of the apparatus is shown in figure1.The condensing chambers CC1 consists of the Gold plated copper tube (GPCT) and the Plain copper tube (PCT) of identical dimensions. Boiler is provided to prepare steam which is allowed inside the chamber. Pump P circulates cooling water through the tubes.
By operating valve V1 and flow control valves V6, V7; experiments are to be carried out at different flow rates. Rotameter R1 and R2 measures water flow rates. T/C T1 measures the water inlet temperature to the system. The two tubes GPCT and PCT are connected in series for the water circuit. Thermocouples T2 ,T3 , T4 and T5 enable measurement of water temperature at the entry and exit points of tube. Thermocouples T8 and T9 measures the surface temperature. T6 and T7 measures vapour temperature. Relief valve RV is provided to prevent build up of high pressure inside the system.
Experimental Procedure:
1. Open valves V1, V6 and V7 . 2. Keep valves V3 , V4 , V5 closed.
3. Fill water in the boiler.
4. Fill water in the sump S.
5. Switch on the electrical heater. Close V2 . 6. Start the pump.
7. Allow steam slowly to the chambers by opening valve V3 .
8. The system is allowed to come to the steady state.
9. Note down the temperatures T1, T2, T3, T4, T5, T6, T7, T8, T9.
Observations:
Ambient temperature = ... °C
Calculations:
i) Surface area of tube(GPCT or PCT) , Ao = π do L Where , do = Outside diameter of the tube =15mm L = Effective length of tube = 150mm ii) Mass flow rate of water, m.
m. = Qw *60 kg/hr
iii) Heat picked up by water, Q Q = m * Cp (∆T)
Dropwise condensation Qdc = m * Cp (T3-T2)
iv) Logmean temperature difference, (∆Tdc)LN
= (∆T1 – ∆T2 )/ ln(∆T1 / ∆T2 ) ∆T1 = Ts - Ti = T6 – T2
∆T2 = Ts - To = T6 – T3
Filmwise condensation Qfc = m * Cp (T5-T4)
(∆Tfc)LN = (∆T1 – ∆T2 )/ ln(∆T1 / ∆T2 )
∆T1 = Ts - Ti = T7 – T4
∆T2 = Ts - To = T7 – T5
S.No Water flow rate,Qw
lpm
Temperatures,°C
T1, T2, T3, T4, T5, T6, T7, T8, T9
Heater, V Volts
Input, A amps
v) Overall heat transfer coefficient, Udc
Udc = Qdc / [Ao * (∆Tdc)LN ] , kcal/ hr m2oC vi) Experimental surface heat transfer
coefficient, hdc
hdc = Qdc / [Ao*(T6 –T8)]
Ufc = Qfc / [Ao * (∆Tfc)LN ] = ...kcal/ hr m2oC
hfc= Qdc / [Ao*(T7 –T9)]
