CLP301: ChE Lab 1- Fluid Mechanics and

Heat Transfer Lab

Semester II, 2020 – 2021

Experiment Name Double effect Evaporator

Group day Monday

Subgroup Number 3

Date/day of submission 20-03-2021 (Saturday)

Date/day of common -
doubt session for the
experiment

Names of group members Aditya Anand(2018ch10216)

Ankur Singh Thakur(2018CH70277)
Amish Mohamed(2018ch10261)
Mehul Garbyal(2018ch70290)
Prajwal Khobragade(2018CH70293)

Name of the lab instructor Prof. Vikram Singh

Marks (to be filled by

instructor / TA)
 Double effect Evaporator
OBJECTIVE:
● To concentrate the sodium carbonate solution.
● To evaluate the economy and capacity of the double effect evaporator at the steady-state.

THEORY:

Basis: time units

Overall Material Balance: Wf= W+ Wv
Solute balance : Wf .Xf= WX
Neglecting heat losses to the surroundings and considering negligible heat of dilution, the
steady state heat balance around the evaporator can be given by:

Ws.λs + Wf .hf = Wv .H + W.h

Where
Ws = flow rate of steam,
λs = latent heat of condensation of steam
Wf = flow rate of feed,
hf = specific enthalpy of feed
Wvv = flow rate of vapor leaving the system,
H = specific enthalpy of vapor leaving the system
W = flow rate of product,
 h = specific enthalpy of product
Capacity: Evaporator capacity can be defined as the amount (kg) of water evaporated per
hour. Economy: Steam Economy is defined as kg of water evaporated per kg of steam used.

Overall Heat Transfer coefficient (U) can be obtained from steady state balance:

Q = Ws. λs = UA(Ts-T)

Where A is the surface area of the evaporator,

TS is the saturated temperature of steam in the shell and
T is the boiling point of the solution at the prevailing pressure inside the evaporator

EXPERIMENTAL SETUP :

The experiment setup i.e. Evaporator (double effect) consists of two evaporator chambers in
series. Diluted solution is fed to the first chamber. Steam generated from any process is passed
on to the first evaporator heat transferred to solution and solution is evaporated. The
 condensed steam is then collected in a trap. The vapour from the first chamber then passed on
to the next evaporator chamber; the residual solution now is in desired concentration. The
steam then evaporates volatile solvent in the next chamber.The vapors of volatile solvent are
condensed in a shell and tube type condenser and the balance non-volatile solute collected in
the accumulator is recycled through the evaporator.

PROCEDURE:

Starting Procedure:
1. Firstly, prepare the calibration curve of concentration vs. density for Na2CO3 by
preparing a 5% solution of sodium carbonate in water and plot the concentration of
sodium carbonate vs density.
2. Make sure that all the switches on the panel are in OFF position and that all the valves in
the setup are closed.
3. Fill the cooling water tank with water and funnel valve and air vent valve of steam
generator 3/4th with water. Close all the valves.
4. Switch ON the power supply to the setup and then switch ON the heater.
5. Set the temperature of the boiler (110 to 120 oC).
6. Open the funnel valve and vent valve of the feed tank, fill the prepared 5% Sodium
Carbonate solution in the feed tank and close the valves.
7. Connect the air supply to the steam tank and adjust the pressure of the feed tank in the
range of 0.5 to 1 kg/cm2 by using a pressure regulator and pressure gauge.
8. Allow the feed supply to the evaporator by opening the feed supply valve and rotameter.
9. Now for allowing the steam supply, first, we need to open the vent valve to release air
and then the steam valve.
10. Stop feed supply through Rotameter when we can see a level of solution through the
front glass of the first evaporator.
11. Open recirculation valve of the first evaporator and wait till observing a rise in
temperature of vapor indicating vapourization.
12. Open vent valve to release air before steam trap of second evaporator. Close the valve.
13. Now allow the feed to the second evaporator by partially opening the product outlet
valve of the first evaporator.
 14. Open feed supply from Rotameter by adjusting the flow rate of feed so that the level of
the solution in the first evaporator remains constant can be seen from the side glass.
15. When a level of the solution is maintained in the second evaporator we can stop the
feed supply.
16. Open recirculation valve of the second evaporator and wait till observing a rise in
temperature of the vapor.
17. Now we need to switch ON the pump and set the flow rate of cooling water using
Rotameter.
18. Open the product outlet valve of the second evaporator partially and then at the same
time open the feed supply using the Rotameter and adjust the flow rate of feed so that
the level of the solution in the side glass of both evaporators remains constant.
19. Measure and note the flow rate of steam condensed by using the measuring cylinder and
stopwatch.
20. Measure and note the evaporated vapour condensation rate by using the measuring
cylinder and stopwatch.
21. When the system is at steady-state, record the temperature of the product at different
points (T1 to T8) and the flow rate of the product using a measuring cylinder and
stopwatch.
22. Measure the concentration of the product using the calibration chart by measuring the
density.

Closing procedure:
1. Turn OFF the Heater.
2. Release pressure from the steam generator by partially opening the vent valve.
3. Switch OFF the pump and also turn OFF the power supply to the panel.
4. Turn OFF the compressed air supply to the setup and release air pressure by using the
regulator.
5. Now open the drain valve provided to drain the feed tank and open the product outlet
valve of the evaporator to drain the evaporator liquid.
6. Open the drain valve provided to drain the condenser.
7. Finally, drain the water present in the steam generator.
CALCULATIONS :
𝑀𝑓 3
Density of feed ρ𝑓 = 𝑉𝑓
* 1000 = 1019 𝑘𝑔/𝑚

𝑀𝑝 3
Density of product ρ𝑝 = 𝑉𝑝
* 1000 = 1053. 2 𝑘𝑔/𝑚
−6
𝑉𝑝*10 *ρ𝑝 3
Flow rate of product 𝑊𝑝 = 𝑡𝑝
= 0. 00526 𝑘𝑔/𝑚
−3
𝐹𝑓*10 *ρ𝑓
Flow rate of feed 𝑊𝑓 = 3600
= 0. 005664 𝑘𝑔/𝑠
−6
𝑉𝑠*10 *ρ𝑤
Rate of steam condensation 𝑊𝑠 = 𝑡𝑠
= 0. 532 𝑘𝑔/𝑠

For first evaporator

−6
𝑉𝑉1*10 *ρ𝑤
Rate of condensation of vapor from condenser 𝑊𝑉1 = 𝑡𝑉1
== 2. 94 𝑘𝑔/𝑠

For second evaporator

−6
𝑉𝑉2*10 *ρ𝑤
Rate of condensation of vapor from condenser 𝑊𝑉2 = 𝑡𝑉2
== 0. 408 𝑘𝑔/𝑠

𝑊𝑉1+𝑊𝑉2
Steam Economy 𝐸 = 𝑊𝑆
= 1. 79

Evaporator Capacity 𝐶 = (𝑊𝑉2 + 𝑊𝑉2) * 3600 = 3. 348 𝑘𝑔/ℎ𝑟

RESULTS:
The economy of the double effect evaporator is 1.79
and the capacity of the double effect evaporator is 3.35Kg/hr

DISCUSSION AND CONCLUSION :

1. In case of multiple-effect evaporators we see that the Steam economy (kg water evaporated/
kg steam) is directly proportional to the number of evaporators.
The potential steam economy of an evaporator with MVR(Mechanical Vapor Recompression)
is around 5–10 times that of a three-effect evaporator with additional thermo compressor.

2. No, it is not possible to extend the evaporator infinitely. This is due to the fact that the
temperature of the liquid in evaporator 2 must be considerably lower than condensation
 temperature for the heat transfer to occur from the condensing vapor (from evaporator 1) to
the boiling liquid in evaporator 2.

3. The expected pressure in the second evaporating chamber should be lower than the 1st
evaporator for the transfer of condensed vapor .

4. Some of the major losses that reduce the efficiency of the system are as below:
a) Presence of low temperature difference and gradient between the condensation
temperature of vapour and boiling temperature of solution which hinders the transfer of heat.
b) Heat loss through the pipes and their instruments .This problem can be tackled using
insulations to prevent the loss.

SOURCES OF ERRORS:
1.Operating the apparatus below 180V and above 240V might give rise to an error.
2. Human errors while performing the experiment
3.Varying rotameter flow during the experiment.

SCOPE FOR FURTHER IMPROVEMENT:

The feeding method can be modified based on the temperature requirement of the feed.
Generally, the forward feed method is used in the double effect evaporator where the thin
liquid is pumped into the first effect, and then it is sent to the other effect. But for subsequent
effects, the evaporation occurs at the decreased temperature. We can use the mixed feed
method where the dilute liquid enters an intermediate effect, flows in forward feed to the end
of the series, and is then pumped back to the first effects for final concentration. This permits
the final evaporation to be done at the highest temperature.
We can use steam ejectors for handling large volumes of low-density vapor than mechanical
vacuum pumps and also they are easier and cheaper to maintain. It improves an evaporator’s
steam economy by an amount equivalent to the addition of another effect, but at a much lower
cost.
By using Mechanical vapor recompression(MVR), a mechanical compressor or fan to
compress low-pressure vapor leaving an evaporator to a higher pressure and temperature for
 reuse as the evaporation heat source, we can eliminates the need for an external condenser
which most evaporator systems need to condense the vapor leaving the last effect.
The overall evaporation process can be enhanced by setting the optimum number of effects. It
must be found from an economic balance between the savings in steam obtained by
multiple-effect operation and the added investment required.

PRECAUTIONS :
1. Strictly make sure that the run power supply to the setup is in range of 180 to 230 volts.
2. Make sure that all the ON/OFF switches on the panel are OFF before switching On the
mains power supply.
3. The temperature indicator should be gently turned OFF.
4. The apparatus should be properly clean and dust free.