COMPARATIVE STUDY OF THE PACKING DESIGN FOR A

COOLING TOWER

JOHNATHAN ORTEGA M.

DANIELA JURADO B.

JUAN CAMILO HENAO D.

MILLER A. PUENTES L.

INTRODUCTION• Currently the mass transfer operations are of great importance and use in the chemical engineering

field. One of the biggest operations based on the heat and mass transfer are humidification operations.

• These operations consist in put in contact a gas flow with a pure liquid in which is completelyimmiscible, in order to achieve the liquid evaporation into the gas and thus increase the concentrationof steam in this gas stream. One of the largest applications of this operation is the cooling of water withair through cooling towers

• The next presentation pretends to show the results and analysis obtained through the experimentalprocedure performed in a packed cooling tower.

• For this, have been evaluated two packing types: one of them corresponds to the original tower packingand the other one to a properly designed packing. In order to compare and evaluate the performanceand effectiveness of these, have been determined the global mass transfer coefficients for each one.

• In addition to this, have been determined the minimum amount of air required to carry out the coolingprocess in order to find the oversizing factor

EXPERIMENTAL METHOD• For this experiment was used a cooling tower PA Hilton brand, of forced draft and crosscurrent.

• For this experience the working conditions were:

o A power of 0.5 KW

o 0% in the fan opening

• The water that enters to the tower is pumped from a reservoir where it is heated. Water flow is also dosed by adistribution system, and it falls by gravity from the top. The water flow temperature is measured at the inlet andoutlet of the tower with a thermocouple. The water flow is measured by a rotameter.

• The air is provided by a fan located at the lower section of the tower and it can be adjusted by a rotatory door. Airflows through the packing in the opposite direction of the water stream and while it rises, its moisture contentincreases. The dry and wet bulb temperature at the air inlet and outlet is measured also by a thermocouple. Theairflow estimation is done by the air speed measurement, which is made using a anemometer.

• During the experiment have been used two different kinds of packing, the original and a properly designedpacking. The original consists of several rows of shingles overlapping each other. The designed packingconsists of plastic and Styrofoam spheres which are randomly added into the tower with a total heightequivalent to the original packing height.

SCHEMATIC DIAGRAM OF THE COOLING TOWER

THEORETICAL METHOD

Mass Transfer Coefficient 𝑲𝒀𝒂:

1. Suppose the water that gets out of the towerhas his temperature 5°C more than the wetbulb temperature at the inlet of the gas.

2. Obtain the Psychometric chart of the air at theMedellin atmospheric pressure.

3. Graph the air saturation enthalpy (axis y)respect to the water flow temperature range(axis x).

4. Plot the operation line. It consists in a straightline between the water temperature at the inletand outlet of the tower (axis x) vs. the enthalpyof the gas in the same points (axis y).

5. Plot the minimum operation line. In order to dothis, it must be taken the outlet conditionspoint (lower point of the operation line) andthen trace a tangent line to the saturationcurve.

Definition:

𝑁𝑡𝑜𝐺 = 𝐻1𝐻2 𝑑𝐻

𝐻∗−𝐻=

𝐾𝑌𝑎𝑍

𝐺𝑠′ (1)

Where:

• 𝐻∗ : air saturation enthalpy .

• 𝐻 : air operation line enthalpy.

• 𝐻1 and 𝐻2 correspond to the operation line extreme values.

6. Solve equation (1). This is done by tabulate the values of 1

𝐻∗−𝐻, and then using a numerical

method.H Saturation (Kj/Kgas) H Operation (Kj/Kgas) 1/(H'-H)

77,1 54,99 0,04523495

81,49 58,95 0,044362817

86,17 62,90 0,042981174

91,16 66,86 0,041151247

96,1 70,81 0,039548827

101,98 74,77 0,036751465

107,52 78,73 0,034728975

From equation (1):

𝐾𝑦𝑎 =𝑁𝑡𝑜𝐺 ∗ 𝐺𝑠

′

𝑍(2)

Where:

• 𝐺𝑠′ : gas flow mass per square meter unit.

𝐺𝑠′ = 𝑉𝑒𝑙𝑜𝑐𝑖𝑡𝑦𝑎𝑖𝑟 ∗ 𝜌𝑎𝑖𝑟 (3)

• Z: packing height.

7. Solve equation (2)

Oversizing factor :

1. Obtain the minimum operation line slope:

𝑚 =(𝐻2′−𝐻1)

(𝑇2−𝑇1)(4)

Definition:

𝑚 =𝐿′𝐶𝑎𝐿𝐺𝑠 𝑚𝑖𝑛′ (5)

Where:

• L': water mass flow

• 𝐶𝑎𝐿: Specific heat of liquid

2. Obtain 𝐺𝑠 𝑚𝑖𝑛′ value from equations (4) and (5)

3. Find oversizing factor:

𝑛 =𝐺𝑠′

𝐺𝑠 𝑚𝑖𝑛′ (6)

RESULTS AND ANALYSIS

Each figure contains the equilibrium curve for the saturated air and also the operating line and the minimum operating line

𝑲𝒀𝒂 , 𝑵𝒕𝑶𝑮 and 𝑯𝒕𝑶𝑮 results for each packing :

• As can be seen, the designed packing has a valueof 𝐾𝑌𝑎 less than the corresponding to the originalpacking, which indicates that the original packingpresents a higher degree of mass-transfer fromthe liquid phase to the gas phase, generating agreater cooling of water. This implies that theoriginal packing of the tower has a higher contactarea between the gas phase and the liquid phase.

• The designed packing has a higher Number ofEnthalpy Transfer Units and therefor a lowerdifficulty for transfer this enthalpy. Also has agreater Enthalpy Transfer Unit Height, whichrepresents a better packing efficiency.

Original Packing Designed Packing

𝐾𝑌𝑎 (𝐾𝑔/𝑚3. 𝑠) 3,3492 2,2758

𝑁𝑡𝑂𝐺 0,9682 0,7802

𝐻𝑡𝑜𝐺 (𝑚) 0,4462 0,5537

Oversizing results:

• It can be seen that for both packing the relationbetween 𝐺′𝑠 and 𝐺′𝑠,𝑚𝑖𝑛𝑖𝑚𝑜 obtains values greaterthan 1, which means that in both cases the tower isoversized in a value greater than twice.

• Therefore, it is possible to conclude that both towershave an oversizing greater than 200%, whichindicates that the air flow used during the toweroperation greatly exceeds the minimum air flowthrough which it would be possible to obtain thesame inlet and outlet conditions.

• In this case, the oversizing is a bit high, which is ofvery important care because it implies anunnecessary increase in the equipment operationcost and air flow wastage.

• This situation can lead to two possible solutions:reduce fan power or increase the packing height inorder to take advantage of this air excess andincrease the cooling effect.

G's (Kg/m^2*s

)

G's min (Kg/m^2*s

)Oversizing

Original Packing 1,4944 0,0159 2,1288

Designed Packing 1,2601 0,0153 2,1297

• Is important see that the results presented could be influenced by errors in measurements and byincorrectly operating mechanisms. Some of these negative factors could be the distribution of the liquidat the inlet to the tower which wasn't uniformly, the contact of the liquid with the walls of the towercould possibly generate wet wall cooling, as may also exist inaccuracies in the measurement method forthe wet bulb temperature. All of this would influence the results for the overall coefficient of masstransfer and for the oversizing

CONCLUSIONS • Molecular mass transport is mainly due to a concentration gradient, but sometimes may be due to a

temperature gradient which additionally generates an energy transfer. Therefore the mass transfer operationsare usually accompanied by a heat transfer.

• The use of packing in cooling towers is of vital importance in these processes, due to they facilitate and providethe necessary conditions for the mass and heat transfer phenomena generation.

• Shape, size and others packing characteristics have a huge influence in the packing effectiveness. Therefore, theproper selection and design of the packing can significantly increase or decrease the cooling process and towerefficiency.

• The consideration and analysis of concepts as 𝐾𝑌𝑎 and 𝐻𝑡𝑂𝐺 in cooling towers designing are of great importance due to they bring a measure of the packing behavior and effectiveness.

• The over sizing factor is a also a very important and useful parameter in designing and improving of cooling towers due to it ensures that the process is carried out at the necessary conditions to obtain the required results. However, it must be taken into account not reach very high values of this parameter due to it could lead to some resource wastage and unnecessary costs