Pump Characteristics Essay, Research Paper

Experiment #1

Prelab Proposal

Wednesday, September 15

COOLING TOWER PERFORMANCE

Jonathan Mettes and Shalin Sanjanwala

Submitted to

Professor Muthanna Al-Dahhan

Teaching Assistant Novica Rados

Chemical Engineering Laboratory – I

ChE 374

Fall 1999

TABLE OF CONTENTS

Page

Table of Contents i

Notation ii

List of Figures iii

Introduction 1

Objectives 2

Experimental Set-Up 2

Experimental Procedure and Data Collection 3

Theory and Calculation Procedure 6

References 15

NOTATION

Letter Symbols

a Contact area per tower volume, ft2/ft3

A Contact area, ft2

BDA Bone Dry Air

Cp Heat capacity, Btu/(lb ?F)

E Voltage, volts

G Air flow rate, lbdry air/(hr ft2)

h Enthalpy, Btu/lb

hgt Height, ft

H Humidity, lbwater/lbBDA

I Current, amperes

K Mass Transfer Coefficient, lbwater/(hr ft2)

L Liquid flow rate, lb/(hr ft2)

m Mass flow rate, lb/hr

NTU Number of Transfer Units

PF Power factor, %

T Temperature, ?F

v Velocity, ft/sec

V Active cooling volume, ft3/ft2 of plain area

Vh Humid volume, ft3moist air/lbBDA

w Width, ft

wt% Weight Percentage

Greek Letters

h Efficiency, %

l Latent heat of vaporization, Btu/lb

LIST OF FIGURES

Page

Figure 1: Process Flow Diagram for Bryan Hall Cooling Tower 3

Figure 2: Counterflow Cooling Diagram for Bryan Hall Cooling Tower 6

INTRODUCTION

The purpose of this experiment is to examine the function and use of cooling towers in industry, while experimentally determining the performance of a particular cooling tower, located on the roof of Bryan Hall. The cooling tower in this case is used in an air-conditioning system, whereby the heat rejected by the heat pump is transferred to a “condenser water” circulation system, and is in turn rejected to the atmosphere via the cooling tower.

The primary measured parameters in this experiment are the temperature of the inlet and outlet water stream, the temperature of the inlet and outlet air stream, the air flow rate, and the liquid flow rate. From these values, other important characteristics of the cooling tower can be evaluated and studied, including: the cooling range, which is the difference between Tw,inlet and Tw,outlet; the cooling-tower approach, calculated as the difference between the Tw,outlet and the inlet air wet-bulb temperature; the number of transfer units (NTU) of the tower, representing the size of the equipment that allows the transfer to come to equilibrium; the rate of water loss, which is the rate of the water that is lost by evaporation, blow down, etc.; the rate of make-up, which is the rate of water added to the circulating system to water loss; and the heat load of the tower, representing the heat that is lost to the atmosphere.

Since a large number of chemical industrial processes employ a heat transfer from a source stream to a heat stream, cooling towers are an important component for the design and construction of these processes. It is necessary to be able to measure and analyze the performance of the cooling tower to assure it meets the needs of a particular process, and if it does not, to be able to correct any problems therein.

OBJECTIVES

1. Develop a cooling diagram for the Bryan Hall cooling tower, and discuss its attributes.

2. Estimate the Number of Transfer Units (NTU) of the cooling tower to evaluate its characteristics.

3. Calculate the flow rate of water into the cooling tower by measuring the horsepower input of the water pump (via the voltage and current), then using the performance graph of the pump that relates horsepower input to pumping rate.

4. Determine the rate of water lost to evaporation, blow down, drift, etc., as well as the rate of make-up.

5. Evaluate the heat lost to the atmosphere from the hot water.

EXPERIMENTAL SET-UP

A process flow diagram of the cooling tower system is shown in Figure 1. The main components are the cooling tower itself, the water pump, and the condenser. The cooling tower in this setup is of the induced draft type. The fan that drives the air flow is located on top of the tower, and the resulting suction pulls in the air through the two side panels, where it cools the water that is trickled down across it. Pump P-101 draws the cooled water from the bottom of the cooling tower, and feeds it through valve V-102a (or V-102b, depending on which condenser is operating), where pump P-102a circulates it into condenser #1. After heat exchange takes place, the warmed water is pumped back up through valve V-104a by pump P-103, and is trickled down the panels in the cooling tower, to be cooled by the atmospheric air blowing across it.

Figure 1: Process Flow Diagram for Bryan Hall Cooling Tower

EXPERIMENTAL PROCEDURE AND DATA COLLECTION

There are three locations from which data need be collected during this experiment: the top of the cooling tower, the bottom of the cooling tower, and the basement of Bryan Hall. At the top of the tower, the inlet water temperature is measured, as well as the humidity of the exiting air using a sling psychrometer. At the bottom of the tower, the outlet water temperature, the humidity of the inlet air (again using a sling psychrometer), the velocity of the inlet air (using an anemometer), and the area of the inlet air vents (using a measuring tape) need to be measured. In the basement of Bryan Hall, a reading from the voltmeter and ammeter on the power line to the pump motor is measured.

Starting at the cooling tower itself, load the equipment you will need (psychrometer, thermometer, books, manual, etc.) into a bucket. Using caution, climb the vertical ladder to the top of the tower, and then pull up the bucket with a rope. Open the metal cover of the water inlet chamber to the right of the fan and take a temperature reading of the inlet water (Twater in1), then do the same for the inlet chamber to the left of the fan (Twater in2). To determine the humidity of the air exiting the tower, whirl the sling psychrometer just above the fan for about 15 seconds, making sure that the reservoir contains distilled water, that the wick covers the wet bulb, and there is no moisture on the dry bulb, then quickly check the temperatures, reading the wet-bulb temperature (Twet-bulb,air out) first, then the dry-bulb (Tdry-bulb,air out). Repeat this procedure until there is no change in the temperatures for three consecutive measurements. Once the wet-bulb, dry-bulb, and inlet water temperatures have been measured and recorded, put the equipment back in the bucket, lower it to the ground, and descend the ladder.

Next, at the bottom of the tower, measure the temperature of the pool of water nearest to the ladder in order to get a value for the exit water stream temperature (Twater out1). Use the sling psychrometer to take the humidity measurements for the inlet air stream (Twet-bulb,air in1 and Tdry-bulb,air in1) by whirling it in front of the vent area, repeating the process as was used for the outlet air stream. To measure the air velocity (vair1), place an anemometer in the inlet air stream so the moving air blows squarely into the entrance face. The velocity should be measured at the centers of nine equal areas over the inlet vent area. Using the measuring tape, determine both the height and width of the vent area face, so that the area can be calculated. Repeat all these measurements for the other side of the cooling tower, as wind humidity, wind speed, etc. may be different for both sides.

Once all the temperatures, the inlet air velocity, and the vent area have been measured and recorded, proceed to the basement in Bryan Hall to get readings for the pump motor. On the circuit box for the pump motor is an ammeter and a voltmeter. Read the values off of the meters and record them in the data sheet. A sample data sheet is shown below:

TOP: Twater in1 (?F) ___________ Twater in2 (?F) ___________

Twet-bulb,air out (?F) ___________ ___________ ___________

___________ ___________ ___________

Tdry-bulb,air out (?F) ___________ ___________ ___________

___________ ___________ ___________

BOTTOM: Twater out1 (?F) ___________

Twet-bulb,air in1 (?F) ___________ ___________ ___________

Tdry-bulb,air in1 (?F) ___________ ___________ ___________

vair in1 (ft/s) ___________ ___________ ___________

___________ ___________ ___________

___________ ___________ ___________

hgtvent (ft) ___________ wvent (ft) ___________

Twater out2 (?F) ___________

Twet-bulb,air in2 (?F) ___________ ___________ ___________

Tdry-bulb,air in2 (?F) ___________ ___________ ___________

vair in2 (ft/s) ___________ ___________ ___________

___________ ___________ ___________

___________ ___________ ___________

BASEMENT: Epump (volts) ___________

Ipump (amps) ___________

THEORY AND CALCULATION PROCEDURE

Development of Cooling Diagram

Figure 2 represents a counterflow cooling diagram for the cooling tower in Bryan Hall. This diagram provides relationships between air and water and shows the driving forces (h’ – h) present in the counterflow cooling tower. Lines CD and AB represent the air and water operating lines, respectively, each being bound by the inlet and outlet water temperatures. The air operating line starts (point C) below point A and at a point corresponding to the enthalpy of the entering wet-bulb temperature. The vertical line AC represents the driving force at the base of the cooling tower. The air operating line (CD) increases with a slope equaling L/G as the heat that is removed from the water is added to the air. The line ceases at the enthalpy corresponding to that of the wet-bulb temperature out.

Figure 2: Counterflow Cooling Diagram for Bryan Hall Cooling Tower

The temperature of the hot water entering the top of the tower corresponds with point B in figure 1. At this point, a film (saturated with water vapor) surrounds the water. As heat is being removed from the water, the film enthalpy follows the water operating line to the temperature of the cold water out (Perry et.al, 12-13).

One item included in the counterflow cooling diagram is the wet-bulb temperature. It is the liquid temperature (at steady state) that the heat needed to evaporate the liquid and heat the vapor to gas temperature is equal to the sensible heat flowing from the gas to the liquid (McCabe et. al, 748). The wet bulb temperature, along with the dry bulb temperature, can be used to find enthalpies of both air in and air out from psychrometric charts.

For air-water mixtures the wet-bulb temperature resembles the adiabatic saturation temperature, the temperature at which a gas comes into equilibrium with any unevaporated water. It can be shown that Twb = Tad.sat..

Eqn. 23.19 and 23.21

Eqn. 23.11

To construct the cooling curve, the wet-bulb temperature in, cold water temperature out, wet-bulb temperature out, and hot water temperature in are measured. From these measurements, the corresponding enthalpies can be found. Psychrometric charts are used to find the enthalpies for the inlet and outlet wet bulb temperatures, which aides in the construction of the air operating line (CD). Point C is plotted at the temperature of the cold water out and the enthalpy of the air corresponding to that of the inlet wet-bulb temperature. Point D is plotted at the temperature of the hot water in and the enthalpy corresponding to that of the outlet wet-bulb temperature. Connecting these two lines gives the air operating line, and the slope of the line is equivalent to the L/G ratio.

To construct the water operating line (AB), the enthalpies of the cold water out and hot water in must be found. Point A is plotted at the temperature of the cold water out and its corresponding enthalpy. Point B is plotted at the temperature of the hot water in and its corresponding enthalpy. A nonlinear line that goes through the inlet and outlet wet-bulb temperatures and their enthalpies connects A and B.

From the cooling curve just developed, the L/G ratio can be found. It is simply the slope of the air operating line.

The hair out and hair in are found on psychrometric charts using the inlet and outlet wet-bulb temperatures.

L/G max occurs when the outlet wet-bulb temperature of air is equal to the inlet water temperature. To calculate this, a line is drawn from C to B and the slope is calculated.

The enthalpy of the inlet and outlet water streams can be found by referring to the moist air charts, such as those in Perry’s Handbook. The enthalpy can be found by reading off the temperature of the stream from the column and obtaining the corresponding enthalpy for that temperature. After the enthalpies of the inlet and outlet water streams are found, driving forces can be calculated. The driving force is the difference between the water operating line and the air operating line at any point along the tower. Driving force calculations are utilized in the calculation of tower characteristics (NTU) utilized in the calculation of tower characteristics (NTU).

The cooling range is the difference between inlet hot water temperature and the outlet cold water temperature and the heat load is fixed. The cooling tower approach is the difference between outlet cold water temperature and entering air wet-bulb temperature. Size and efficiency of the tower fix approach. Large towers that have average efficiency do not “approach” given wet-bulb temperatures any closer than smaller towers with better efficiency (Hensley 22).

Estimation of NTU

The Number of Transfer Units (NTU) can be estimated in two different ways: numerical solution or adiabatic humidification assumption. The numerical solution involves using the cooling diagram and different methods of integration to estimate NTU. The adiabatic humidification assumption involves the use of formulas based on either mass or heat transfer.

Estimating NTU numerically involves using the cooling diagram since NTU is related to the area of ABCD. The values of the different driving forces (h’-h) vs. T are taken from the cooling diagram within the ABCD area. The inverse of the driving force 1/(h’-h) is plotted versus temperature. The area under the curve of 1/(h’-h) vs. T is calculated using Trapezoidal or Simpson’s rule using the bounds Tcw,out and Thw,in. The result is equal to NTU.

NTU can be estimated two different ways when using the adiabatic humidification assumption. Based on mass transfer:

where H is humidity. The humidity can be read off the psychrometric charts using the dry-bulb and wet-bulb temperatures. NTU can also be calculated based on heat transfer:

The size of the equipment that allows a transfer to come to equilibrium is what NTU measures.

Estimation of Water Circulating Flow Rate

Since there is no flowmeter on the circulated water line and therefore no direct way to measure the water circulating flow rate, it is necessary to use the power input to the pump to indirectly calculate the flow rate of water in the system. Using the formula:

the power input to the pump can be calculated, where Imeasured is the measured current and Emeasured is the measured voltage from the circuit box of the pump in the basement of Bryan. PF is the power factor and can be determined from Table 2 (pg. 18, Lab Manual) by the following equation:

where Erated = 230V. After calculating Icorrect from this formula, PF (%) can be obtained and then plugged into equation 6 to calculate power input. The efficiency of the pump (h) can also be obtained from Table 2, and the power output of the pump can be calculated from:

Once power output is calculated, the flow rate of the water in gallons per minute can be determined from Figure 4 (pg. 22, Lab Manual) and converted to lb/(hr ft2) using the following equation:

Using the power output to estimate the circulating flow rates from Figure 4 is more accurate than using the power input, since the power output is directly correlated to the flow rate, whereas some of the power input is lost to friction, heat, etc. A possible source of error in the estimation of the water flow rate lies in Figure 4 (output vs. power input) itself. This graph is merely an experimentally determined correlation and is not an exact relation, so some error is introduced into the experiment. A better way would be to install a flowmeter on the water line to directly measure the flow rate. There is only one pump operating in Bryan Hall because one pump can provide the necessary water flow rate capacity of the cooling tower. The tower can only handle a certain amount of water per unit time, so if all three of the pumps in the basement of Bryan Hall were in operation, it would just overload the tower.

Determination of Water Loss or Make-up

The water loss of the tower is the rate of the water that is lost by evaporation, blow down, etc., and is measured in lb/hr. It can be calculated by taking the difference in the inlet and outlet humidity of the atmospheric air used in the tower multiplied by the mass flow rate of the air:

Where Hair out, c is the humidity of the outlet air stream, after correction to the operating pressure, and Hair in, c is the humidity of the inlet air stream after pressure correction. The rate of water make-up is equal to the rate of water loss. To calculate the make-up percentage of the circulating water flow rate:

It is better for the rate of water loss to be small so that you do not have to replenish the circulating condenser water as often. If the rate of water loss is too large, the tower should be checked to make sure that there are no severe leaks in the piping system, that the trickle plates are not damaged, etc.

Evaluation of Heat Load

The heat load of the cooling tower is the total heat to be removed from the circulating water by the cooling tower per unit time, and can be calculated two ways, both of which are in units of BTU/hr. The first way is based on the amount of heat that is released from the water, and is calculated by determining the heat (in BTU) available in the inlet water versus the heat (in BTU) available in the outlet water:

The second way is based on the amount of heat that is absorbed by the ambient air fed through the cooling tower, which is calculated by the difference in heat in the outlet air and the inlet air:

where DT1 = Tair out – Treference, and DT2 = Tair in – Treference. lavg is the average latent heat, calculated from the latent heat for the inlet and outlet conditions:

The unit “tons of refrigeration” is a unit used to measure the amount of heat removed by a chiller per unit time, and is equal in value to 12000 BTU/hr. It is a refrigerating effect equal to melting one ton of ice in 24 hours. Using the following equation expresses the heat load calculated by either equation 12 or equation 13 in tons of refrigeration:

It is important to know the heat load of the cooling tower in order to evaluate its overall capacity to reject heat from the “condenser water”, so that (in this particular case) accurate calculations can be made as to how much volume the air conditioning system can effectively cool. The heat load should be a large number, as the larger the heat load, the larger the amount of heat that can be removed from the water per hour, and the more condenser water that can be pumped through to absorb the heat rejected from the heat pump.

RESULTS AND DATA ANALYSIS

Cooling Diagram

The cooling diagram for the Bryan Hall cooling tower as constructed from the raw data listed in Appendix C is shown in figure 3. In developing this diagram, a counterflow assumption was made, even though the tower exhibits a crossflow pattern. In assuming counterflow, air and water conditions are assumed to be constant across any horizontal section of the tower. This differs from crossflow towers since both air and water conditions vary vertically and horizontally in crossflow towers. Some errors result from these differences. For instance, colder water can be obtained from a counterflow tower (Burger, 1999). Colder water would effect both of the temperature and air operating lines. If temperature of water out was cooler, the slope of the air operating line (L/G) and the driving force would also be smaller, therefore increasing the number of transfer units. Since in this case the number of transfer units for the counterflow tower would be larger than for a crossflow tower, using the counterflow theory overestimates the tower characteristics.

Figure 3. Counterflow cooling diagram for Bryan Hall Cooling Tower

On the cooling diagram, the water operating line exhibits a curved shape that increases as temperature increases. As water falls vertically, it tends to move towards colder air. The water approaches the wet-bulb temperature as a limit (Baker and Shryock, 1961). The saturation line also lies on the water operating line. This is true because it is assumed that at points on this line, a film (saturated with water vapor) surrounds each water particle. If a particle is at conditions above this line, it exists as water. The water operating line displays the same behavior as the saturation line.

The air operating line on the cooling diagram exhibits a straight line. Air that moves through a section always moves towards hotter water and this can be seen in the cooling diagram (figure 3). The air approaches the hot water temperature as a limit. As the hot air enters the bottom of the cooling tower, it wants to move towards hotter water (hot water is located at the top of the tower). This is why the air operating line goes from cold water temperature to hot water temperature.

The slope of the air operating line was used to calculate L/G and (L/G)max. Equations 1 and 2 (calculated in Appendix A), respectively, were used to calculate these values and they are presented in Table 1:

Table 1. Results for L/G and (L/G)max.

L/G .7738 Btu/lb ?F

(L/G)max 1.5344 Btu/lb ?F

The L/G ratio is simply the ratio of the mass flow rates of water to air. The L/G ratio increases as the air flow rate decreases and the driving force is decreased. Therefore, the NTU would be increased. This is not a desired situation since the extent of the equipment that allows the transfer to come to equilibrium is large. Therefore it is more desirable to have a low L/G ratio. A low ratio means the gas flow rate is much larger than the liquid flow rate. More heat can be transferred from the water to the air faster if the gas flow rate is larger.

The driving forces at the top and bottom of the tower were calculated using eq. 3 (calculations in appendix A). Table 2 summarizes the results of the driving forces:

Table 2: Driving Forces at top and bottom of cooling tower.

Location Driving Force

Top 13.39 Btu/lb BDA

Bottom 9.44 Btu/lb BDA

The driving force increases when going from the bottom of the cooling tower to the top of the cooling tower.

The cooling range is the difference between inlet hot water temperature and the outlet cold water temperature and the heat load is fixed. It is better if the range is low because then the L/G ratio will be small, which in turn means the required coefficient (NTU) is small and performance of the tower is better. Thus, a larger cooling range means a smaller extent of equipment is needed for the transfer to come to equilibrium, which is desirable. The cooling tower approach is the difference between outlet cold water temperature and entering air wet-bulb temperature. Size and efficiency of the tower fix approach.

Number of Transfer Units

The number of transfer units measures the size or the extent of the equipment that allows the transfer to come to equilibrium. It was calculated three different ways: 1) numerical estimation; 2) adiabatic assumption based on mass transfer; 3) adiabatic assumption based on heat transfer. In order to solve the equations 4 and 5, the Hsat,inlet and Tsat,inlet were found (the values are found in Appendix C). In the numerical estimation, equations for the water operating line and air operating line (both labeled in figure 3) were found using regression analysis. Table 3 summarizes the results of NTU calculations (refer to appendix A for sample calculations):

Table 3: Results for NTU calculations.

Method NTU

Numerical Estimation 1.895

Mass Transfer Basis .0117

Heat Transfer Basis -.2097

The numerical estimation gives the most reliable value out of the three. In estimation using mass transfer, the Hairout was not able to be found. This is because the dry-bulb temperature of the air out was higher than the dry-bulb temperature coming in. And with the adiabatic humidification assumption, it was not possible to find Hairou. Having the outlet dry-bulb temperature higher than the inlet dry-bulb temperature also led to error in estimation of NTU using heat transfer basis.

NTU is an important value for the tower performance because it is a measure of the degree-of-difficulty of the problem (Baker and Shryock, 1961). It is better for NTU to be small. A smaller NTU means it is easier for the transfer to come to equilibrium.

REFERENCES

Al-Dahhan, Muthanna. ChE 374 Laboratory Manual: Experiments in Heat-Mass-

Momentum Transport. Washington University, 1997.

Baker, Donald, and Shryock, Howark. Journal of Heat Transfer. “A comprehensive approach to the analysis of cooling tower performance.” August, 1961

Hensley, J.C., ed. Cooling Tower Fundamentals. The Marley Cooling Tower Co. 1982.

McCabe, W.L., Smith, J.C., and Harriott, P. Unit Operations of Chemical Engineering.

5th edition. McGraw-Hill, 1993.

Perry, R., Green, D., and Maloney, J. Perry’s Chemical Engineers’ Handbook. 6th

edition. McGraw-Hill, 1984.

Smith, J.M., and Van Ness, H.C. Introduction to Chemical Engineering

Thermodynamics. 4th edition. McGraw-Hill, 1987.

Welty, James R., Wicks, Charles E., and Wilson, Robert E. Fundamentals of Momentum,

Heat, and Mass Transfer. 3rd edition. John Wiley & Sons, 1984.

Convert Crossflow to Counterflow – National Engineer by Bob Burger

Bibliography

Al-Dahhan, Muthanna. ChE 374 Laboratory Manual: Experiments in Heat-Mass-

Momentum Transport. Washington University, 1997.

Baker, Donald, and Shryock, Howark. Journal of Heat Transfer. “A comprehensive approach to the analysis of cooling tower performance.” August, 1961

Hensley, J.C., ed. Cooling Tower Fundamentals. The Marley Cooling Tower Co. 1982.

McCabe, W.L., Smith, J.C., and Harriott, P. Unit Operations of Chemical Engineering.

5th edition. McGraw-Hill, 1993.

Perry, R., Green, D., and Maloney, J. Perry’s Chemical Engineers’ Handbook. 6th

edition. McGraw-Hill, 1984.

Smith, J.M., and Van Ness, H.C. Introduction to Chemical Engineering

Thermodynamics. 4th edition. McGraw-Hill, 1987.

Welty, James R., Wicks, Charles E., and Wilson, Robert E. Fundamentals of Momentum,

Heat, and Mass Transfer. 3rd edition. John Wiley & Sons, 1984.

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