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: