EXP 7- cooling tower Report final PDF

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Experiment 7: Mass and Energy Balance on aForced Draft Cooling TowerCHEE2820: Transfer Processes LaboratoryFaculty of Engineering and the Built EnvironmentAbstractEnergy and mass balances over a P.A Forced Draft Cooling Tower were carried out.This allowed the determination of wet and dry bulb temper...

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CHEE2820 Mass and Energy Balance on a Forced Draft Cooling Water Tower:

Experiment 7: Mass and Energy Balance on a Forced Draft Cooling Tower CHEE2820: Transfer Processes Laboratory Faculty of Engineering and the Built Environment

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CHEE2820 Mass and Energy Balance on a Forced Draft Cooling Water Tower:

Abstract Energy and mass balances over a P.A.Hilton Forced Draft Cooling Tower were carried out. This allowed the determination of wet and dry bulb temperatures, and the temperatures of the input and output flows over the system. Four sets of experimental conditions were tested each varying in the air flow rate, water flow rate, load of the heater on the system increasing the systems energy all compared with base conditions. The flow rate of the water was increased from 10g.s-1 to 30g.s-1, heat loads between 1000W and 1500W were tested and air flow rates between 4.5 and 12 mmH2O were also varied. Through increasing the heater load on the system the efficiency of the cooling appeared to increase while in increasing the flowrate of the air stream decreased the overall efficiency. Efficiency also increased when the rate of flow of the water decreased, increasing the amount of cooling time available. The results of the energy and mass balances were determined both theoretically and experimentally. Experimentally derived results employed the use of a psychrometric chart to calculate specific enthalpy, volume and absolute humidity of the flows based upon the wet and dry bulb temperatures from the air flows. The results of the energy balances were compared to theoretically determined values and were within good agreement. The experimentally determined values contained a large error margin this was due to the propagation of errors in measurements throughout the calculations. The losses of energy from the system to the surroundings through natural convection were found to be negligible due to the presence of other erroneous values.

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CHEE2820 Mass and Energy Balance on a Forced Draft Cooling Water Tower:

Table of Contents Abstract................................................................................................................................................. 2 Introduction...........................................................................................................................................5 Theory................................................................................................................................................... 7 Figure 1: Experimental system boundaries taken in Experiment.......................................................9 Experimental.......................................................................................................................................12 Apparatus and Equipment:...............................................................................................................12 Figure 2: Schematic diagram Bench Top Cooling Tower H892 (P. A. Hilton Ltd, 2006).....................13 Experimental Procedure:.................................................................................................................14 Table 1: Base experimental conditions for the forced draft cooling tower system...............................14 Table 2: Experimental conditions tested for the forced draft sooling tower system.............................15 Data Analysis:..................................................................................................................................15 Results and Discussion........................................................................................................................16 Table 3: Steady State Temperatures and Experimental Configurations...............................................16 Table 4: Inlet/Outlet Air Stream Properties from estimated from psychometric chart..........................17 Table 5: Theoretical and experimental flow rates for both of the air and water streams.....................17 Table 6: Energy Balance on Overall System including losses due to convection.................................18 Table 7: Energy Balance across Boundary 2.......................................................................................19 Table 8: Energy Balance across Boundary 3.......................................................................................20 Conclusions and Recommendations....................................................................................................22 Nomenclature......................................................................................................................................23 References...........................................................................................................................................24 Appendix............................................................................................................................................. 25 Appendix A-Raw Data.....................................................................................................................25 Appendix B-Sample Calculations....................................................................................................26

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CHEE2820 Mass and Energy Balance on a Forced Draft Cooling Water Tower: Appendix C-Error Calculations.......................................................................................................28

Table of Figures Figure 1: Experimental system boundaries taken in Experiment. . .Error: Reference source not found Figure 2: Schematic diagram Bench Top Cooling Tower H892 (P. A. Hilton Ltd, 2006)...............13

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CHEE2820 Mass and Energy Balance on a Forced Draft Cooling Water Tower:

Introduction Water cooling towers are an integral part of the chemical process industry and are used as an inexpensive means of transferring waste heat from a process to the atmosphere. Towers consist of feeding water in at the top of the column and allowing it to cascade down over a large number of slats while air is forced up through the tower in an opposing direction, Iveson 2009. The purpose of the slats is to increase the surface area of the water, while at the same time creating turbulence in the water flow and increasing the time the water is contacted with the air. Slats are usually constructed from hydroscopic materials helping to increase water cooling time in the tower. This therefore allows for greater cooling of the water. This report describes an experiment that was carried out as a component of second year Chemical Engineering at the University of Newcastle. The experiment utilised a P.A Hilton Ltd. Forced Draft Bench Top Cooling Tower H892 which introduced students to the operation of cooling tower systems. The experiment also allowed students to apply mass and energy balances to a real life situation. From the experiment a set of temperatures for both the air and water streams over a period of time were obtained. Cooling tower systems are vastly used and as a result, the properties of air and water are readily available. These properties, along with the recorded temperatures and calculated flow rates allowed the calculation of the specific enthalpy of both the inlet and outlet water streams. These calculated values were used to construct mass and energy balances around the overall system boundaries. These experimental values were then compared to theoretical values which had been calculated previously.

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CHEE2820 Mass and Energy Balance on a Forced Draft Cooling Water Tower:

The equations used for the calculation of specific enthalpy, absolute humidity and specific volume can be found in the theoretical section of the report. The theoretical section also contains the air and mass flow rate equations, energy balance equations and the natural convective heat loss equation. This section is then followed by a detailed description of the experimental procedure. The acquired results are detailed and the calculations are presented in the discussion. Conclusions were then made based on the results and calculations.

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CHEE2820 Mass and Energy Balance on a Forced Draft Cooling Water Tower:

Theory The H891 Forced Draft Bench Top Cooling Tower, Figure 2, is used to measure the temperatures of the wet and dry bulbs, the inlet and outlet air stream and the water. These air and water properties are used to compare the theoretical and experimental values of the material and energy balances carried out for the control volume. The wet bulb temperature and the dry bulb temperature are both measured for the same stream and from this experimental data the humidity of the system can be measure by the use of the Psychometric Chart. The specific enthalpy and the volume can also be calculated from the chart. Cooling towers are used to recirculate the water and use it as a coolant. This is achieved through removal of any waste heat generated in the water stream. Initially the water is pumped from the load tank to the top of the column, from here is it distributed evenly over the top of the cooling tower. The water then moves down the column and as it does so, a small amount of the water evaporates into the air stream. This allows for the rest of the water stream to cool as energy is removed as latent heat. The cooled water falls into the basin and the bottom of the tower before it is then fed back into the load tank where it is then reheated and recirculated. The load tank is topped up by the make-up tank which accounts for the minimal evaporation that occurs in the cooling tower. The air passes the wet and dry bulb thermometers before entering the column. The moisture content is highest at the top of the column as the air is pumped up the column before being released to the atmosphere. The process can be seen diagrammatically in Figure 2.

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CHEE2820 Mass and Energy Balance on a Forced Draft Cooling Water Tower:

With the extrapolated values for specific enthalpy, volume and humidity the air flow rate can be calculated for each of the different operating parameters measured in the experiment.

(1)

Where ma is the mass flow rate of the dry air (kg dry air.s-1), x is the orifice manometer reading (mm H2O), Y is the specific humidity of the outlet air (kg H2O .kg-1 of dry air) and va is the specific volume of the air released at the top of the column (m3 total .kg-1 of dry air). Due to the evaporation within the cooling tower water needs to be replaced within the tower and is done by using the water from the make-up tank. The rate at which the water is refilled is proportional the mass flow rate which is calculated by using equation 1 above. Therefore the theoretical value for the mass flow rate can be calculated through: ṁ (theory) = ma (ωb – ωa)

(2)

Equating the formulas of 1 and 2 the complete equation for calculating the theoretical mass balance is given by: ṁ (theory) = 0.0137√(x/((1+Y)va) x (ωb – ωa)

(3)

Where ma is the mass flow rate of the dry air (kg dry air/s), x is the orifice manometer reading (mm H2O), Y is the specific humidity of the outlet air (kg H2O . kg-1 of dry air) and va is the specific volume of the air released at the top of the column (m3 total .kg-1 of dry air). The experimental mass flow of air value can be calculated by: ṁ(exp) = amount of water used / time taken to use that water. (4)

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CHEE2820 Mass and Energy Balance on a Forced Draft Cooling Water Tower:

This is the amount of water used in the make-up tank divided by the time taken for the water to be used within the tank. We can assume that the amount of water that is added to the system should then be equal to the rate of evaporation of the water in the system. This can be assumed due to the Law of Conservation of Mass. The energy balance values also use the Psychometric Chart to obtain the values for the moisture content and the specific heat of the air. The same humidity levels are used in finding the experimental mass flow rate is used to find the theoretical values of the energy balance.

In our experiment we are going to analyse the cooling tower in specific boundaries shown in figure 2 below

Figure 1: Experimental system boundaries taken in Experiment

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CHEE2820 Mass and Energy Balance on a Forced Draft Cooling Water Tower:

The cooling tower can be analysed with energy balances. The general energy balance equation for an open non-steady-state system can be written as (Himmelblau, 1996): dE  ( K  P  H)   in m dt



out

 ( K  P  H)  Q  W m

(5)

 (kg.s-1) is the mass flow rate of where E (J) is the total energy of the system at time t, m

stream i, K (J) is kinetic energy of the stream, P (J) is the potential energy of the stream, and H (J.kg-1) is the specific enthalpy of the stream i. Q and W respectively are the rate of heat flow into the system and work done by the surroundings on the system (J.s-1 or W). For all the systems that we are going to assume that the height of the cooling tower is sufficiently small as to provide negligible potential energy and kinetic energy gain, so those terms have been removed. Assuming there is no accumulation of mass or energy the energy is no longer a function of time. For boundary (1) we are ignoring the work term as all work done is internal. The inlet air stream is assumed to be dry air. The composition of the outlet air stream is assumed to be dry air stream and evaporated water. The equation becomes: (6)

 a (kg.s-1), is the mass flowrate of dry air, m  m (kg.s-1) is the make-up water rate, H i, where m

Ho and Hm (J.kg-1) are the enthalpies of the inlet, outlet and water streams, and Q (W) is the rate of heat flow into the system. One Source of error could be the heat lost through the convection through the wall to the surrounding air. We attempt to account for this using newtons law of cooling

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CHEE2820 Mass and Energy Balance on a Forced Draft Cooling Water Tower: q hA(Twall  Tair )

(7)

where A is the area of the wall of the outside of the cooling tower (m2), h (W.m-2K-1) is the convective heat transfer coefficient, and Twall and Tair (ºC) are the temperatures of the wall and the air in the laboratory, respectively.

For System boundary 2, we include the power of the pump, and ignore the air streams as it does not enter this boundary, and thus the inlet water, outlet water and make up water energy streams are included in the energy balance. The equation for system 2 becomes (8)

 m (kg.s-1) is the make-up water rate, m  w (kg.s-1) is the mass flowrate through the where m

pump (re-circulated water), h wi, hwo and hw (J.kg-1) are the enthalpies of the inlet outlet and water streams, Q (W) is the heat provided by the heater, and W (W) is the work done by the pump. Finally for boundary 3, the energy balance is only over the cooling tower. The balance here only concerns the inlet and outlet water as well as the inlet and outlet air streams. The equation becomes : (9)

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CHEE2820 Mass and Energy Balance on a Forced Draft Cooling Water Tower:

Experimental Apparatus and Equipment: The apparatus utilised throughout the experiment consisted of a P.A. Hilton Ltd. Bench Top Cooling Tower H892. The standard layout for the apparatus is outlined in Figure 1. The Bench top Cooling Tower consisted of an air pump, air flow manometer, water pump and heating unit, variable area rotameter, cooling tower column, digital temperature data logger which displays the temperature readings of the inlet flows, wet and dry bulbs. The 1.2m tall cooling tower, made from a transparent P.V.C plastic, was constructed with eight levels of inclined laminated plastic baffles. Fitted to the top of the water cooling tower is a droplet arrester made of a mesh like material which collects small droplets of water vapour from the exiting air stream and returns them to the cooling tower. Wet and dry bulbs are located top of the cooling tower to record the temperature of the exiting cooled air stream and at the base of the cooling tower where the air enters the tower to be cooled. The Cooling tower utilises both an air stream and a water stream in a counter current arrangement. The air is pumped via an air pump located at the base of the apparatus up through the tower and exits at the top via a discharge orifice. The flow rate of the air displayed by a manometer and can be controlled via an intake damper setting. As the air rises up through the cooling tower the moisture content increases as the water is cooled. Most of this excess water vapour is collected at the top of the tower in the droplet arrester.

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CHEE2820 Mass and Energy Balance on a Forced Draft Cooling Water Tower:

Heated water is pumped from a make-up water tank to the top of the cooling tower. During such time the water passes through a control valve and the flow rate is measured using a variable area rotameter. At the top of the tower the water flows over a thin film into the cooling tank where it falls down through the baffles and is collected at the base of the cooling tower there it flows through a thermostat, of heating capacities 1.0 kW and/or 0.5 kW, which heats the water it can then be recirculated through the system again. The temperature of the water is recorded at the inlet to the thermostat and at the entry to the cooling tower. As the water falls down through the tower it is cooled via the air flow and some of the water evaporates thus the make-up tank level is recorded and cannot be allowed to fall below the marked minimum level on the tank (approximately 25mm).

Figure 2: Schematic diagram Bench Top Cooling Tower H892 (P. A. Hilton Ltd, 2006)

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CHEE2820 Mass and Energy Balance on a Forced Draft Cooling Water Tower:

Experimental Procedure: The temperatures values for the air and water flows were initially recorded using the temperature data logger which was accurate to ±00.1⁰C. The air and water flows for the cooling tower were switched on and the water level for the make-up tank and wet bulbs checked. For re-filling both tanks distilled water was always used. Initial conditions for the cooling tower were programmed into the equipment:

Table 1: Base experimental conditions for the forced draft cooling tower system. Heat Rating: 1.0 kW

Air Flow rate: 11 mmH2O

Water Flow Rate: 30 g.s-1

Water of a known mass and temperature was added to the system via the make-up tank until the level of water was above the mark on the tank. When the water level for the make-up tank reached the line marked timing of the experiment started. Water of a known mass and temperature was added continually throughout the experimental run. The temperatures for each of the thermometers where recorded on a two minute interval. Timing continued until the variation in temperature for each of the thermometers levelled out. This signified the system equilibrium. The timing was stopped when the water level reached the mark on the make-up tank. The time taken for the system to reach equilibrium was also recorded.

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CHEE2820 Mass and Energy Balance on a Forced Draft Cooling Water Tower:

The above procedure was repeated several times until the effects of each of the variables, air flow rate, water flow rate and the temperature of the water could be determined. The following settings were performed on the unit:

Table 2: Experimental conditions tested for the forced draft sooling tower system. Setting Trial: 1 2 3

Heat Rating: 1.0 kW 1.0 kW 1.5 kW

Air Flow Rate: 4.5 mmH2O 11 mmH2O 11 mmH2O

Water ...