Colegio San Agustin BacolodCollege of Engineering

ChELab 1: Chemical Engineering Laboratory 1

Laboratory Experiment #1:Heat Loss in Bare, Finned, & Lagged Pipes

Group 3:Aries, Allen JerryBedrio, Shaira KarleneCabuguason, Paul LyndonJardenil, Stephen Paul

Instructor:Engr. Leovigildo Diopenes

Date Performed: June 19, 2015

ABSTRACT

Table of Contents

Introduction and BackgroundHeat Transfer PrinciplesThe transport phenomena of energy or Heat Transfer occur in the unit processes and operations in many industries. Different heat transfer mechanisms are used in the cooling or heating of process materials in heat exchange equipment or more commonly known as heat exchangers. Also, accounting for the heat loss in process plant is essential in minimizing the economic value of energy being wasted. Hence, the knowledge of heat transfer principles is important in the chemical engineering. As in the transfer of momentum and mass, heat transfer occurs due to a driving force and is impeded by a resisting quantity or dimension. The driving force is the temperature difference between two bodies. Whenever, there is an imbalance between the temperatures of two bodies heat transfer occurs. From the Kinetic Molecular Theory, the hotter body contains more energy than the cold body and thus energy transfers from a hot body to a cold body which. This also conforms to the Second Law of Thermodynamics which dictates that the normal flow of heat. In many practical applications, heat transfer is assumed to be steady-state. The most basic law which governs heat transfer is Fouriers Law of Conduction which is mathematically expressed as:

Where There are three ways in which heat can be transferred and these are conduction, convection, and radiation. Conduction occurs when two materials with different temperatures come into contact with each other. It is well understood by the Fouriers Laws of Convection. Fouriers first law is usually used in steady-state conduction and the second law is used in the analysis of unsteady-state conduction which is given below:

Where The expression for Fouriers Law for Unsteady-state Conduction involves partial differential equations and requires special techniques to obtain particular solutions.Convection is also similar to conduction. However, due to the property of fluids to change its density with temperature complicates the analysis. The equivalent equation to describe convection is:

Where There are two kinds of convection: Forced Convection and Natural Convection. Forced Convection involves a fluid-motive device such as a fan or blower which forces the fluid to flow past through a solid surface. On the other hand, in natural convection, the warmer or cooler fluid next to the solid surface causes a circulation due to density differences resulting from the temperature differences in the fluid. Radiation is quite unique compared to conduction and convection because there is no physical medium used in the transfer of heat. Thermal energy in here travels in the form of electromagnetic waves. Heat Transfer through a Hollow CylinderFor a hollow cylinder such as a pipe with the specified dimensions in Figure 1, Fouriers Law of Conduction can be rewritten as:

Figure 1Insulated SurfaceAn insulated pipe

Extended SurfaceFins are added to bare pipe heat exchangers to increase the surface area that will allow transfer of heat. Also finned exchangers have relatively high heat-transfer coefficients compared to the bare ones. The most common type of fin attached to the outside of a tube wall is a longitudinal fin shown below:

Figure 2The heat loss of the finned exchanger is given by the expression:

Where

Objectives1. To determine the heat loss of the hot air-heated bare pipe, finned pipe, insulated pipe and compare their heat losses.2. To estimate its over-all heat transfer coefficient, based on the experimental results of its heat loss. 3. To calculate its average natural (or free) convective heat transfer coefficient.

Materials and MethodsThe following are the materials, supplies, equipment, and apparatuses used in the conduct of the experiment: The Heat Exchangers set-up - Bare pipe / Insulated Pipe / Finned Pipe HE of CSA-B Thermometers, range 0-100oC (7 pieces.) Anemometer ( 1 set ) Stop watch GLX Explorer instrument for surface temperature measurement Masking tape

Thermometers were placed in both ends of the pipe for the measurement of the inlet and outlet hot air temperature. Five points (T1, T2, T3, T4 & T5) were equally located for the measurement of the surface temperature. T1 is located near the hot air inlet thermometer and T2 near the hot air outlet. A thermometer was hanged at a convenient location within the vicinity of the heat exchanger for measurement/monitoring the ambient temperature. The air supply valves of the pipe being used were slightly opened. The main breaker was then switched on. Then the air blowers were switched on. Each air heaters were switched on one at a time for every minute with a valve opening for Trial 1 to open and for Trial 2 to full open. The air temperatures at the inlet at outlet of the pipe were monitored andrecorded until it became constant. GLX Explorer was used in measuring the surface temperature after the inlet and outlet temperature of the pipes are stabilized. Temperature readings were recorded for each point or location ((T1, T2, T3, T4 & T5). Three readings at different spots were taken for each location for its average surface temperature.Time-air speed was measured at the outlet of the pipe with the use of an anemometer and a stopwatch with a 2-3 readings and getting its average. The same procedure was made for Trial 2, but this time the opening of the valve at the air inlet was to full open. Heaters, air-blowers and the main switch were switched off after use and the area was cleared and cleaned up after the experiment.

Tair,in T1 T2 T3 T4 T5 Tair,outTHERMOMETERS B. THERMOMETERSTair,in T1 T2 T3 T4 T5 Tair,out C. FINNED PIPETHERMOMETERSTair,in T1 T2 T3 T4 T5 Tair,outDIAGRAM OF SET-UP

Results and DiscussionTemperature DataThe tables below show the temperature readings in degrees Celsius of the ambient air, air inlet, different surface temperatures, and air outlet.Bare PipeTrialAmbient Air InletSurface TemperatureAir Outlet

01234567

1304640.536.936.135.635.240

2306543.440.538.637.836.852.5

Lagged PipeTrialAmbient Air InletSurface TemperatureAir Outlet

01234567

130.546.535.534.734.333.633.445

230.56740.136.235.635.334.163.8

Finned PipeTrialSurfaceAmbient Air InletSurface TemperatureAir Outlet

01234567

1Fin30.544.739.435.935.735.535.139

Pipe4137.13635.835.3

2Fin30.562.54442.739.939.53950.5

Pipe4846.342.440.840.1

The foregoing surface temperature data were graphed to see the temperature profiles:

The average velocity of air in meters per second flowing inside the pipes has been calculated from the anemometer data and stopwatch:Trial\PipeBareLaggedFinned

11.531.631.91

21.932.072.69

The average of the inlet and outlet air temperatures in each case has been evaluated and is used as the basis temperature in determining the physical properties the air:

PipeTrialAverage T,Density, Viscosity, Heat Capacity, Thermal Conductivity(W/m-K)Prandtl Number,

Bare143.251.11590.0000192981007.10.0273130.71155

259.751.06050.000020051008.00.0284990.70917

Lagged145.51.1080.0000194021007.20.0274760.71121

264.41.04580.0000202581008.30.0288290.70856

Finned141.851.12090.0000192331007.00.0272110.71177

256.51.07090.0000199031007.80.0282670.70961

Source: http://www.mhtl.uwaterloo.ca/old/onlinetools/airprop/airprop.html To determine the flow regime and for the proceeding calculations, the Reynolds Number in each pipe and n each trial has been calculated. Also, the flow rates have been calculated:PipeTrialPipe ID(m)Average Velocity(m/s)Volumetric Flow Rate(m3/s)Mass Flow Rate(kg/s)Reynolds NumberFlow Regime

Bare10.0551.530.0036350.0040564866Turbulent

20.0551.930.0045850.0048635615Turbulent

Lagged10.040641.630.0021140.0023433783Transient

20.040642.070.0026850.0028084343Turbulent

Finned10.0551.910.0045380.0050866122Turbulent

20.0552.690.0063910.0068447961Turbulent

Heat Loss from each pipe can be calculated using the equation:

Where rate of heat loss from the pipemass flow rate of airheat capacity of airdifference in the inlet and outlet temperatures of air

PipeTrialMass Flow Rate(kg/s)Heat Capacity (J/kg-K)

Bare10.0040561007.16.526.55

20.0048631008.014.571.07

Lagged10.0023431007.21.02.36

20.0028081008.31.23.40

Finned10.0050861007.05.729.20

20.0068441007.812.082.77

Comparing the calculations, the rate of heat loss in the lagged pipe is lower compared to the bare pipe. This is due to the higher resistance to heat transfer in the lagged pipe due to the insulation. The heat loss in the finned pipe showed the highest rate which conforms with theory that extended surfaces allow higher rates of heat transfer.Overall heat-transfer coefficients were calculated using the equation

Where rate of heat loss from the pipeoverall heat-trasnfer coefficient heat transfer area temperature driving forcePipeTrialT2T6Delta TAU

Bare140.535.25.30.17278759628.99542572

243.436.86.60.17278759662.32399425

Lagged135.533.42.10.12767432510.31248329

240.133.46.70.1276743253.971996905

Finned14135.35.70.17278759629.6436997

24840.17.90.17278759660.63625986

The Dittus-Boelter correlation is used to solve for the convective heat-transfer coefficient.

The calculated convective-heat transfer coefficient is tabulated as follows:PipeTrial

Bare19.08

210.61

Lagged110.11

211.83

Finned110.87

213.92