Download Lab 27 Thermal Resistance - Insulation

LAB 27: THERMAL RESISTANCE - INSULATION
OVERVIEW:
Heat energy is an indication of the vibrational motion of the atoms and molecules that make up
the material. As Heat energy enters the material the atoms and molecules vibrate faster. Heat
energy is transferred through the material by collision of atoms/molecules with adjacent
atoms/molecules. In this way the Heat energy moves from the "hot end" of the material towards
the cold end. A good conductor, such as metal, will allow the rapid movement of heat energy
from one end of the material to the other in very short time. A good insulator is a poor conductor
and heat energy moves very slowly through the material.
Resistance in thermal systems opposes the flow of thermal or heat energy through a
substance. Thermal Resistance limits heat flow rate just as electrical resistance limits current
(charge flow rate) and fluid resistance limits fluid flow rate. An insulator is a material which has
high thermal resistance and thus restricts the flow of heat energy through it. A conductor is a
material which has low thermal resistance and thus allows heat energy to rapidly flow through it.
Thermal resistance, 𝑅𝑇 , follows the basic equation for resistance of a force or force-like
quantity divided by a rate.
𝛥𝑇
𝑅𝑇 =
𝑄𝑅
T is the difference between the temperatures on each side of the material, and 𝑄𝑅 is the
rate of heat flow.
The units of resistance will be °C/(cal per second) in the SI system, and °F/(Btu per hour)
in the British system.
In this Lab you will fill two identical pipes with hot water and measure the temperatures
in each pipe over a period of time. One pipe is insulated while the other is not insulated. You will
find the heat flow rate for each pipe and then calculate the thermal resistance of each pipe. You
will see the effect of insulation in decreasing the heat flow rate of the insulated pipe.
1
LAB 27: THERMAL RESISTANCE - INSULATION
OBJECTIVES:
A) Find the rate of heat flow out of a pipe assembly.
B) Find the thermal resistance of a pipe assembly.
C) Observe the effect of insulating a pipe assembly.
EQUIPMENT REQUIRED:
Thermal Pipe Assembly
Two Thermocouples
Thermometer
Universal Lead Set
Digital Multimeter (DMM) `
Stop Watch
Hot Plate
Pan
DPDT Switch
Circuit Panel
Circuit Panel Easel
Funnel
PROCEDURE:
A) Lab Setup The basic setup for this lab is shown in Figure 1.
1. Place the Celsius thermometer on the lab table, and let it rest undisturbed.
2. Put about 1000 ml of water in the pan. Place the pan on the hot plate and turn the temperature
control to High. Continue with the following, procedure, as the water heats up. When it reaches a
boil, turn it low and put the lid on so the water just barely continues to boil.
3. Insert a thermocouple probe into the hole of each single-hole stopper. Push the probe through
the hole until it extends about one inch out of the bottom of the stopper. See Figure 2.
4. Mount the DPDT (Double-Pole Double-throw) switch to the circuit panel.
5.Wire the thermocouples to the DPDT switch and the multimeter. Follow the wiring diagram in
Figure 3.
6. Set the DMM to measure voltage in millivolts. Turn it on.
B) Collecting Data
1. Read the thermometer temperature. Record this value in Data Table 1 as the room
temperature.
2. Pour boiling water into each pipe up to the line where the pipe changes inner diameter. Place
the stoppers into the hole of each pipe assembly. See Figure 4.
3. Each pipe holds 350 ml of water. Record the volume of water in Data Table 1.
4. Allow several minutes to elapse before taking the initial thermocouple readings. This will
prevent the heat absorbed by the pipes from being included with the heat flowing out of the pipes
into the room.
2
LAB 27: THERMAL RESISTANCE - INSULATION
5. The DPDT switch is used to select which thermocouple probe is being measured by the DMM.
Use the switch to select the insulated pipe probe, and read the thermocouple voltage. Record the
reading as the initial thermocouple voltage in Data Table 1.
6. Repeat the voltage reading for the uninsulated pipe. Record the value in Data Table 1,.
7. Start the stopwatch.
8. After 20 minutes, take another voltage reading for each pipe. Record these readings as the
final thermocouple voltage in Data Table 1.
9. Take apart the lab setup. Allow the hot plate and pan to cool thoroughly before storing.
C) Calculations
1. Using the thermocouple Calibration Table, find the room temperature voltage for a reference
of 0° C. Record the value in Data Table 1.
2. Add the room temperature voltage to each thermocouple reading. This gives you the total
thermocouple voltage for each reading. By adding the two values, you determine the voltage
based on a reference of 0°C. Record the values in Data Table 1.
3. Using the calibration table, find the water temperature for each total voltage. Record each
temperature in Table 2.
4. Find the temperature change for the water in each pipe. This is equal to the difference between
the initial and final water temperature for each pipe. Record the values in Table 2.
𝛥𝑇𝑊𝐴𝑇𝐸𝑅 = 𝑇𝑖𝑛𝑖𝑡𝑖𝑎𝑙 − 𝑇𝑓𝑖𝑛𝑎𝑙
5. Find the average water temperature for each pipe, using the equation below. Record the values
in Data Table 2.
𝛥𝑇𝐴𝑉𝐸 =
〖𝑇𝑖𝑛𝑖𝑡𝑖𝑎𝑙 − 𝑇𝑓𝑖𝑛𝑎𝑙 )
2
D) REPORT FORM CALCULATIONS The following calculations should be recorded on the
analysis page of your report sheet.
𝐻
To find the heat flow rate, you will use the equation 𝑄𝐻 = 𝑡 . First, though, you must find the
amount of heat, H, which the water lost.
1. Determine the heat lost by the water in each pipe by the equation 𝐻 = 𝑚 • 𝑐 • 𝛥𝑇.
The
ΔT in this equation refers to the temperature change of the water, which is recorded in Table 2. c
is the specific heat of water, 1.0 cal/(g • C°), and m is the mass of water in each pipe.
𝐻
2. Find the heat flow rate, 𝑄𝐻 , in calories per second by using the equation 𝑄𝐻 = 𝑡 , where H is
the heat loss computed in part 1, and t is the time in seconds. Record this value in Table 3.
3
LAB 27: THERMAL RESISTANCE - INSULATION
𝛥𝑇
3. The equation for thermal resistance, 𝑅𝑇 , is 𝑅𝑇 = 𝑄 The ΔT value in this equation is the
𝐻
difference in temperature between the hot water and the room. Since the water temperature
changed during the cooling process, the average water temperature must be used. For each pipe,
calculate ΔT = Tavg - TROOM .
4. Use the calculated values of QH and ΔT from steps 2 & 3 to calculate the thermal resistance
RT for both the insulated and uninsulated pipes. Record these values in table 3.
4
LAB 27: THERMAL RESISTANCE - INSULATION
ANALYSIS SHEET
1. Show how to determine the heat lost by the water in each pipe by the equation
H = mcT.
2. Show how to determine the heat flow rate QH in calories per second by using the
𝐻
equation QH = .
𝑡
3. Show how to calculate the difference between the average water temperature and the
room temperature using T = Tavg - Troom
4. Show how to calculate the thermal resistance, RT =
T
𝑄𝐻
.
5
LAB 27: THERMAL RESISTANCE - INSULATION
5. Explain the two different meanings of “T” in the equations used in this lab. Be specific.
________________________________________________________________________
________________________________________________________________________
________________________________________________________________________
6. How are heat flow and thermal resistance related?
________________________________________________________________________
________________________________________________________________________
7. How are thermal resistance and thermal conductivity related?
________________________________________________________________________
________________________________________________________________________
8. What is the relationship between thermal resistance and the “R-factor” specified for
insulation?
________________________________________________________________________
________________________________________________________________________
6