HEAT TRANSFER

There are three modes of heat transfer: conduction, convection, and radiation.

BASIC HEAT TRANSFER RATE EQUATIONS Conduction

Fourier's Law of Conduction , where

Q kA

dx

= - dT o

Qo = rate of heat transfer (W)

k = the thermal conductivity [W/(m•K)]

A = the surface area perpendicular to direction of heat transfer (m²)

Convection

Newton's Law of Cooling

, where Qo = hA T_{_ w}- T3_i

h = the convection heat transfer coefficient of the fluid [W/(m2•K)]

A = the convection surface area (m²) T_w = the wall surface temperature (K) T_∞ = the bulk fluid temperature (K) Radiation

The radiation emitted by a body is given by ,

Qo = fvAT⁴ where ε = the emissivity of the body σ = the Stefan-Boltzmann constant = 5.67 × 10^-8 W/(m2•K⁴) A = the body surface area (m²) T = the absolute temperature (K) CONDUCTION

Conduction Through a Plane Wall

T1

Q T2

k

L

, where

Q L

kA T₂ T₁

= -

-o ^ h

A = wall surface area normal to heat flow (m²) L = wall thickness (m)

T₁ = temperature of one surface of the wall (K) T₂ = temperature of the other surface of the wall (K)

Conduction Through a Cylindrical Wall

T1 Q

T2

r2

r1

k

Cylinder (Length = L)

ln Q

r r kL T T 2

1 2

1 2

= r

-o

d

^ n

h

Critical Insulation Radius

r h

k

cr= insulation 3

Thermal Resistance (R)

Q R

T

total

= D

o

Resistances in series are added: Rtotal= RR where, Plane Wall Conduction Resistance (K/W): R ,

kA

= L where L = wall thickness

Cylindrical Wall Conduction Resistance (K/W): , ln

R kL

r r 2

1 2

= r

d n where

L = cylinder length

Convection Resistance (K/W) : R hA

= 1 Composite Plane Wall

L_B

T₁ T ₂

h2

Q

T2

T3

L_A

k_B

k_A Fluid 2

T ₁ ∞

h1

Fluid 1

∞

L

k A k A A

L A

B A

A B

T ₂

h₁ h₂

Q

T₃ T₂

T_{1 ∞}

T _∞1

1 1

To evaluate surface or intermediate temperatures:

Q R

T T

R

T T

A B

1 2 2 3

=

-=

-o

h_∞

r insulation

kinsulation

118 HEAT TRANSFER

Transient Conduction Using the Lumped Capacitance Model

The lumped capacitance model is valid if kA ,

hV 1

Biot number, Bi where

s

= %

h = the convection heat transfer coefficient of the fluid [W/(m2•K)]

V = the volume of the body (m³)

k = thermal conductivity of the body [W/(m•K)]

A_s = the surface area of the body (m²)

A_s

Body

ρ, V, c , T P

h,T Fluid

∞

Constant Fluid Temperature

If the temperature may be considered uniform within the body at any time, the heat transfer rate at the body surface is given by

,

Q hA T T V c

dt

dT where

s P

= - 3 = - t

o ^ h ^ hb l

T = the body temperature (K) T_∞ = the fluid temperature (K) ρ = the density of the body (kg/m³)

c_P = the heat capacity of the body [J/(kg•K)]

t = time (s)

The temperature variation of the body with time is ,

T- T₃=_T_i- T e₃i ^{- bt} where

Vc hA

P

= s

b t

and s where 1

time constant

=

=

b x

x ^ h

The total heat transferred (Q_total) up to time t is ,

Q_total= tVc_{P_}T_i- T_i where T_i = initial body temperature (K) Fins

For a straight fin with uniform cross section (assuming negligible heat transfer from tip),

, tanh

Qo = hPkA T_{c_ b}- T_{3i _}mL_ci where

h = the convection heat transfer coefficient of the fluid [W/(m2•K)]

P = perimeter of exposed fin cross section (m) k = fin thermal conductivity [W/(m•K)]

A_c = fin cross-sectional area (m²) T_b = temperature at base of fin (K) T_∞ = fluid temperature (K)

m kA

hP

c

= L_c = L

P A_c

+ , corrected length of fin (m)

Rectangular Fin

L w

w

= 2w + 2t P

= t

Tb

Ac

T , h_∞

t

Pin Fin

L

D

2

= π

P D

4

= πD

Tb

Ac

T , h_∞

CONVECTION Terms

D = diameter (m)

h = average convection heat transfer coefficient of the fluid [W/(m2•K)]

L = length (m)

Nu = average Nusselt number Pr = Prandtl number =

k c_Pn u_m = mean velocity of fluid (m/s) u_∞ = free stream velocity of fluid (m/s) µ = dynamic viscosity of fluid [kg/(s^•m)]

ρ = density of fluid (kg/m³) External Flow

In all cases, evaluate fluid properties at average temperature between that of the body and that of the flowing fluid.

Flat Plate of Length L in Parallel Flow

. <

. >

Re

Re Pr Re

Re Pr Re

u L

Nu k

hL

Nu k

hL

0 6640 10

0 0366 ^. 10

L

L L L

L L L

1 2 1 3 5

0 8 1 3 5

=

= =

= =

n t ₃

_ _

i i

119 HEAT TRANSFER

Cylinder of Diameter D in Cross Flow

, Re

Re Pr u D

Nu k

hD C where

D

D D

n 1 3

=

= =

n t ₃

Re_D C n

1 – 4 0.989 0.330

4 – 40 0.911 0.385

40 – 4,000 0.683 0.466

4,000 – 40,000 0.193 0.618

40,000 – 250,000 0.0266 0.805

Flow Over a Sphere of Diameter, D

. . ,

< < , ; . < <

Re Pr

Re Pr

Nu k

hD 2 0 0 60

1 70 000 0 6 400

D D

D

1 2 1 3

= = +

^ h

Internal Flow

Re u D

D= tnm

Laminar Flow in Circular Tubes

For laminar flow (Re_D < 2300), fully developed conditions Nu_D = 4.36 (uniform heat flux)

Nu_D = 3.66 (constant surface temperature)

For laminar flow (Re_D < 2300), combined entry length with constant surface temperature

1.86 Re Pr

, Nu

D

L where

1 3 0.14

D

D

s

nb

=

f p

^dn ⁿ

L = length of tube (m) D = tube diameter (m)

µ_b = dynamic viscosity of fluid [kg/(s^•m)] at bulk temperature of fluid, T_b

µ_s = dynamic viscosity of fluid [kg/(s•m)] at inside surface temperature of the tube, T_s

Turbulent Flow in Circular Tubes

For turbulent flow (Re_D > 10⁴, Pr > 0.7) for either uniform surface temperature or uniform heat flux condition, Sieder-Tate equation offers good approximation:

.Nu 0 023Re ^. Pr

.

D D

s 0 8 1 3 b

0 14

= d nnn

Noncircular Ducts

In place of the diameter, D, use the equivalent (hydraulic) diameter (D_H) defined as

D wetted perimeter 4 cross sectional area

H #

-=

Circular Annulus (D_o > D_i)

In place of the diameter, D, use the equivalent (hydraulic) diameter (D_H) defined as

D_H= D_o- D_i

Liquid Metals (0.003 < Pr < 0.05)

. . Re Pr

Nu_D= 6 3+ 0 0167 ^{0 85}_D^{. 0 93.} (uniform heat flux)

. . Re Pr

NuD 7 0 0 025 D^{. .} 0 8 0 8

= + (constant wall temperature) Boiling

Evaporation occurring at a solid-liquid interface when T_solid > Tsat, liquid

q" = h(T_s – T_sat) = hΔT_e , where ΔT_e = excess temperature Pool Boiling – Liquid is quiescent; motion near solid surface is due to free convection and mixing induced by bubble growth and detachment.

Forced Convection Boiling – Fluid motion is induced by external means in addition to free convection and bubble-induced mixing.

Sub-Cooled Boiling – Temperature of liquid is below saturation temperature; bubbles forming at surface may condense in the liquid.

Saturated Boiling – Liquid temperature slightly exceeds the saturation temperature; bubbles forming at the surface are propelled through liquid by buoyancy forces.

♦

CONVECTION NUCLEATEFREE TRANSITION FILM

JETS AND COLUMNS ISOLATED

BUBBLES

LEIDENFROST POINT, q"_min

∆T_e = T_s − T_sat (°C)

∆T_{e, D}

∆T_{e, C}

∆T_{e, B}

∆T_{e, A} A ONB

B P

C CRITICAL HEAT FLUX, q"max

D q"_max

q"_min

5 10 30 120 1,000

10³1 10⁴ 10⁷ 10⁶

10⁵ q"s (W/m2)

BOILING REGIMES

Typical boiling curve for water at one atmosphere: surface heat flux q"s as a function of excess temperature, ∆Te= T_s– T_sat

Free Convection Boiling – Insufficient vapor is in contact with the liquid phase to cause boiling at the saturation temperature.

Nucleate Boiling – Isolated bubbles form at nucleation sites and separate from surface; vapor escapes as jets or columns.

♦ Incropera, Frank P. and David P. DeWitt, Fundamentals of Heat and Mass Transfer, 3rd ed., Wiley, 1990. Reproduced with permission of John Wiley & Sons, Inc.

120 HEAT TRANSFER

♦ For nucleate boiling a widely used correlation was proposed in 1952 by Rohsenow:

q h g Pr

C h

c T T

nucleate

1/2

sat 3 l fg

l v

sf fg l

n pl s

n v

t t

= − −

o < _ i^F > _ iH

qonucleate = nucleate boiling heat flux, W/m² µ_l = viscosity of the liquid, kg/(m•s) h_fg = enthalpy of vaporization, J/kg g = gravitational acceleration, m/s² ρ_l = density of the liquid, kg/m³ ρ_v = density of the vapor, kg/m³

σ = surface tension of liquid-vapor interface, N/m c_pl = specific heat of the liquid, J/(kg^•°C)

T_s = surface temperature of the heater,°C T_sat = saturation temperature of the fluid,°C

C_sf = experimental constant that depends on surface-fluid combination

Pr_l = Prandtl number of the liquid

n = experimental constant that depends on the fluid

♦ Peak Heat Flux

The maximum (or critical) heat flux (CHF) in nucleate pool boiling:

qmax C h g

2 1/4

cr fg v t v tl tv

= −

o 8 _ iB

C_cr is a constant whose value depends on the heater geometry, but generally is about 0.15.

The CHF is independent of the fluid-heating surface combination, as well as the viscosity, thermal conductivity, and specific heat of the liquid.

The CHF increases with pressure up to about one-third of the critical pressure, and then starts to decrease and becomes zero at the critical pressure.

The CHF is proportional to h_fg, and large maximum heat fluxes can be obtained using fluids with a large enthalpy of vaporization, such as water.

♦

Values of the coefficient C_cr for maximum heat flux (dimensionless parameter L* = L[g(ρ_l − ρ_v)/σ]^1/2

Heater Geometry Large horizontal flat heater Small horizontal flat heater¹ Large horizontal cyclinder Small horizontal cyclinder Large sphere

Small sphere

0.149 18.9 K₁ 0.12 0.12 L*^−0.25 0.11 0.227 L*^−0.5

Width or diameter Width or diameter Radius

Radius Radius Radius

L* > 27 9 < L* < 20 L* > 1.2 0.15 < L* < 1.2 L* > 4.26 0.15 < L* < 4.26 C_cr Charac. Dimension

of Heater, L Range of L*

1K₁ =σ/[g(ρ_l − ρ_v)A_heater]

♦ Minimum Heat Flux

Minimum heat flux, which occurs at the Leidenfrost point, is of practical interest since it represents the lower limit for the heat flux in the film boiling regime.

Zuber derived the following expression for the minimum heat flux for a large horizontal plate

0.09

q h g

min 2

1/4 v

l v

l v

t fg

t t

v t t

= +

o

-_ _

i

> iH

The relation above can be in error by 50% or more.

Transition Boiling – Rapid bubble formation results in vapor film on surface and oscillation between film and nucleate boiling.

Film Boiling – Surface completely covered by vapor blanket;

includes significant radiation through vapor film.

♦ Film Boiling

The heat flux for film boiling on a horizontal cylinder or sphere of diameter D is given by

D

0.4

0.62 for horizontal cylinders 0.67 for spheres

q C

T T

gk h c T T

T T

C

film film

v sat

3

sat 1/4

sat

film

s

v v l v fg pv s

n s

t t t

= −

− + −

−

=

o _

_

_ _

i

i

i i

>

^{8 B}

H

* 4

Film Condensation of a Pure Vapor On a Vertical Surface

. ,

Nu k

h L

k T T

0 943 gh L where

.

sat L

l l l s

l fg

2 3 0 25

n

= = t

_ - i

> H

ρ_l = density of liquid phase of fluid (kg/m³) g = gravitational acceleration (9.81 m/s²) h_fg = latent heat of vaporization [J/kg]

L = length of surface [m]

µ_l = dynamic viscosity of liquid phase of fluid [kg/(s•m)]

k_l = thermal conductivity of liquid phase of fluid [W/(m^•K)]

T_sat = saturation temperature of fluid [K]

T_s = temperature of vertical surface [K]

Note: Evaluate all liquid properties at the average temperature between the saturated temperature, T_sat, and the surface temperature, T_s.

Outside Horizontal Tubes

0.729 ,

Nu k

h D

k T T

gh D where

sat

2 3 0.25

D

l l s

l fg

n

= = t

` − j

> H

D = tube outside diameter (m)

Note: Evaluate all liquid properties at the average temperature between the saturated temperature, T_sat, and the surface temperature, T_s.

♦ Cengel, Yunus A., and Afshin J. Ghajar, Heat and Mass Transfer, 4th ed., McGraw-Hill, 2011.

121 HEAT TRANSFER

Natural (Free) Convection

Vertical Flat Plate in Large Body of Stationary Fluid

Equation also can apply to vertical cylinder of sufficiently large diameter in large body of stationary fluid.

,

h C

Lk Ra where_Lⁿ r = b l

L = the length of the plate (cylinder) in the vertical direction

Ra_L = Rayleigh Number = Pr v

g T_s T L

2

- 3

b_{_ 3i} T_s = surface temperature (K)

T_∞ = fluid temperature (K)

β = coefficient of thermal expansion (1/K) (For an ideal gas:

T T

2

s

= +

b

3 with T in absolute temperature) υ = kinematic viscosity (m²/s)

Range of Ra_L C n

10⁴ – 10⁹ 0.59 1/4

10⁹ – 10¹³ 0.10 1/3

Long Horizontal Cylinder in Large Body of Stationary Fluid ,

h C

Dk Ra whereⁿ_D

= b l

v Pr

g T T D

RaD

s 2

3

= b_{_} - _3i

Ra_D C n

10^–3 – 10² 1.02 0.148

10² – 10⁴ 0.850 0.188

10⁴ – 10⁷ 0.480 0.250

10⁷ – 10¹² 0.125 0.333

Heat Exchangers

The rate of heat transfer in a heat exchanger is ,

Qo = UAF TD _lm where

A = any convenient reference area (m²)

F = correction factor for log mean temperature difference for more complex heat exchangers (shell and tube arrangements with several tube or shell passes or cross-flow exchangers with mixed and unmixed flow); otherwise F = 1.

U = overall heat transfer coefficient based on area A and the log mean temperature difference [W/(m2•K)]

∆T_lm = log mean temperature difference (K)

Overall Heat Transfer Coefficient for Concentric Tube and Shell-and-Tube Heat Exchangers

, ln

UA h A A

R

kL D D

A R

h A

1 1

2

1 where

i i i

fi i

o

o fo

o o

= + + r + +

d n

A_i = inside area of tubes (m²) A_o = outside area of tubes (m²) D_i = inside diameter of tubes (m) D_o = outside diameter of tubes (m)

h_i = convection heat transfer coefficient for inside of tubes [W/(m2•K)]

h_o = convection heat transfer coefficient for outside of tubes [W/(m2•K)]

k = thermal conductivity of tube material [W/(m•K)]

R_fi = fouling factor for inside of tube [(m2•K)/W]

R_fo = fouling factor for outside of tube [(m2•K)/W]

Log Mean Temperature Difference (LMTD) For counterflow in tubular heat exchangers

ln T

T T

T T

T T T T

lm

Hi Co

Ho Ci

Ho Ci Hi Co

=

-- -

-D

d

_ _

n

i i

For parallel flow in tubular heat exchangers , ln

T

T T

T T

T T T T

where

lm

Hi Ci

Ho Co

Ho Co Hi Ci

=

-- -

-D

d

_ _

n

i i

∆T_lm = log mean temperature difference (K) T_Hi = inlet temperature of the hot fluid (K) T_Ho = outlet temperature of the hot fluid (K) T_Ci = inlet temperature of the cold fluid (K) T_Co = outlet temperature of the cold fluid (K) Heat Exchanger Effectiveness, ε

Q Q

maximum possible heat transfer rate actual heat transfer rate

max

= =

f o

o

C T T

C T T

C T T

C T T

or

min Hi Ci min

H Hi Ho

Hi Ci

C Co Ci

=

-- =

-f f

-_ _

_ _ i

i

i i where

C = mco P = heat capacity rate (W/K) C_min = smaller of C_C or C_H

122 HEAT TRANSFER

Number of Transfer Units (NTU)

NTU C

UA

min

=

Effectiveness-NTU Relations

C C

C heat capacity ratio

max min

r= =

For parallel flow concentric tube heat exchanger exp

ln C

NTU C

NTU C

C 1

1 1

1

1 1

r r

r r

= +

- - +

= - +

- +

f

f

^

^

h h 8

8

B

B

For counterflow concentric tube heat exchanger

<

<

exp exp

ln

C NTU C

NTU C

C

NTU

NTU C

NTU C C C

NTU C

1 1

1 1

1

1 1

1 1

1

1 1

1 1

r r

r

r

r

r r r

r

= - -

-- -

-= + =

= - -

-= - =

f

f

ff f f

^

^ ^

^

c ^

^ h

h h

h

m h

h 8

8

B B

RADIATION Types of Bodies Any Body

For any body, α + ρ + τ = 1 , where

α = absorptivity (ratio of energy absorbed to incident energy) ρ = reflectivity (ratio of energy reflected to incident energy) τ = transmissivity (ratio of energy transmitted to incident energy)

Opaque Body

For an opaque body: α + ρ = 1 Gray Body

A gray body is one for which

α = ε, (0 < α < 1; 0 < ε < 1), where ε = the emissivity of the body

For a gray body: ε + ρ = 1

Real bodies are frequently approximated as gray bodies.

Black body

A black body is defined as one that absorbs all energy incident upon it. It also emits radiation at the maximum rate for a body of a particular size at a particular temperature. For such a body

α = ε = 1

Shape Factor (View Factor, Configuration Factor) Relations

Reciprocity Relations A_iF_ij = A_jF_ji, where A_i = surface area (m²) of surface i

F_ij = shape factor (view factor, configuration factor); fraction of the radiation leaving surface i that is intercepted by surface j; 0 ≤ F_ij ≤ 1

Summation Rule for N Surfaces Fij 1

j N

1

=

!=

Net Energy Exchange by Radiation between Two Bodies Body Small Compared to its Surroundings

, Q12 A T1 T where

4 2

= fv - 4

o ` j

Qo = the net heat transfer rate from the body (W)₁₂ ε = the emissivity of the body

σ = the Stefan-Boltzmann constant [σ = 5.67 × 10^-8 W/(m2•K⁴)]

A = the body surface area (m²)

T₁ = the absolute temperature (K) of the body surface T₂ = the absolute temperature (K) of the surroundings

123 HEAT TRANSFER

Net Energy Exchange by Radiation between Two Black Bodies

The net energy exchange by radiation between two black bodies that see each other is given by

Qo₁₂= A F_{1 12}v`T₁⁴- T₂⁴j

Net Energy Exchange by Radiation between Two Diffuse-Gray Surfaces that Form an Enclosure

Generalized Cases

A₂ T₂ ε2

12

, ,

A2,T2,ε2

A₁,T₁,ε1

A₁,T₁,ε1

Q Q₁₂

Q

A A F A

T T

1 1 1

12

1 1 1

1 12 2 2

2 1

4 2

4

=

-+ +

-f

f

f f

o v` j

One-Dimensional Geometry with Thin Low-Emissivity Shield Inserted between Two Parallel Plates

A2

ε2

T2

, ,

A3,T3

ε3, 1

ε3, 2

A1

ε1

T1

, ,

Q12 Radiation Shield

Q

A A F A A A F A

T T

1 1 1 1 1 1

, ,

, , 12

1 1 1

1 13 3 1 3

3 1

3 2 3 3 2

3 32 2 2

2 1

4 2

4

=

-+ +

-+

-+ +

-f

f

f f

f f

f f

o v` j

Reradiating Surface

Reradiating Surfaces are considered to be insulated or adiabatic Q_ o_R= 0i.

A_R,T_R,ε_R

A2,T2,ε2

A₁,T₁,ε₁

Q12

Q

A

A F A F A F

A

T T

1

1 1

1 1

R R

12

1 1 1

1 12 1 1 2 2

1 2 2

2 1

4 2

4

=

-+

+ +

+

-f

f

f f v

-o

c c

`

m m

j

= G

124 INSTRUMENTATION, MEASUREMENT, AND CONTROLS

INSTRUMENTATION, MEASUREMENT, AND CONTROLS

MEASUREMENT Definitions

Transducer – a device used to convert a physical parameter such as temperature, pressure, flow, light intensity, etc. into an electrical signal (also called a sensor).

Transducer Sensitivity – the ratio of change in electrical signal magnitude to the change in magnitude of the physical parameter being measured.

Resistance Temperature Detector (RTD) – a device used to relate change in resistance to change in temperature. Typically made from platinum, the controlling equation for an RTD is given by:

, R_T=R₀81+a_T-T₀iB where

R_T = resistance of the RTD at temperature T (in °C) R₀ = resistance of the RTD at the reference temperature

T₀ (usually 0°C)

α = temperature coefficient of the RTD

The following graph shows tolerance values as a function of temperature for 100-Ω RTDs.

±2.5

±2.0

±1.5

±0.5 0

±1.0

−200 −100 0 100 200 300 400 500 600 700800850

±5.0

±4.5

±4.0

±3.5

±3.0

±2.5

±2.0

±1.5

±1.0

±0.5 0 Temperature (°C)

Tolerance Values (Ω)

Class B (

°C) Class B (

Ω)

Class A (°C) Class A (Ω)

Tolerance Values (°C)

From Tempco Manufactured Products, as posted on www.tempco.com, July 2013.

Thermocouple (TC) – a device used to relate change in voltage to change in temperature. A thermocouple consists of two dissimilar conductors in contact, which produce a voltage when heated.

70 60

1,000 10

20 30 40 50

0 0 500 1,500 2,000 2,500 3,000 3,500 4,000 4,500 5,000 Temperature (°F)

EMF Output (mV) Ref. Junction at 32°F

Chromel vs. ConstantanIron vs. Constantan Copper vs. Constantan

Chromel vs.

Alumel

Tungsten 5% Rhenium vs.

Tungsten 26% Rhenium

Tungsten vs. Tungsten 26% Rhenium

Platinum vs. Platinum 13% Rhodium Platinum vs. Platinum 10% Rhodium

From Convectronics Inc., as posted on www.convectronics.com, July 2013.

OUTER JACKET + LEAD

− LEAD

Typical Thermocouple (TC) Cable

From Convectronics Inc., as posted on www.convectronics.com, July 2013.

Alloy Combination and Color

+ Lead − Lead

ANSI

Code Thermocouple

Leads

Extension Cable

Maximum Thermocouple Temperature Range

Environment

J

IRON Fe (magnetic)

White

CONSTANTAN COPPER-NICKEL

Cu-Ni Red

Brown Black −346 to 2,193°F

−210 to 1,200°C

Reducing, Vacuum, Inert.

Limited use in Oxidizing at High Temperatures.

Not Recommended for Low Temperatures

K

NICKELCHROMIUM Ni-Cr Yellow

NICKEL-ALUMINUM Ni-Al (magnetic)

Red

Brown Yellow −454 to 2,501°F

−270 to 1,372°C

Clean Oxidizing and Inert.

Limited Use in Vacuum or Reducing.

T

COPPER Cu Blue

CONSTANTAN COPPER-NICKEL

Cu-Ni Red

Brown Blue −454 to 752°F

−270 to 400°C

Mild Oxidizing, Reducing Vacuum or Inert. Good where

moisture is present.

E

NICKELCHROMIUM Ni-Cr Purple

CONSTANTAN COPPER-NICKEL

Cu-Ni Red

Brown Purple −454 to 1,832°F

−270 to 1,000°C Oxidizing or Inert. Limited Use in Vacuum or Reducing.

Outer Jacket Color

From Omega Engineering, as posted on www.omega.com, July 2013.

125 INSTRUMENTATION, MEASUREMENT, AND CONTROLS

Strain Gage – a device whose electrical resistance varies in proportion to the amount of strain in the device.

Gage Factor (GF) – the ratio of fractional change in electrical resistance to the fractional change in length (strain): ,

GF L L

R R R R

where

= D = f

D D

R = nominal resistance of the strain gage at nominal length L ΔR = change in resistance due the change in length ΔL

ε = normal strain sensed by the gage

The gage factor for metallic strain gages is typically around 2.

1

1

2 1

3

1

3 4

2

1

2

1

3

1 2

4

3 1 1

1

3 4 21

Gage Setup Bridge

Type

Sensitivity

@ 100 µεmV/V Details

Strain

1/4

1/2

1/2

Full

1/4

1/2

Full

1/2

Full

0.5

0.65

2.0 1.0 0.5 1.3 1.0

2.0 1.0

Good: Simplest to implement, but must use a dummy gage if compensating for temperature. Also responds to bending strain.

Better: Temperature compensated, but it is sensitive to bending strain.

Better: Rejects bending strain, but not temperature. Must use dummy gages if compensating for temperature

Best: More sensitive and compensates for both temperature and bending strain.

Good: Simplest to implement, but must use a dummy gage if compensating for temperature. Responds equally to axial strain.

Better: Rejects axial strain and is temperature compensated.

Best: Rejects axial strain and is temperature compensated. Most sensitive to bending strain.

Good: Gages must be mounted at 45 degrees from centerline.

Best: Most sensitive full-bridge version of previous setup. Rejects both axial and bending strains.

AxialBendingTorsional and Shear

From National Instruments Corporation, as posted on www.ni.com, July 2013.

126 INSTRUMENTATION, MEASUREMENT, AND CONTROLS

A simple piezoelectric transducer generates a voltage that is proportional to the change in its ceramic's volume, or will change volume proportional to the applied voltage.

Dimensional changes are usually very small and can be predominantly in one dimension.

♦ Pressure Sensors – can alternatively be called pressure transducers, pressure transmitters, pressure senders, pressure indicators, piezometers, and manometers.

♦

Pressure Relative

Measurement Types Comparison

Absolute Relative to 0 Pa, the pressure in a vacuum Gage Relative to local atmospheric pressure Vacuum Relative to either absolute vacuum (0 Pa)

or local atmospheric pressure

Differential Relative to another pressurized container Sealed Relative to sea level pressure

• pH Sensor – a typical pH meter consists of a special measuring probe connected to an electronic meter that measures and displays the pH reading.

E_el = E⁰- S(pH_a- pH_i) E_el = electrode potential E⁰ = zero potential

S = slope (mV per pH unit)

pH_a = pH value of the measured solution pH_i = pH value of the internal buffer

♦ From National Instruments Corporation, as posted on www.ni.com, July 2013.

• From Alliance Technical Sales, Inc., as posted on www.alliancets.com, July 2013.

Wheatstone Bridge – an electrical circuit used to measure changes in resistance.

R₁

R2

R₃

R4

+

V_IN

−

− V₀ +

WHEATSTONE BRIDGE

If RR R R

2 1

4

= 3 then V₀= 0 V and the bridge is said to be balanced.

If R₁= R₂= R₃= Rand R₄= R + ∆R,where DR%R then, .

V R

R V

4 ^IN

0. D :

Piezoelectric Transducers – often comprised of a special ceramic that converts the electrical energy to mechanical energy, or electrical voltage to mechanical force, and vice versa. When an electric field is applied to the material, it will change dimension. Conversely, when a mechanical force is applied to the material, an electric field is produced.

Piezoelectric transducers can have multiple layers and many different geometries.

Sensor Type Principle Materials Analyte

Semiconducting oxide sensor

Conductivity impedance

SnO₂, TiO₂, ZnO₂, WO₃, polymers

O₂, H₂, CO, SO_x, NO_x, combustible hydrocarbons, alcohol, H₂S, NH₃

Electrochemical sensor (liquid electrolyte)

Amperiometric composite Pt, Au catalyst H₂, O₂, O₃, CO, H₂S, SO₂, NO_x, NH₃, glucose, hydrazine

Ion-selective electrode (ISE) Potentiometric glass, LaF₃, CaF₂ pH, K⁺, Na⁺, Cl⁻, Ca², Mg²⁺, F⁻, Ag⁺ Solid electrode sensor Amperiometric

Potentiometric

YSZ, H⁺-conductor YSZ, β-alumina, Nasicon, Nafion

O₂, H₂, CO, combustible hydrocarbons, O₂, H₂, CO₂, CO, NO_x, SO_x, H₂S, Cl₂ H₂O, combustible hydrocarbons Piezoelectric sensor Mechanical w/

polymer film

quartz combustible hydrocarbons, VOCs

Pyroelectric sensor Calorimetric Pyroelectric + film Vapors Colorimetric

fluorescence

optical fiber/indicator dye Acids, bases, combustible hydrocarbons, biologicals Optical sensors

Catalytic combustion sensor Calorimetric Pt/Al₂O₃, Pt-wire H₂, CO, combustible hydrocarbons Examples of Common Chemical Sensors

Reprinted with permission from Journal of The Electrochemical Society, 150 (2), ©2003, The Electrochemical Society.

127 INSTRUMENTATION, MEASUREMENT, AND CONTROLS

CONTROL SYSTEMS

The linear time-invariant transfer function model represented by the block diagram

X(s)

G(s)

Y(s)

INPUT OUTPUT

can be expressed as the ratio of two polynomials in the form

X s

Y s G s

D s

N s K

s p

s z

n n

N m m

M

1

= = = 1

-=

^ =

^ ^

^^

_ _

hh h

hh

i i

%

%

where the M zeros, z_m, and the N poles, p_n, are the roots of the numerator polynomial, N(s), and the denominator polynomial, D(s), respectively.

One classical negative feedback control system model block diagram is

R(s)

H(s)

−

+ G₁(s)

L(s)

+

+ G₂(s) Y(s)

where G1(s) is a controller or compensator, G₂(s) represents a plant model, and H(s) represents the measurement dynamics.

Y(s) represents the controlled variable, R(s) represents the reference input, and L(s) represents a disturbance. Y(s) is related to R(s) and L(s) by

Y s G s G s H s

G s G s

R s G s G s H s

G s L s

1 1 2 1

1 2

1 2

= 2

+ +

^ +

^^ ^^ ^ ^

^ ^ ^ ^ ^

h hh hh h h

h h h h h

G₁(s) G₂(s) H(s) is the open-loop transfer function. The closed-loop characteristic equation is

1 + G₁(s) G₂(s) H(s) = 0

System performance studies normally include

1. Steady-state analysis using constant inputs based on the Final Value Theorem. If all poles of a G(s) function have negative real parts, then

dc gain = lim G s

s"0 ^ h

Note that G(s) could refer to either an open-loop or a closed-loop transfer function.

For the unity feedback control system model

+ –

R(s) E(s)

G(s) Y(s)

SAMPLING

When a continuous-time or analog signal is sampled using a discrete-time method, certain basic concepts should be considered. The sampling rate or frequency is given by

f t

s= 1 D

Nyquist's (Shannon's) sampling theorem states that in order to accurately reconstruct the analog signal from the discrete sample points, the sample rate must be larger than twice the highest frequency contained in the measured signal. Denoting this frequency, which is called the Nyquist frequency, as

f

_N, the sampling theorem requires that

f_s> 2f_N

When the above condition is not met, the higher frequencies in the measured signal will not be accurately represented and will appear as lower frequencies in the sampled data. These are known as alias frequencies.

Analog-to-Digital Conversion

When converting an analog signal to digital form, the resolution of the conversion is an important factor. For a measured analog signal over the nominal range [VL,V_H], where V_Lis the low end of the voltage range and VH is the nominal high end of the voltage range, the voltage resolution is given by

V V

V = H2-n L

f

where n is the number of conversion bits of the A/D converter with typical values of 4, 8, 10, 12, or 16. This number is a key design parameter. After converting an analog signal, the A/D converter produces an integer number of n bits. Call this number N. Note that the range of N is [0 , 2ⁿ– 1]. When calculating the discrete voltage, V, using the reading, N, from the A/D converter the following equation is used.

V= f_VN+ V_L

Note that with this strategy, the highest measurable voltage is one voltage resolution less than ,V_H orV_H- f_V.

Signal Conditioning

Signal conditioning of the measured analog signal is often required to prevent alias frequencies from being measured, and to reduce measurement errors.

MEASUREMENT UNCERTAINTY

Suppose that a calculated result R depends on measurements whose values are x1± w1, x₂± w₂, x₃± w₃, etc., where R = f(x1, x₂, x₃, ... xn), x_i is the measured value, and w_i is the uncertainty in that value. The uncertainty in R, wR, can be estimated using the Kline-McClintock equation:

w w

x

f w

x

f w

x f

R n

n 1

1 2

2 2

2 2

2 2

2

2 g 2

= d n + d n + + d 2 n

문서에서 FE Reference Handbook (페이지 123-137)