a

interfacial area per volume, ft²/ft³ or m²/m³

a_p

surface area/volume, m²/m³

a_p1, a_p2, a_p3, a_T1, a_T2, a_T6

constants in Eq. (2-30) and Table 2-3

A,B,C

constants in Antoine Eq. (2-34)

A,B,C,D,E

constants in Eq. (2-60)

A,B,C,D

constants in matrix form of mass balances, Eqs. (6-13) and (12-58)

A_E, B_E, C_E, D_E

constants in matrix form of energy balances, Eq. (6-34)

A_active

active area of tray, ft² or m²

A_c

cross sectional area of column, ft² or m²

A_d

downcomer area, ft² or m²

A_du

flow area under downcomer apron, Eq. (10-28), ft²

A_hole

area of holes in column, ft²

A_I

interfacial area between two phases, ft² or m²

A_net

net area, Eq. (10-13), ft² or m²

b

equilibrium constant for linear equilibrium, y = mx + b

B

bottoms flow rate, kg mole/hr or lb mole/hr

C

number of components

C_BM

bare module cost, Chapter 11

C_fL

vapor load coefficient, Eq. (15-38)

C_o

orifice coefficient, Eq. (10-25)

C_p

heat capacity, Btu/lb° F or Btu/lbmole° F or cal/g° C or cal/g mole° C, etc.

C_p

base purchase cost, Chapter 11

C_p,size

packing size factor, Table 10-5

C_pW

water heat capacity

C_s

capacity factor at flood, Eq. (10-48)

C_sb

capacity factor, Eq. (10-8)

d

dampening factor, Eq. (2-57)

D

dampening factor, Eq. (2-57)

D

diffusivity, cm²/s or ft²/hr

D, Dia

diameter of column, ft or m

D′_col

column diameter, see Table 15-2, ft

D_total

total amount of distillate (Chapter 09), moles or kg distillate flow rate, kg mole/hr or lb mole/hr

e

absolute entrainment, moles/hr

E

extract flow rate (Chapters 13 and 14), kg/hr

Ê

mass extract, kg

E_k

value of energy function for trial k, Eq. (2-51)

E_ML, E_MV

Murphree liquid and vapor efficiencies, Eqs. (4-58) and (4-59)

E_o

Overall efficiency, Eq. (4-56)

E_pt

point efficiency, Eq. (10-5) or (15-76a)

E_settler

Holdup extract phase in settler, kg

f = V/F

fraction vaporized

f(x)

equilibrium function, Chapter 09

f_k(V/F)

Rachford-Rice function for trial K, Eq. (2-42)

F

packing factor, Figure 10-25 and Tables 10-3 and 10-4

F

degrees of freedom, Eq. (2-2)

F

charge to still pot (Chapter 09), moles or kg

F

mass of feed in batch extraction, kg

F

feed flow rate, kgmole/hr or lbmol/hr or kg/hr etc.

F_D

diluent flow rate (Chapter 13), kg/hr

F_lv, FP

, flow parameter

F_m

material factor for cost, Table 11-1

F_p

pressure factor for cost, Eqs. (11-5) and (11-6)

F_q

quality factor for cost, Eq. (11-7)

F_s,F_solv

flow rate solvent (Chapter 13), kg/hr

F_solid

solids flow rate in leaching, kg insoluble solid/hr

F_w

modification factor, Eq. (10-26) and Figure 10-20

gap

gap from downcomer apron to tray, Eq. (10-28), ft

g

32.2 ft/s², 9.81 m/s²

G

flow rate carrier gas, kgmole/hr or kg/hr

G′

gas flux, lb/s ft²

h

pressure drop in head of clear liquid, inches liquid

h

height of liquid on stage (Chapter 15), ft

h

height, m or ft

h

liquid enthalpy, kcal/kg, Btu/lb mole, etc.

h

step size in Euler’s method = Δt, Eq. (8-29)

pure component enthalpy

h_f

enthalpy of liquid leaving feed stage

h_F

feed enthalpy (liquid, vapor or two-phase)

h_o

hole diameter, inches

h_p

packing height, ft or m

h_total

height of flash drum, ft or m

H

Henry’s law constant, Eqs. (8-8) and (12-1)

H

molar holdup of liquid on tray, Eq. (8-27)

H

vapor enthalpy, kcal/kg, Btu/lbmole, etc.

H_G

height of gas phase transfer unit, ft or m

H_L

height of liquid phase transfer unit, ft or m

H_OG

height of overall gas phase transfer unit, ft or m

H_OL

height of overall liquid phase transfer unit, ft or m

HETP

height equivalent to a theoretical plate, ft or m

HTU

height of a transfer unit, ft or m

J_A

flux with respect to molar average velocity of fluid

individual mass transfer coefficients in liquid and vapor phases, see Table 15-2

k′_y

mass transfer coefficient in concentrated solutions, Eq. (15-43)

k_x, k_y

individual mass transfer coefficient in weight units

K_D

y/x, distribution coefficient for dilute extraction

K, K_i

y_i/x_i, equilibrium vapor-liquid ratio

K_drum

parameter to calculate u_perm for flash drums, Eq. (2-59)

K_x, K_y

overall mass transfer coefficient in liquid or vapor, lbmoles/ft² hr

l_w

weir length, ft

L

liquid flow rate, kgmoles/hr or lbmoles/hr

mass liquid flow rate, lb/hr (Chapter 15)

L′

liquid flux, lb/(s)(ft²)

L_g

liquid flow rate in gal/min, Chapter 10

m

linear equilibrium constant, y = mx + b

m

local slope of equilibrium curve, Eq. (15-56)

m

ratio HETP_practical/HETP_packing Eq. (10-46)

M

flow rate of mixed stream (Chapter 14), kg/hr

M

multiplier times (L/D)_min (Chapter 07)

MW

molecular weight

n

moles

n_G

number of gas phase transfer units

n_L

number of liquid phase transfer units

n_OG

number of overall gas phase transfer units

n_OL

number of overall liquid phase transfer units

n_org

moles organic in vapor in steam distillation

n_w

moles water in vapor in steam distillation

N

number of stages

N_A

flux of A, lbmoles/(hr)(ft²) or kgmoles/(hr)(m²)

N_f,N_feed

feed stage

N_min

number of stages at total reflux

N_feed,min

estimated feed stage location at total reflux

NTU

number of transfer units

O

total overflow rate in washing, kg/hr

p, p_tot

pressure, atm, kPa, psi, bar etc.

, p_B

partial pressure

P

Number of phases

Q

L_F/F = (L − L)/F, feed quality

Q

amount of energy transferred, Btu/hr, kcal/hr etc.

Q_c

condenser heat load

Q_flash

heat loss from flash drum

Q_R

reboiler heat load

r

radius of column, ft or m

R

gas constant

R

raffinate flow rate (Chapts. 13 and 14), kg/hr

mass raffinate, kg

Raffinate holdup in settler, kg

Raffinate holdup in tank, kg

S

solvent flow rate (Chapter 08) kgmoles/hr or lbmoles /hr

S

tray spacing, inches, Eq. (10-47)

S

moles second solvent in constant-level batch distillation

Ŝ

mass of solvent, kg

S

solvent flow rate (Chapter 14), kg/hr

S_cL

Schmidt number for liquid = µ/ρD

S_cv

Schmidt number for vapor = µ/ρD

t

time, s, min, or hr

t_batch

period for batch distillation, Eq. (9-27)

t_down

down time in batch distillation

t_operating

operating time in batch distillation

t_res

residence time in downcomer, Eq. (10-30), s

t_tray

tray thickness, inches

T

temperature,° C,° F, K or° R

T_ref

reference temperature

u

vapor velocity, cm/s or ft/s

u_flood

flooding velocity, Eq. (10-8)

u_op

operating velocity, Eq. (10-11)

u_perm

permissible vapor velocity, Eq. (2-59)

U

underflow liquid rate, (Chapter 13), kg/hr

v

superficial vapor velocity, ft/s

v_o

vapor velocity through holes, Eq. (10-29), ft/s

v_o,bal

velocity where valve is balanced, Eq. (10-36)

V

vapor flow rate, kgmoles/hr or lbmoles/hr

V

molal volume Eq. (13-6)

V_max

maximum vapor flow rate

V_settler

volume settler, m³

V_tank

volume tank, m³

V_surge

surge volume in flash drum, Figure 2-11, ft³

VP

vapor pressure, same units as p

W_L

liquid flow rate, kg/hr or lb/hr

W_L

liquid mass flux, lb/s ft² or lb/hr ft², (Chapter 15)

W_V

vapor flow rate, kg/hr or lb/hr

x

weight or mole fraction in liquid

x

[L/D − (L/D)_min]/(L/D + 1) in Eqs. (7-42)

x*

equilibrium mole fraction in liquid

x_i,k, x_i,k+1

trials for integration, Eq. (8-29)

x_I

interfacial mole fraction in liquid

x*_out

liquid mole fraction in equilibrium with inlet gas, Eq. (15-35b)

X

weight or mole ratio in liquid

y

weight or mole fraction in vapor

y*

equilibrium mole fraction in vapor

y*_out

vapor mole fraction in equilibrium with inlet liquid in counter current system, Eq. (15-35a) or in equilibrium with outlet liquid in cocurrent contactor, Eq. (15-71)

y_I

interfacial mole fraction in vapor

mass fraction in vapor

Y

weight or mole ratio in vapor

z

weight or mole fraction in feed

z

axial distance in bed (Chapter 15)

α_AB

K_A/K_B, relative volatility

β

A_hole/A_active

γ

activity coefficient

δ_p

characteristic dimension of packing, inch, Eq. (10-39a)

δ_i

solubility parameter, Eq. (13-6)

Δ

change in variable

ΔE_v

latent energy of vaporization, Eq. (13-6)

ε

limit for convergence

η

fraction of column available for vapor flow

θ

angle of downcomer, Figure 10-18B

λ

latent heat of vaporization, kcal/kg, Btu/lb, Btu/lbmole etc.

µ

viscosity, cp

µ_w

viscosity of water, cp

ρ_L

liquid density, g/cm³ or lb/ft³ or kg/m³

ρ_V

vapor density

σ

surface tension, dynes/cm

φ

liquid phase packing parameter, Eq. (15-38)

φ_dc

relative froth density in downcomer, Eq. (10-29)

ψ

ρ_water/ρ_L, Chapter 10

ψ

e/(e + L), fractional entrainment, Chapter 10

ψ

packing parameter for gas phase, Eq. (15-37)

a, a_j

term in quadratic equations for well-mixed membrane systems, Eqs. (16-10b), (16-74a) and (16-74a)

constant in expression to calculate osmotic pressure, kPa/mole fraction, Eq. (16-15a)

a′

constant in expression to calculate osmotic pressure, kPa/weight fraction, Eq. (16-15b)

a_i

activities, Eq. (16-51)

A

membrane area available for mass transfer, cm² or m²

b, b_j

term in quadratic equations for well-mixed membrane systems, Eqs. (16-10c), (16-74b), and (16-74b)

c, c_j

term in quadratic equations for well-mixed membrane systems, Eqs. (16-10d), (16-74c) and (16-74c)

c

concentration, g solute/L solution

c_out

outlet concentration of solute, g/L

c_p

permeate concentration of solute, g/L

c_w

concentration of solute at wall, g/L

c′

water concentration in permeate in Figure 16-17

C_PL,p

liquid heat capacity of permeate, kJ/(kg°C)

C_PV,p

vapor heat capacity of permeate, kJ/(kg°C)

d_t

diameter of tube, cm

d_tank

tank diameter, cm

D

diffusivity in solution, cm²/s

D_m

diffusivity in the membrane, cm²/s

F_p

volumetric flow rate of permeate, cm³/s

F_out

volumetric flow rate of exiting retentate, cm³/s

F_solv

volumetric flow rate of solvent in RO, cm³/s

molar flow rate, gmole/s, gmole/min, etc.

F′

mass flow rate, g/s, g/min, kg/min, etc.

h

½ distance between parallel plates, cm

h_in

enthalpy of inlet liquid stream in pervaporation, kJ/kg

h_out

enthalpy of outlet liquid retentate stream in pervaporation, kJ/kg

H_A

solubility parameter, cc(STP)/[cm³ (cm Hg)]

H_p

enthalpy of vapor permeate stream in pervaporation, kJ/kg

k

mass transfer coefficient, typically cm/s, Eq. (16-33)

K_solv

permeability of the solvent through membrane, L/(atm m² day) or similar units

j

counter for stage location in staged models in Figure 16-20

J

volumetric flux, cm³/(s cm²) or m³/(m² day), Eq. (16-1b)

J′

mass flux, g/(s cm²)or g/(m² day), Eq. (16-1c)

mole flux, gmole/(s cm²) or kg mole/(day m²), Eq. (16-1d)

K′_A

solute permeability, g/(m day wt frac)

K_m,i

rate transfer term for multicomponent gas permeation, dimensionless, Eq. (16-11d)

L

tube length, cm

M

concentration polarization modulus in wt. fraction units, dimensionless, Eq. (16-17)

M_c

concentration polarization modulus in concentration units, dimensionless, Eq. (16-48)

MW

molecular weight, g/gmole or kg/kg mole

N

number of well-mixed stages in models in Figure 16-20

p

pressure, Pa, kPa, atm, mm Hg, etc.

p_A

partial pressure of species A, Pa, atm, mm Hg, etc.

p_p

total pressure on the permeate (low pressure) side, Pa, kPa, atm, mm Hg, etc.

p_r

total pressure on the retentate (high pressure) side, Pa, kPa, atm, mm Hg, etc.

P_A

permeability of species A in the membrane, cc(STP) cm/[cm² s cm Hg]

R

rejection coefficient in wt frac units, dimensionless, Eq. (16-24)

R°

inherent rejection coefficient (M = 1), dimensionless, Eq. (16-24)

R_c

rejection coefficient in conc. units, dimensionless, Eq. (16-48)

R

tube radius, cm

Re

Reynolds number, dimensionless, Eq. (16-35b)

Sc

Schmidt number, dimensionless, Eq. (16-35c)

Sh

Sherwood number, dimensionless, Eq. (16-35a)

t_ms

thickness of membrane skin doing separation, µm, mm, cm or m

T

temperature,°C

T_ref

reference temperature,°C

u_b

bulk velocity in tube, cm/s

v_solvent

partial molar volume of the solvent, cm³/gmole

x

wt frac of retentate in pervaporation. In binary system refers to more permeable species.

x_g

wt frac at which solute gels in UF

x_p

wt frac solute in liquid permeate in RO and UF

x_r

wt frac solute in retentate in RO and UF

y

wt frac of permeate in pervaporation. In binary system refers to more permeable species.

y_p

mole fraction solute in gas permeate for gas permeation

y_r

mole fraction solute in gas retentate for gas permeation

y_r,w

mole fraction solute in gas retentate at membrane wall

y_t,A

mole fraction solute A in gas that transfers through the membrane

α

selectivity, dimensionless, Gas Permeation: Eq. (16-4b), RO: Eq. (16-20), pervaporation: Eq. (16-53a)

Δx

difference in wt frac of solute across the membrane

Δπ

difference in the osmotic pressure across the membrane, Pa, atm, mm Hg, etc.

π

osmotic pressure, Pa, kPa, atm, mm Hg, etc.

θ

, with flows in molar units, dimensionless

θ′

cut = F′_p/F′_in in flows in mass units, dimensionless

µ

viscosity, centipoise or g/(cm s)

ν = µ/ρ

kinematic viscosity, cm²/s

ρ_solv

mass solvent density, kg/m³

molar solvent density, kg mol/m³

λ_p

mass latent heat of vaporization of the permeate in pervaporation determined at the reference temperature, kJ/kg

Ω

stirrer speed in radians/s

a

constant in Langmuir isotherm, same units as q/c, Eq. (17-6c)

a

argument for error function, dimensionless, Eq. (17-70), Table 17-6

a_p

surface area of the particles per volume, m⁻¹

A_c

cross sectional area of column, m²

A_w

wall surface area per volume of column for heat transfer, m⁻¹

b

constant in Langmuir isotherm, (concentration)⁻¹, Eq. (17-6c)

c_A

concentration of species A, kg/m³, kg moles/ m³, g/liter, etc.

c_i

concentration of species i, kg/m³,kg moles/ m³, g/liter, etc., or

c_i

concentration of ion i in solution, typically equivalents/m³

concentration of species i that would be in equilibrium with , same units as c_i

average concentration of solute in pore, same units as c_i

c_pore

fluid concentration at surface of adsorbent pores, same units as c_i

c_i,surface

fluid concentration at surface of particles, ε_p = 0, same units as c_i

c_Ri

concentration of ion i on the resin, typically equivalents/m³

c_RT

total concentration of ions on the resin, typically equivalents/m³

c_T

total concentration of ions in solution, typically equivalents/m³

C_i

constant relating solute velocity to interstitial velocity, dimensionless, Eq. (17-15e)

C_P,f

heat capacity of the fluid, cal/(g°C), cal/(g mol°C), J/(g K), etc.

C_P,p

heat capacity of particle including pore fluid, same units C_P,f

C_P,s

heat capacity of the solid, same units as C_P,f

C_P,w

heat capacity of the wall, same units as C_P,f

d_p

particle diameter, cm or m

D

desorbent rate in SMB, same units as F

D/F

desorbent to feed ratio in SMB, dimensionless

D_col

column diameter, m or cm

D

diffusivity including both molecular and Knudsen diffusivities, m²/s or cm²/s

D_effective

effective diffusivity, m²/s or cm²/s, Eq. (17-4)

D_K

Knudsen diffusivity, m²/s or cm²/s, Eq. (17-51)

D_molecular

molecular diffusivity in free solution, m²/s or cm²/s

D_s

surface diffusivity, m²/s or cm²/s, Eq. (17-53)

erf

error function, Eq. (17-70) and Table 17-7

E_D

axial dispersion coefficient due to both eddy and molecular effects, m²/s or cm²/s

E_DT

thermal axial dispersion coefficient, m²/s or cm²/s

E_eff

effective axial dispersion coefficient, same units E_D, Eq. (17-68)

F

volumetric feed rate, e.g., m³/h, cm³/min, liter/h

h_p

particle heat transfer coefficient, J/(K s m²) or similar units

h_w

wall heat transfer coefficient, J/(K s m²) or similar units

HETP

height of equilibrium plate, cm/plate, Eq. (17-78b)

k_f

film mass transfer coefficient, m/s or cm/s

k_m,c

lumped parameter mass transfer coefficient with concentration driving force, m/s or cm/s, Eqs. (17-56a) and (17-57a)

k_m,q

lumped parameter mass transfer coefficient with amount adsorbed driving force, m/s or cm/s, Eqs. (17-56b) and (17-57b)

K_AB

mass action equilibrium constant for monovalent-monovalent ion exchange, dimensionless, Eq. (17-40a)

K_A,c

adsorption equilibrium constant in terms of concentration, units are (concentration)⁻¹

K′_i,c

linearized adsorption equilibrium constant in terms of concentration, units are units of q/c, Eq. (17-6b)

K_Ao

pre-exponential factor in Arrhenius Eq, (17-7a), same units as K_A

K_A,p

adsorption equilibrium constant in terms of partial pressure, units are (pressure)⁻¹

K′_A,p

linearized adsorption equilibrium constant in terms of partial pressure, units are units of q_A/p_A, Eq. (17-5b)

K_d

size exclusion parameter, dimensionless

K_DB

mass action equilibrium constant for divalent-monovalent ion exchange, same units as c_T/c_RT, Eq. (17-41)

K_DE

Donnan exclusion factor, dimensionless, Following Eq. (17-44)

L

length of packing in column, m or cm

L_MTZ

length of mass transfer zone, Figure 17-23, m or cm

M

molecular weight of solute, g/g mole or kg/kg mole

M_i

multipliers in Eqs. (17-29), dimensionless

N

equivalent number of plates in chromatography, Eq. (17-78)

N_Pe

Peclet number, dimensionless, Eq. (17-62)

p_A

partial pressure of species A, mm Hg, kPa, or other pressure units

p_h

high pressure, mm Hg, kPa, or other pressure units

p_L

low pressure, mm Hg, kPa, or other pressure units

Pe_L

Peclet number based on length, dimensionless, Eq. (17-78a)

q_A

amount of species A adsorbed, kg/kg adsorbent, gmoles/kg adsorbent, or kg/liter

q_A,max

maximum amount of species A that can adsorb, kg/kg adsorbent, gmoles/kg adsorbent, or kg/liter

amount adsorbed in equilibrium with feed concentration, same units as q_A average amount of species i adsorbed, kg/kg adsorbent, gmoles/kg adsorbent, or kg/liter

amount adsorbed that would be in equilibrium with fluid of concentration c_i, same units as q_A

Q

volumetric flow rate, m³/s, liter/min, etc.

r_p

pore radius, m or cm

R

resolution, dimensionless, Eq. (17-82)

R

gas constant (e.g., )

Re

Reynolds number, dimensionless, Eq. (17-60)

Sc

Schmidt number, dimensionless, Eq. (17-60)

Sh

Sherwood number, dimensionless, Eq. (17-60)

t

time, s, minutes or hours

t_br

breakthrough time, s, minutes or hours

t_center

time center of pattern exits column, s, min or hours, Eq. (17-85b)

t_elution

elution time, s, minutes or hours

t_F, t_feed

feed time, s, minutes or hours

t_MTZ

time of mass transfer zone, Figure 17-23, s, minutes or hours

t_R

retention time, s, minutes or hours

t_sw

switching time in SMB, s, minutes or hours

T

temperature,°C or K

T_amb

ambient temperature,°C or K

T_s

solid temperature,°C or K

u_ion,i

velocity of ion i, m/s or cm/s

u_s

average solute velocity, m/s or cm/s

average of solute velocities for A and B, cm/s, Eq. (17-83)

u_s,ion,i

diffuse wave velocity of ion i, m/s or cm/s

u_sh

shock wave velocity, m/s or cm/s

u_sh,ion,i

shock wave velocity of ion i, m/s or cm/s

u_th

thermal wave velocity, m/s or cm/s

u_{total_ion}

velocity of total ion wave, m/s or cm/s

v_A,product

interstitial velocity of A Product if it was in the column, m/s or cm/s = (A Product)/(ε_e A_c)

v_B,product

interstitial velocity of B Product if it was in the column, m/s or cm/s = (B Product)/(ε_e A_c)

v_D

interstitial velocity of desorbent if it was in the column, m/s or cm/s = D/(ε_e A_c)

v_Feed

interstitial velocity of feed if it was in the column, m/s or cm/s = F/(ε_e A_c)

v_inter

interstitial velocity, m/s or cm/s, Eq. (17-2b)

v_super

superficial velocity, m/s or cm/s, Eq. (17-2a)

V_available

volume available to molecule, m³, Eq. (17-1c)

V_column

column volume, m³

V_feed

volume feed gas, m³

V_fluid

volume available to fluid, m³, Eq. (17-1a)

V_purge

volume purge gas, m³

w_A, w_B

width of chromatographic peak, s, min or hours

W

weight of the column per length, kg/m

x

deviation from the location of the peak maximum, dimensionless Eq. (17-79)

x_l

deviation from peak maximum in length units, Eq. (17-80b)

x_t

deviation from peak maximum in time units, Eq. (17-80a)

x

weight or mole fraction solute in liquid, kg solute/kg liquid or kg mole solute/kg mole liquid, dimensionless

x_i

= c_i/c_T equivalent fraction of ion in solution, dimensionless

X_breakthrough (z,t)

general solution for column breakthrough for linear isotherms, same units as c, Eq. (17-72)

y

weight or mole fraction solute in gas, kg solute/kg gas or kg mole solute/kg mole gas, dimensionless

y_i

= c_Ri/c_RT equivalent fraction of ion on resin, dimensionless

Z

axial distance in column, m or cm. (Measured from closed end for PSA pressure change calculations)

β_strong

ratio velocities of strong and weak solutes, Eq. (17-27), dimensionless

Δc

change in solute concentration, same units as c

ΔH_ads

heat of adsorption, J/kg, cal/gmole, etc.

Δp_A

change in partial pressure, kPa, atm, etc.

Δq

change in amount adsorbed, kmole/kg adsorbent, kg/kg adsorbent, kmol/m³ or kg/ m³

Δt

change in time, s, min or hour

ΔT_f

change in fluid temperature,°C or K

Δz

increment of column length, m

γ

volumetric purge to feed ratio in PSA, dimensionless, Eq. (17-26)

ε_e

external porosity, dimensionless

ε_p

internal or pore porosity, dimensionless

ε_T

total porosity, dimensionless, Eq. (17-1b)

ρ_b

bulk density of adsorbent, kg/m³, Eq. (17-3b)

ρ_f

fluid density, kg/m³

molar density of fluid, kg mole/m³

ρ_p

particle density, kg/m³, Eq. (17-3a)

ρ_s

structural density of solid, kg/m³

σ

standard deviation of Gaussian chromatographic peak, Eq. (17-79)

σ_l

standard deviation in length units, m or cm, Eq. (17-80b)

σ_t

standard deviation in time units, min or s, Eq. (17-80a)

τ

tortuosity, dimensionless, Eq. (17-4)

ζ

Greek letter zeta used as dummy variable in Eq. (17-70)