The viscosity of a fluid is its resistance to shear or flow, and is a measure of the fluids adhesive/cohesive or frictional properties. This arises because of the internal molecular friction within the fluid producing the frictional drag effect. There are two related measures of fluid viscosity which are known as dynamic and kinematic viscosity. Dynamic viscosity is also termed "absolute viscosity" and is the tangential force per unit area required to move one horizontal plane with respect to the other at unit velocity when maintained a unit distance apart by the fluid. Centipoise (CPS) Millipascal(mPas) Poise (P) Centistokes (cSt) Stokes (S) SayboltSeconds Universal(SSU) 1 0.01 1 0.01 31 2 0.02 2 0.02 34 4 0.04 4 0.04 38 7 0.07 7 0.07 47 10 0.1 10 0.1 60 15 0.15 15 0.15 80 20 0.2 20 0.2 100 25 0.24 25 0.24 130 30 0.3 30 0.3 160 40 0.4 40 0.4 210 50 0.5 50 0.5 260 60 0.6 60 0.6 320 70 0.7 70 0.7 370 80 0.8 80 0.8 430 90 0.9 90 0.9 480
Transcript
8/8/2019 The Viscosity of a Fluid is Its Resistance to Shear or Flow
The viscosity of a fluid is its resistance to shear or flow, and is a measure of the fluidsadhesive/cohesive or frictional properties. This arises because of the internal molecular friction withinthe fluid producing the frictional drag effect. There are two related measures of fluid viscosity which areknown as dynamic and kinematic viscosity.
Dynamic viscosity is also termed "absolute viscosity" and is the tangential force per unit arearequired to move one horizontal plane with respect to the other at unit velocity when maintained a unit
distance apart by the fluid.
Centipoise(CPS)
Millipascal (mPas)
Poise(P)
Centistokes
(cSt)
Stokes(S)
Saybolt SecondsUniversal
(SSU)
1 0.01 1 0.01 31
2 0.02 2 0.02 34
4 0.04 4 0.04 38
7 0.07 7 0.07 47
10 0.1 10 0.1 60
15 0.15 15 0.15 80
20 0.2 20 0.2 100
25 0.24 25 0.24 130
30 0.3 30 0.3 160
40 0.4 40 0.4 210
50 0.5 50 0.5 260
60 0.6 60 0.6 320
70 0.7 70 0.7 370
80 0.8 80 0.8 430
90 0.9 90 0.9 480
8/8/2019 The Viscosity of a Fluid is Its Resistance to Shear or Flow
pH can be viewed as an abbreviation for power of Hydrogen - or more completely, power of theconcentration of the Hydrogen ion.
The mathematical definition of pH is a bit less intuitive but in general more useful. It says that the pH isequal to to the negative logarithmic value of the Hydrogen ion (H+) concentration, or
pH = -log [H +
]
pH can alternatively be defined mathematically as the negative logarithmic value of theHydroxonium ion (H3O
+) concentration. Using the Bronsted-Lowry approach
pH = -log [H 3O+ ]
pH values are calculated in powers of 10. The hydrogen ion concentration of a solution with pH 1.0 is10 times larger than the hydrogen concentration in a solution with pH 2.0. The larger the hydrogen ionconcentration, the smaller the pH.
• when the pH is above 7 the solution is basic (alkaline)
• when the pH is below 7 the solution is acidic
In pure neutral water the concentration of hydrogen and hydroxide ions are both 10-7 equivalents per liter.
pHIon Concentration(gram equivalent
per liter)Type of Solution
0 1.0
Acid Solution- Hydrogen ions -
H+
1 0.1
2 0.01
3 0.001
4 0.0001
5 0.00001
6 0.000001
7 0.0000001 Neutral Solution
8 0.000001 Basic (alkaline)Solution
- Hydroxide ions -
9 0.00001
8/8/2019 The Viscosity of a Fluid is Its Resistance to Shear or Flow
• Combustion Efficiency - indicates a burners ability to burn fuel measured by unburned fuel
and excess air in the exhaust• Thermal Efficiency - indicates the heat exchangers effectiveness to transfer heat from thecombustion process to the water or steam in the boiler, exclusive radiation and convection losses
• Fuel to Fluid Efficiency - indicates the overall efficiency of the boiler inclusive thermalefficiency of the heat exchanger, radiation and convection losses - output divided by input.
Boiler Efficiency is in general indicated by either Thermal Efficiency or Fuel to Fluid Efficiencydepending the context.
Boiler Efficiency
Boiler Efficiency related to the boilers energy output to the boilers energy input can be expressed as:
Boiler efficiency (%) = heat exported by the fluid (water, steam ..) / heat provided by the fuel x 100 (1)
Heat Exported from the Boiler to the Fluid
If a fluid like water is used to export heat from the boiler, exported heat can be expressed as:
q = ( m / t ) c p dT (2)
where
8/8/2019 The Viscosity of a Fluid is Its Resistance to Shear or Flow
dT = temperature difference between inlet and outlet of the boiler ( oC)
For a steam boiler the heat exported as evaporated water at the saturation temperature can beexpressed as:
q = ( m / t ) he (3)
where
m = mass flow of evaporated water (kg)
t = time (s)
he = evaporation energy in the steam at the saturation pressure the boiler is running (kJ/kg)
Heat Provided by Fuel
The energy provided by fuel may be expressed in two ways 'Gross' or 'Net' Calorific Value.
Gross Calorific Value
This is the theoretical total of the energy in the fuel. The gross calorific value of the fuel includes the
energy used for evaporating the water in the combustion process. The flue gases from boilers are ingeneral not condensed. The actual amount of heat available to the boiler plant is therefore reduced.
• Gross Calorific Values of some common Fuels
An accurate control of the air supply is essential to the boilers efficiency.
• to much air cools the furnace and carries away useful heat
• too little air and the combustion will be incomplete. Unburned fuel will be carried over andsmoke produced
Net calorific value
This is the calorific value of the fuel, excluding the energy in the water vapor discharged to the stackin the combustion process. The combustion process can be expressed as:
[C + H (fuel)] + [O2 + N 2 (Air)] -> (Combustion Process) -> [CO2 + H 2 O + N 2 (Heat)]
Remember, always check local authorities before final design.
Stoichiometric or Theoretical Combustion is the ideal combustion process where fuel is burnedcompletely.
A complete combustion is a process burning all the carbon (C) to (CO2), all the hydrogen (H) to (H2O)and all the sulphur (S) to (SO2).
With unburned components in the exhaust gas, such as C, H2, CO, the combustion process isuncompleted and not stoichiometric .
The combustion process can be expressed as:
[C + H (fuel)] + [O2 + N 2 (Air)] -> (Combustion Process) -> [CO2 + H 2 O + N 2 (Heat)]
where
C = Carbon
H = Hydrogen
O = Oxygen
N = Nitrogen
To determine the excess air or excess fuel for a combustion system we starts with the stoichiometricair-fuel ratio. The stoichiometric ratio is the perfect ideal fuel ratio where the chemical mixingproportion is correct. When burned all fuel and air is consumed without any excess left over.
Process heating equipment are rarely run that way. "On-ratio" combustion used in boilers and hightemperature process furnaces usually incorporates a modest amount of excess air - about 10 to 20%more than what is needed to burn the fuel completely.
If an insufficient amount of air is supplied to the burner, unburned fuel, soot, smoke, and carbonmonoxide exhausts from the boiler - resulting in heat transfer surface fouling, pollution, lower combustion efficiency, flame instability and a potential for explosion.
8/8/2019 The Viscosity of a Fluid is Its Resistance to Shear or Flow
To avoid inefficient and unsafe conditions boilers normally operate at an excess air level. This excessair level also provides protection from insufficient oxygen conditions caused by variations in fuelcomposition and "operating slops" in the fuel-air control system. Typical values of excess air areindicated for various fuels in the table below.
• if air content is higher than the stoichiometric ratio - the mixture is said to be fuel-lean
•
if air content is less than the stoichiometric ratio - the mixture is fuel-rich
• Excess air of different fuels
Example - Stoichiometric Combustion of Methane - CH4
The most common oxidizer is air . The chemical equation for stoichiometric combustion of methane -CH4 - with air can be expressed as
CH 4 + 2(O2 + 3.76N 2 ) -> CO2 + 2H 2 O + 7.52N 2
If more air is supplied some of the air will not be involved in the reaction. The additional air is termedexcess air , but the term theoretical air may also be used. 200% theoretical air is 100% excess air.
The chemical equation for methane burned with 25% excess air can be expressed as
CH 4 + 1.25 x 2(O2 + 3.76 N 2 ) -> CO2 + 2H 2 O + 0.5O2 + 9.4N 2
Excess Air and O2 and CO2 in Flue Gas
Aproximate values for CO2 and O2 in the flue gas as result of excess air are estimated in the tablebelow:
Excess Air %
Carbon Dioxide - CO2 - in Flue Gas (% volume)Oxygen inFlue Gas
Note! Failure to use an adequate size may result in fire. Always secure a wire with a fuse.
• 1 ft (foot) = 0.3048 m
Wire Gauge Design Procedure
1. calculate the total length of the wire from the source to the device and back again2. determine the amount of current in the wire3. correct wire gauge is in the intersection of amps and feet
Note! The wire size is required for a 3% voltage drop in 12 Volt circuits. Oversize the wire if thevoltage drop is critical
8/8/2019 The Viscosity of a Fluid is Its Resistance to Shear or Flow
American Wire Gauge (AWG) is a U.S. standard set of wire conductor sizes. The "gauge" is related tothe diameter of the wire.
The AWG standard includes copper, aluminum and other wire materials. Typical household copper wiring is AWG number 12 or 14. Telephone wire is usually 22, 24, or 26. The higher the gaugenumber, the smaller the diameter and the thinner the wire.
The table below can be used to convert American Wire Gauge (AWG) to square mm cross sectionalarea.
If undersized wire is used between the motor and the power source, the starting and load carrying capabilities of the motor will be limited.
There are three primary categories of steam traps:
• mechanical
• thermostatic
• thermodynamic
Popular traps in these categories includes the inverted bucket steam trap, the float steam trap, thethermostatic steam trap and the thermodynamic disc steam trap.Which one is preferred depends onthe application.
A steam trap prime missions is to remove condensate and air preventing escape of live steam from thedistribution system. The steam trap must adapt to the application. A disc thermodynamic steam trapshould never be used together with a modulating heat exchanger - and a floating ball steam trap isoverkill for draining steam pipes.
The table below can be used as a short guide for the selection of steam traps:
Type of SteamTrap
OperationNormalFailureModeNo or little
loadLight Load
NormalLoad
Heavy Load
8/8/2019 The Viscosity of a Fluid is Its Resistance to Shear or Flow
Dribble May dribble Intermittent Continuous Variable
BimetalThermostatic
No ActionUsuallyDribbleAction
May blast athigh
pressuresContinuous Open
ImpulseSmall
Dribble
Usuallycontinuouswith blast athigh loads
Usuallycontinuouswith blast athigh loads
Continuous Open
ThermodynamicDisc
No Action Intermittent Intermittent Continuous Open
Inverted Bucket Steam Trap
The inverted bucket is the most reliable steam trap operating principle known. The heart of its simpledesign is a unique leverage system that multiplies the force provided by the bucket to open the valveagainst pressure. Since the bucket is open at the bottom, it resists damage from water hammers, andwearing points are heavily reinforced for long life.
• intermittent operation - condensate drainage is continuous, discharge is intermittent
• small dribble at no load, intermittent at light and normal load, continuous at full load
• excellent energy conservation
• excellent resistance to wear
• excellent corrosion resistance
• excellent resistance to hydraulic shocks
• vents air and CO2 at steam temperature
• poor ability to vent air at very low pressure
• fair ability to handle start up air loads
• excellent operation against back pressure
• good resistance to damage from freezing
• excellent ability to purge system
• excellent performance on very light loads
• immediate responsiveness to slugs of condensate
•excellent ability to handle dirt
• large comparative physical size
• fair ability to handle flash steam
• open at mechanical failure
Thermostatic Steam Traps
8/8/2019 The Viscosity of a Fluid is Its Resistance to Shear or Flow
There are two basic designs for the thermostatic steam trap, a bimetallic and a balanced pressuredesign. Both designs use the difference in temperature between live steam and condensate or air tocontrol the release of condensate and air from the steam line.
In an thermostatic bimetallic trap it is common that an oil filled element expands when heated to closea valve against a seat. It may be possible to adjust the discharge temperature of the trap - oftenbetween 60oC and 100oC.
This makes the thermostatic trap suited to get rid of large quantities of air and cold condensate at thestart-up condition. On the other hand the thermostatic trap will have problems to adapt to the variationscommon in modulating heat exchangers.
• intermittent operation
• fair energy conservation
• fair resistance to wear
• good corrosion resistance
• poor resistance to hydraulic shocks (good for bimetal traps)
• do not vent air and CO2 at steam temperature
• good ability to vent air at very low pressure
• excellent ability to handle start up air loads• excellent operation against back pressure
• good resistance to damage from freezing
• good ability to purge system
• excellent performance on very light loads
• delayed responsiveness to slugs of condensate
• fair ability to handle dirt
• small comparative physical size
• poor ability to handle flash steam
• open or closed at mechanical failure depending of the construction
Float Steam Traps
In the float steam trap a valve is connected to a float in such a way that a valve opens when the floatrises.
The float steam trap adapts very well to varying conditions as is the best choice for modulating heatexchangers, but the float steam trap is relatively expensive and not very robust against water hammers.
• continuous operation but may cycle at high pressures
• no action at no load, continuous at full load
• good energy conservation
• good resistance to wear
• good corrosion resistance
• poor resistance to hydraulic shocks
• do not vent air and CO2 at steam temperature
• excellent ability to vent air at very low pressure
• excellent ability to handle start up air loads
• excellent operation against back pressure
• poor resistance to damage from freezing
• fair ability to purge system
• excellent performance on very light loads
• immediate responsiveness to slugs of condensate
8/8/2019 The Viscosity of a Fluid is Its Resistance to Shear or Flow
The thermodynamic trap is an robust steam trap with simple operation. The trap operates by means of the dynamic effect of flash steam as it passes through the trap.
• intermittent operation
• poor energy conservation
• poor resistance to wear
• excellent corrosion resistance
• excellent resistance to hydraulic shocks
• do not vent air and CO2 at steam temperature
• not recommended at low pressure operations
• poor ability to handle start up air loads
• poor operation against back pressure• good resistance to damage from freezing
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