May 28, 2009

Typical Heat Transfer Coefficient - Values

Some typical heat transfer coefficients for different systems.

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May 20, 2009

Assessing ESP Performance

Electrostatic precipitators are among the most effective pollution control devices for removing particles from gases. They are used in chemical process plants including thermal power plants, cement plants and pulp & paper industries.

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These devices called ESP’s, enjoy a relatively low payback period – often 3 - 5 years – especially those that are able to recover valuable solids for sale / reuse.

Calculating collection efficiency:

ESP’s are used to treat particulate laden gases at high flow rate alone or together with Bag house Filters, Cyclones, Hydro-cyclones and Wet scrubbers and have following advantages.

  • Low pressure drop (about 2.5 cm H2O compared to 5-10 cm H2O for other methods)

  • High removal efficiency (> 97+%) even for flows with particles < 2m.

  • Handling gases with high moisture content (even 30%).

  • Easy separation and recovery of collected material when it is reused.

During ESP operation, first the airborne particles are electrically charged. The charged particles then migrate to the collector (negatively changed electrode) where they are neutralized and collected in a hopper. The high voltage system works at 25-100 KV, and a corona type discharge occurs at the negatively charged electrode.

The theoretical efficiency of an ESP can be determined by equation-1, known as Deutsch equation given as under:

Eff = 1- exp(- 2VL/RU) (1)

Where L = collector length, m
V = Particle velocity towards the electrode, usually 0.03-0.21m/s
R = Collector radius, m
U = Net gas velocity, m/s

Collector area : A = 2 Pi RL - (1a)
Volumetric flow rate Q = U R^2 - (1b)

Substituting 1a & 1b in eqaution (1), it can be re-written as:

Eff = 1- exp(- AVL/Q) - (2)

Higher collection efficiencies can be obtained by increasing the throughput velocity (V), collector area (A) or by reducing the gas flow rate (Q). However, increasing collector area is not always feasible.

To increase efficiency from 90 to 99%, the area needs to be doubled; while from 90 – 99.9%, three times area corresponding to 90% efficiency, is required.

Increase in flow rate will increase the particle loading at the outlet. In a typical ESP, reduction in the collection efficiency from 99 to 97% will triple the particle loading in the exhaust.

The Deutsch equation was later modified (equation 3) and now widely used to calculate collection efficiency with varying operating parameters.

Eff = 1- exp(- AVK/Q) - (3)

Where VK is the effective migration velocity and given by equation (4).

VK = v ln (1/1-Eff) … (4)

Where v is the effective migration velocity of particles areas across the inter electrode space, as computed by equation (5). Accordingly to electrostatic field theory,

v = [PDE^2 (1 + J L/D)]/ 36x10^7 Pi Mu (5)

Where D = Particle diameter, m
E = Electrostatic force field, V/m
J = Average free distance run by the gas molecules, as given by equation (6), m
L = Length between electrodes, cm
Mu= Absolute gas viscosity, Poise
P = A unit less parameters, given by equation (7), where C is the di-electric constant, coulombs/gm
J = 1.764 + 0.562 e-0.785 D/L - (6)
P = 3C/(C+2) - (7)

Equation (5), used for calculating effective migration velocity for particles, is based on three assumptions:
  • Particles rapidly reach to their final velocity while moving towards the negatively charged electrode.

  • For particles with a diameter similar to, or lower than, the average free distance run by gas molecules, the hydrodynamic buoyancy can be calculated using ‘Stokes law’ with ‘Cunnighum Correction’ factor.

  • The accumulated electrostatic charge nearly instantaneously reaches its maximum, limit value.

Theoretical determination of ESP efficiency though straight forward, but involves theoretical formulations for variables like V, VK and J, which is often difficult to get in industrial situations.

“Practical” efficiency can be determined by equation (8) involving volumetric flow rate and particulate content, which can be determined using an Isokinetic method.

Eff = (Cpi.Qi – Cpo.Qo)/ Cpi.Qi - (8)

Where Cp = Particulate content of the gas, mg/m3
Q = Volumetric flow rate, m3/hr
i = inlet
o = outlet.

Isokinetism correlates gas velocity with sampling velocity. 100% isokinetism means gas velocity equals sampling velocity. For practical purpose (stack sampling), 90-110% isokinetism gives fairly good value.

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May 12, 2009

Viscosity of Pure Gases at High Pressure

In the previous posts, I discussed about methods called Chapman & Enskog and also Yoon & Thodos. Both were applicable for finding out viscosities of pure gases at low pressures up to 5 atm. What about the viscosity at high pressure because gas properties change significantly with pressure and behaviour becomes more & more real deviating from ideal gas condition.

So in this part I will discuss the methods which are apllicable for gases at high pressure.

In such cases, mostly correlations are linked with reduced density i.e. based on the standard definition of reduced property is density at given condition / density at critical point.

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Also, you must remember that mostly the properties at high pressure are reported in terms of residual properties which is either consider as the differential value of property at given pressure and atmospheric pressure (vis1 - vis0) or as their ratio i.e. vis1/vis0. So in both the cases you must know vis0 at atm pressure, which you can find out in my previous articles Viscosities of Pure Gases at low pressure - Part-I and, Viscosities of Pure Gases at low pressure - Part-II.

The method is called Jossi, Stiel & Thodos method.

So for Non Polar Gas the equation is -

[(Vis1 - vis0)*zeta +1]^0.25 = 1.0230 + 0.23364 Dr + 0.58533 Dr^2 - 0.40758 Dr^3 + 0.0933 Dr^4

Dr = Reduced density with its usual definition i.e Dr = D / Dc
vis0 = already given in previous post as viscosity at atm pressure.
zeta is given by the following equation.

zeta = [Tc ^ (1/6)]/ [ M ^ 0.5 * Pc ^ (2/3) ]

Its valid for 0.1 < Dr < 3.0

Now for Polar Gases there are three parts depending on Dr value. The equations from Stiel & Thodos are -

Dr <= 0.1

(Vis - vis0) x zeta = 1.656 Dr^ 1.111

0.1< Dr <= 0.9

(vis - vis0) x zeta = 0.0607 * (9.045 Dr + 0.63)^ 1.739

0.9< Dr <= 2.6

log [4 - log {mod (vis-vis0)*zeta}] = 0.6439 - 0.1005 Dr - Delta

Where Delta is as below

Delta = 0 if 0.9 < Dr < 2.2
Delta = 0.000475 (Dr^3 -10.65)^2 if 2.2 < Dr < 2.6

Yes again the unit of viscosity here is micropoise.

List of other property estimation methods on this Blog.

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May 04, 2009

LED - Energy Saving Lighting Option

LEDs are growing in the current market scenario due to their higher efficiencies, their widely versatile uses & applications and due to the current technological advancements they are now available as source for lighting media also.

Due to their various applications in the industry & now even in day to day life I thought to write some thing on this topic for the benefit of all.
Currently we have three major options for lighting purpose.

1. Incandescent Lamps
2. Fluorescent Lamps
3. Light Emitting Diodes (LED)

Incandescent lamps or old conventional light bulbs produce light by converting electric energy into heat energy thereby generating illumination of visible range. They are very easy to produce but the disadvantage is that they are very less efficient in terms of energy consumption which is generally measures as illumination / watt OR Lumens/watt or Lux.

Fluorescent lamps on the other hand work on UV light which is generated by passing current through mercury vapors. The ultraviolet light is then absorbed by a phosphor coating inside the lamp, causing it to glow, or fluoresce. They are generally costlier by 5 to 10 times compared to incandescent lamps. Their efficiency is still poor & they also involve the risk of some mercury vapors (generally very less).

LEDs work on semiconductor technology & are much more efficient than any other form. LEDs produce more light per watt compared to any other lamp or bulb. They can emit light of any type of desired color without the use of color filters that traditional lighting methods require. This is more efficient and can lower initial costs.

Advantages of LEDs
  • LEDs can be very small.

  • LEDs are Ideal for use where frequent on-off cycling is required. Fluorescent lamps burn out much more quickly in this condition.

  • LEDs can very easily be dimmed. This is a very good property of LEDs where most of other lights may fail.

  • In contrast to most light sources, LEDs emit very less heat which may be damaging to the surroundings due to its IR nature.

  • LEDs life is more than 2 to 3 times compared to fluorescent tubes while it may be 5 times or greater compared to incandescent lamps.

  • LEDs do not contain mercury or any other harmful vapors which are unfriendly to the environment.

  • LEDs are now available in the size of more than 12W or even up to 20W.

Applications of LEDs
Decorative lights on interiors, exteriors, parks, walls etc., Night lights, walkway, stair lighting, ceiling lights, porch and landscaping lights.

Car bulbs as LEDs are now available in high power of more than 5W, may be 12W and as high as 20W also sometimes.

Underbody car LED kits come in a range of exciting colors and forms. These kits are equipped with remote controls and are capable of operating in different modes, for example flashing, strobing, chasing, phasing etc. Modern high quality underbody kits also come with sound systems and they can be set to 12 different functions. Multi-color strobe lights for cars are liked by car lovers across the globe because they look amazing. These are the latest advancement in the car parts industry and have been predicted to replace ordinary car kits in the future. They come with high-intensity, wide angle LED’s housed in strong and long-lasting tubes.

LED lighting for cars is not limited to underbody car kits. Exterior car lights are also quite popular and well-liked. These lights include bumper guard lights, third-brake lights, running lights, and multi-purpose light tube strips. In addition to these, popular manufacturers are also providing position and side marker lights for cars. All these lights are quite inexpensive and can be installed easily. High power LED modules come in various colors including cool and warm white, blue, green, red and yellow. LED multiplexing is the future of LED lighting in cars which will produce brighter, more efficient and more flexible lights.

LED light bulbs are used for a variety of purposes in homes. Recessed ceiling lights can be used for lighting up a hallway or passage. These bulbs come in three shapes: MR (Multi Reflector), Universal, and PAR type. MR and PAR types are normally used for focused or directional lighting. They are ideal to be used in night or reading lamps. Universal LED bulbs, on the other hand, can be used for general lighting purposes. These bulbs replace ordinary incandescent bulbs and range in wattage from 3.3 Watt to 15 Watt. Most of these lights are made waterproof and come in various levels of brightness.

Linear LED lights used in homes are available as light bars, rope lights, net & string lights, track lights, and signage & back lights. These types of lights can light up an entire cabinet or shelf. They are known as linear because they consist of a number of small LED’s aligned in a case. Rope lights are perfect for parties and outdoor decoration. They come in a variety of colors and look amazing stretched out on the floor. The latest rope lights come in durable glass tubing and simple electrical connections. They are very flexible and quite easy to install.

Architectural and track lights can also be used to decorate and light up the interior of an office. These lights produce enough illumination for all kinds of operations. Doorways, closets and lobbies can be lit up by using signage lights. These lights are available in the form of daisy chains and are very high power. Sign boards can be effectively constructed by using these lights which are totally waterproof and robust. LED architectural lights are available as street lights, wall washers, path lights and panel lights. LED street lights save money and energy and are very reliable.

Wall washers are extensively used in buildings and homes to illuminate an entire wall. They look beautiful from a distance and can be used to make a building stand out from the rest. The best thing about these lights is that they are not very expensive. The intensity of these lights can be controlled through a remote control along with their color and speed.

Deco, underground, lawn, ceiling, lamp, pool and many other types of LED lighting are admired by people all over the world. These lights are not only affordable but also last for many years. LED’s scatter heat quickly and therefore do not heap up like incandescent bulbs. They can be fitted anywhere because of their small size and adjustable design. Lawn lights look really pretty and they come in various colors including white, green, red and yellow.

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