December 25, 2007

How much you know your Boilers or Steam Generation System?

I am starting this open discussion for the benefit of all the readers / visitors of this site. Many students & even process engineers do not fully understand the concept of efficiency calculation methods especially for steam system and in particular steam generation system.

So let me ask you One simple question.....Will the boiler efficiency increase if I increase combustion air temperature?

If your answer is Yes then probably you need to read & discuss the entire issue again.

If your answer is No then again you need to define the terms & explain them.

So I am not putting much here I am leaving it open ended for discussions among all of you...........& will come back later on this topic.

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December 21, 2007

Turbines for Saturated Steam??? - More Energy - More Savings

Traditionally, the cogeneration of steam and electricity has been restricted to plants that generate superheated steam, making the recovery of energy losses from saturated steam impossible for many industrial sites ranging from distilleries to pharmaceutical plants and pulp-and-paper mills.

Pennat International Corp promised to change that with the introduction of its new Energy Conversion System (ECS) Series of saturated steam turbines.

Yes Steam turbines which operate with saturated steam as motive fluid.

The turbines, which can also function with superheated steam, generate incidental electrical power while regulating and maintaining steam pressure like a conventional pressure-reduction station. They are intended to be installed in parallel with the existing pressure-reducing valve (See Diagram below) or pressure-reducing desuperheater station and process-control software tasked with managing the pressure consistency of reduced-pressure process streams.

With steam typically produced at significantly higher temperatures and pressures than needed for its intended process application, the turbines promise greater resource efficiency, reduced energy consumption and associated environmental benefits.

For the past eight years, Pennant has installed specially commissioned versions of its saturated-steam turbines for facilities in India, Dubai and elsewhere. The costs of system and installation vary based on steam throughput, but can be around $400,000for a 77,000-lb/h saturated steam flow and $250,000 for a 22,000-lb/h flow.

However, with power savings of $325,000/ yr and $135,000/yr, respectively, Pennant estimates payback periods of less than two years (assuming in both cases an operation time of 8,000 h/yr and an energy cost of 6.33 ¢/kWh). The ECS Series is a slightly more modular concept based on the needs of the typical industries that would use them and function within a pressure range of 50–700 psi. Units will be ready to ship within months.

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December 15, 2007

HVAC - Quick Calculation of refrigeration load for Rooms

A building or room gains heat from many sources. Inside occupants, computers, copiers, machinery, and lighting all produce heat. Warm air from outside enters through open doors and windows, or as ‘leakage’ though the structure. However the biggest source of heat is solar radiation from the sun, beating down on the roof and walls, and pouring through the windows, heating internal surfaces.

The sum of all these heat sources is know as the heat gain (or heat load) of the building, and is expressed either in BTU (British Thermal Units) or kW (Kilowatts).

For an air conditioner to cool a room or building its output must be greater than the heat gain. It is important before purchasing an air conditioner that a heat load calculation is performed to ensure it is big enough for the intended application.

Quick calculation for offices
For offices with average insulation and lighting, 2/3 occupants and 3/4 personal computers and a photocopier, the following calculations will suffice:

Heat load (BTU) = Length (ft.) x Width (ft.) x Height (ft.) x 4

Heat load (BTU) = Length (m) x Width (m) x Height (m) x 141

For every additional occupant add 500 BTU.

If there are any additional significant sources of heat, for instance floor to ceiling south facing windows, or equipment that produces lots of heat, the above method will underestimate the heat load. In which case the following method should be used instead.

A more accurate heat load calculation for any type of room or building

The heat gain of a room or building depends on:

  1. The size of the area being cooled.

  2. Sze and position of windows, and whether they have shading./li>
  3. number of occupants.

  4. generated by equipment and machinery.

  5. generated by lighting .

By calculating the heat gain from each individual item and adding them together, an accurate heat load figure can be determined.

Step One

Calculate the area in square feet of the space to be cooled, and multiply by 31.25

Area BTU = length (ft.) x width (ft.) x 31.25

Step Two

Calculate the heat gain through the windows. If the windows don’t have shading multiply the result by 1.4.

North window BTU = Area of North facing windows (m. sq.) x 164

If no shading, North window BTU = North window BTU x 1.4

South window BTU = Area of South facing windows (m. sq.) x 868

If no shading, South window BTU = South window BTU x 1.4
Add the results together.

Total window BTU = North window + South window

Step Three
Calculate the heat generated by occupants, allow 600 BTU per person.

Occupant BTU = number of people x 600

Step Four

Calculate the heat generated by each item of machinery - copiers, computers, ovens etc. Find the power in watts for each item, add them together and multiply by 3.4

Equipment BTU = total equipment watts x 3.4

Step Five

Calculate the heat generated by lighting. Find the total wattage for all lighting and multiply by 4.25

Lighting BTU = total lighting watts x 4.25

Step Six
Add the above together to find the total heat load.

Total heat load BTU = Area BTU + Total Window BTU + Occupant BTU + Equipment BTU + Lighting BTU

Step Seven

Divide the heat load by the cooling capacity of the air conditioning unit in BTU, to determine how many air conditioners are needed.

Number of a/c units required = Total heat load BTU / Cooling capacity BTU

You can download a demo version of HVAC calculation software here.
Download ComfortAir HVAC Software

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December 08, 2007

Never Ignore Pump Bypasses?

Many of you, especially who are handling projects engineering, may be surprised when & why should I consider bypass for pumps. What is the utility for considering bypasses? Why not only relief valves are sufficient for safeguard against high pressure in the line? The list may be little longer than my expectation…. Here are few things to remember….

Here is the list of few important considerations for Bypasses.
  1. For all Positive displacement type pumps (PD pumps) full size bypasses are always essential in the form of safety relief valves back to the suction line or suction tank.

  2. All the equipments or instruments before the bypasses in item-1 above should be designed up to 1.5 times of the set value of relief valve.

  3. To prevent excessive temperature rise at prolonged low flow operation compared to design value, a bypass is necessary for

    a. High discharge head pumps due to higher rate of temperature rise at low flow and,

    b. Liquids operating near saturation temperature. Normally, BFW (boiler feed water) pumps are equipped with automatic regulators for bypass flow due to availability of both these factors in one pump.

    c. Usually 10% of normal flow is sufficient in such cases as bypass flow.

  4. To avoid unstable operation below certain point in case of some pumps it is necessary to provide bypass. This condition arises out due to internal re-circulation which may result in vortex cavitations, low frequency pulsation and, vibrations. This normally occurs if Ns (Suction specific speed) is very high at 10000 or more.

  5. Automatic bypass is necessary for pumps, which are controlling level or temperature instead of flow. In such cases variations are usually large & prolonged and it may have to run at shut-off also due to process disturbances or during start-up / shutdown.

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December 05, 2007

How to Calculate Viscosity of Liquid Mixture?

Have you ever faced a problem of calculating the Viscosity of a Muixture?
Most of us feel that there is no approx formula for calculating Viscosity of mixture of liquids.

Now here is a useful co-relation. Thanks to Milt, It is his effort which can be very useful for many of us.

Calculating the viscosity of a blended liquid consisting of two or more liquids having different viscosities is a three step procedure. The first step involves calculation of the Viscosity Blending Index (VBI) of each component of the blend using the following equation (known as a Refutas equation):

(1) VBI = 14.534 × ln[ln(v + 0.8)] + 10.975

where v is the viscosity in centistokes and ln is the natural logarithm (Loge).

The second step involves using this blending equation:

(2) VBI-blend = [wA × VBIA] + [wB × VBIB] + ... + [wX × VBIX]

where w is the weight fraction (i.e., % ÷ 100) of each component of the blend. In using the above blending equation, it is necessary that all viscosities are determined at the same temperature, for example, 100 °C.

The third and final step is to determine the viscosity of the blend by using the invert of equation (1):

(3) v = (ee(VBI - 10.975) ÷ 14.534) − 0.8

where VBI is the Viscosity Blending Index of the blend and e is the transcendental number 2.71828, also known as Euler's number.

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