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Boiler Design Life

Most of the cases.. Custumer ask for expected boiler life & we tend to say approx 10 years..

How one arrive at this figure? I’ve asked this question so many time to may expert inthis field but not able to get any satisfactory answer.

Recently i went thru ASME VIII Division1 , where the Fatigue requirements are discussed.

If the boiler pressure variation is less than 20% then we need not to consider those variation in fluctuating load. Only boiler cold stat-up & shut down need to be considred while calculating the fluctuating cycle.

i.e for 12 bar(g) design pressure boiler, the maximum operating pressure is 90% of design pressure i,e 10.8 bar(g) 20% of pressure variation means , pressure below 8.6 bar(g) will be counted for fluctuating load.

also delta temperature is important while calculating the fluctuation temperature.
for Delta of 200 (Star-up condition) the equivalent cycle is 4 per star-up & for
delta of 100 (Shut-down condition) the equivalent cycle is 1 per shut down, i.e if boiler is closing 15 time per month, it 12 per year, the cycle it will go thru is (4+1) x 15 x 12 = 900 cycles.

Approx, 10000 cycle boiler can take by design, i.e boiler design life will be = 10000/900 = 11.11 year approx.

Convection Continuied..

OK..
Now the big question.. the flow parameter Re & thermal parameter Pr, how they connects to HEAT TRANSFER???

So for our help.. Mr. Nusselt has created Nusselt Number!!

The traditional dimensionless form Nusselt number Nu, which may be defined as the ratio of convection heat transfer to fluid conduction heat transfer under the same conditions.

Nusselt number Nu can be calculated as

Nu = X . Re^y.Pr^z

Where X , y, z parameters are depends upon geometry, nature of flow, flow regime etc

Nusselt Number can also calculated as

So equating these two equation.. one can find heat transfer co-efficient on fluid side.

Hope you have understood the above co-relations..

Heat Transfer : CONVECTION At Glance

Lets See.. “CONVECTION HEAT TRANSFER”
Heat transfer occur when there is Delta T Across the two bodies.. heat always flow from hot side to cold side. Heat transfer may occur under natural draft or under forced circulation.

Industrial heat transfer equipment are mostly occur under forced condition.

As discussed above, one can conclude that the flow characteristics of the fluid on any side will pay an important role!!

We may recall our old books and some jargon like ‘ boundary layer’, ‘Lamina Flow’ , ‘Turbulent Flow’ etc.

Lay-Mann says: As we break the boundary layer.. more new fluid molecule come in contact with the surface & more heat they will carry.. So in Laminar flow the heat transfer will be less as fluid wall touching the heating surface is not leaving the place & core will remain unaffected! while in turbulent flow the boundary wall will brake & better heat transfer will occur.

This ‘Turbulent’ thing can be identified scientifically using Reynolds analogy . Same is represented with Reynold’s Number.

Reynold’s number can be calculated as:

While there is one more parameter which plays an important role in heat transfer which is ‘Prandtl number’, which shows how heat will diffuse in fluid during convectio heat transfer..

Prandtl Number can be calculates as

Where v is Kinematic Viscosity & k is thermal conductivity of fluid

As you have noticed, Reynold’s number is depends on carrier geometry & fluid properties, while Prandle number is depends on fluid property only.

Now how to use these number to evaluate heat transfer??

Wait for my next Scrap..

About Shell & Tube heat Exchangers

The discussion is IS 4503 Oriented.
IS 4503 is the code for heat exchanger ..

Type of heat exchanger

1. Fixed Tube Plate
2. U Tube
3. Floating head

Classification on Pressure

1. 2.5 kg/cm²(g)
2. 6.3 kg/cm²(g)
3. 10 kg/cm²(g)
4. 16 kg/cm²(g)
5. 25 kg/cm²(g)
6. 40 kg/cm²(g)

Classification on Temperature
Max. Allowable Metal Temperature

• Carbon Steel : 250ºC
• SS : 120ºC
• Non-ferrous : 65ºC

Max. Fluid Temperature

• Carbon Steel : 540ºC
• SS : 590ºC
• Non-ferrous : 200ºC

Corrosion allowance : 3mm minimum

Tube Pitch : 1.25 times the diameter of the tube

Tube Plate thickness is depend on the Tube outside diameter

Spacing of tube plate support (baffle) : minimum 50mm (varies from 0.6m to 2.5m)

Baffle distance to be decided to avoid any flow induced vibration which will lead to tube to tube plate cracking. TEMA has very indepth calculation to avoid these kind of ‘ harmonic’ vibrations.

Feed Pump Selection

Lets do the Quick Caclulation
Inputs
Actul Flow in m3/hr ?
Design head MWC ?
Operating head MWC ?

Minimum Flow required for safer operating of the pump ?
How the by pass is maintained, using Orifice or control Valve ?
Let Boiler Steam flow rate is 31000 kg/hr

Flow
32.488 m3/hr X 1.05 (design factor) + Min. bypass flow via orifice = 40.87 m3/hr
Head
(Boiler pressure + pr. drop in system ) x 1.05 (design factor) = 207.38 MLC

Select the pump for 40.87 m3/hr & 207MLC & 228MLC Condition

Superheater – Gas Flow

Design of any heat exchanger is manly depend upon the flow configuration.
As discussed before the configuration could be of three type (majourly)

1. Parallel Flow
2. Counter Flow
3. Cross Flow

The compactness increases from Parallel flow to Counter Flow to Cross Flow

While complexity increases in reverse direction.

The superheater has flue gas on one side & Ideal gas on other. As steam has less capacity of carring the heat than water, we have to give more area.

The velocity of steam & flue gas plays a majour role in deciding the compact ness of heat exchanger.
More the velocity, more will be reynold’s number, higher will be nusselt number, higher will be heat transfer co-efficient which leads to higher over all heat transfer co-efficient & hence
lesser Area.

Superheater – Performance Optimization

One thing we must remember while designing super heater.. Better the steam dryness compact will be the super heater.

One must always ensure that dry steam (99%) is coming in the superheater.
Just to give you the picture..

i.e if you give 99% dry.. 61.5-56.1 = 5.4Kcal/kg i.e for 6000kg/hr its 40KW less energy is required.

Lower heat demand.. means lower heat transfer area.

Now Dryness can be improved by giving dimister pad or giving external equipment to improve steam dryness.