Economiser, some FAQs

Technical, ,

Guide for better economiser design

Is it essential to pre heat feed water?

How one preheats they boiler feed water? We have two options,

  • Via economiser, mounted on exhaust of boiler, and heating incoming feed water by absorbing lost heat in flue gases
  • Via Deareator tank, adding steam to deareator and heating the water temperature. Though the purpose of deareator is as name suggest remove the “oxygen” from the water, by heating it. Nevertheless it also increases the temperature of water in this process.

Let’s talk about economiser

Now that’s the question, one should not ask, if possible, one should always use economiser. It generally speaking will increase boiler efficiency by 3-5 percentage points.

It also helps reduce the thermal stress on boiler as a whole.

Then why this question?

Challenges with economisers are few, apart from how much heat we are recovering vs how much expenditure we are doing in is installation, maintenance and qualification.

Assuming the economics works well, the next question is if you absorb too much heat, and condensation happens, it will lead to sulphur corrosion, distorting not only boiler but also chimney in long run.

Criteria for designing economiser from user perspective

  • Define maximum hot water temperature we expect out of economiser, this has to be above sulphur due point
  • Define maximum pressure drop allowed. As more pressure drop will add more duty to the Blower, adding to running energy cost
Acid Dewpoint Corrosion

Criteria for design from Designer’s perspective

  • Configuration of path, with multiple branches to make water flow, it helps get maximum efficiency but also adds to pressure drop and duty on feed pump
  • Profile of fins for heat exchanger, some profiles of fins, for example serrated fins, has huge heat transfer to area ratio, and are highly efficient, but also adds to pressure drop on air side, and blower duty increases accordingly.
  • Number of passes of hot flue gases, this ensures more residential time for the flue gas, so that we can extract more heat, however, this will also cost us more pressure drop and added duty to the blower.
  • Minimum feed water temperature and maximum outlet temperature, both factors are important to ensure we get better efficiency but also to ensure we have less issues with corrosion (oxygen pitting

Typical failure reasons

Typically in boiler, the flue gas is mostly utilised to super heat the steam, and then passed to economiser, in such cases, The temperature differentials between the flue gas and water are quite low. To maximize heat transfer, water temperatures at the end of the economizer run should be very close to the saturation temperature. If the temperature difference is very low, then it some time can lead to steaming in economiser. Steaming not only reduces the efficiency drastically but also lead to knocking and failure of weld during operation due to thermal impact.

Second reason for failure could be quenching effect. This happens, when boiler is stand by mode, that is steam is not consumed. Water in economiser reduces drastically, and sudden surge in demand, make feed pump, pump cold water to economiser, leading to thermal stresses, even if the delta of water differential is low, this will lead to failure after some time. This issue can be over come by losing some water in boiler itself via intermediate blow down, small water circulation in economiser helps avoid such quenching issues.

Energy Required to Heat Air

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Recently I came across this requirement of calculating heat required to heat air (for AHU), I came across two simplified formulas, as follows.

Please also note the learning from this workout at the bottom!

image

Learning!

for delta T, Centigrade vs Fahrenheit are different!

I was thinking, as long as its Delta (Difference between two temperature), °F and °C doesn’t matter! I was WRONG… See following example

Raise temperature of air from 10 °C to 110°C, Temperature difference is 100°C

Where as in Fahrenheit, its 50°F (10°C) to 230°F (110°C) i,e difference is 180°F!

Tube Holes in Tubesheet : Point to ne noted while designing heat exchanger

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Tube hole finish affects the mechanical strength and leak tightness of an expanded tube-to-tubesheet joint. In general:
(1) A rough tube hole provides more mechanical strength than a smooth tube hole. This is influenced by a complex relationship of modulus of elasticity, yield strength and hardness of the materials being used.
(2) A smooth tube hole does not provide the mechanical strength that a rough tube hole does, but it can provide a pressure tight joint at a lower level of wall reduction.

(3) Very light wall tubes require a smoother tube hole finish than heavier wall tubes.

(4) Significant longitudinal scratches can provide leak paths through an expanded tube-to-tubesheet joint and should therefore be removed.

Hence its important to show roughness during design, and ensure it durign fabrication.

Good day!

Design of heat exchanger step :5 : Parameter Calculations

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In the process of evaluation of U, we need to calculate various parameters. Below paragraph mentions few of them.

1. Reynolds number (Re) Reynolds number, which relates inertial forces to viscous forces and thereby characterizes the type of flow regime
2. Prandtl number (Pr), which relates the thermal properties of the fluid to the conductivity of the pipe.
3. Nusselt number (Nu) , a dimensionless group defining the relative significance of the film heat transfer coefficient to the conductivity of the pipe wall

All above three parameters are linked as shown below
Calculation of Reynold’s Number

where:

Dh is the hydraulic diameter of the pipe; its characteristic travelled length, , (m).

Q is the volumetric flow rate (m3/s).

A is the pipe cross-sectional area (m²).

v is the mean velocity of the object relative to the fluid (SI units: m/s).

mu is the dynamic viscosity of the fluid (Pa·s or N·s/m² or kg/(m·s)).

nue is the kinematic viscosity ( (m²/s).

rho is the density of the fluid (kg/m³).

Calculation of Prandtle number :

Where

Based on these parameter, we can now calculated Heat transfer co-efficient on either side.

Where kw is thermal conductivity of the bulk fluid.

With this you are now equipped to calculate over all heat transfer.
 

Differences Between The Eighth And Ninth Edition Of The TEMA

Technical,
Ok, TEMA Ninth edition is here.

Other Than complete change in Page Indexing (Page number), Following are note worthy changes from eigth edition :
Followings are Prominent Changes and Listed at TEMA website
1. New rules for flexible shell elements (expansion joints), which are based on a Finite Element Analysis (FEA) approach.

2. Tables for tube hole drilling have been expanded to 3” diameter tubes.
3. Guidelines for performing Finite Element Analysis (FEA) had been added.
4.Rules for the design of shell intersections (with large nozzle to cylinder ratios) subjected to pressure and external loadings have been added.
5.Foreign material cross-reference linking material specifications from various international codes has been added.
6.Rules for the design of longitudinal baffles have been added

Followings are my observation (And I’ll keep it upto date as more I go in detail)
1. Page numbering & Indexing changed completely
2. Tube Plate thickness calculation is now moved to Appendix A, and detail and simplified approach is seen.
3. Section T (Thermal Relations), Section P (Physical properties of fluids), Section D (General Information) and Section RGP (Recommended Good Practice) added

Design of heat exchanger step :4 : Thermal Design

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Next step, do a detailing.

To Calculate LMTD, we need to understand the flow and type of process (Isothermal or not)

Generally we have complication, when either of fluid is showing isothermal characteristics. see the following variation.

Two fluids are separated by a heat transfer surface (wall), these fluids ideally do not mix, and there are no moving parts.In this blog the thermal design theory of recuperates is presented. In a heat exchanger, when hot and cold fluids are maintained at constant temperatures of Th and Tc as shown in above Fig(a)..The driving force for overall heat transfer in the exchanger, referred to as mean temperature difference (MTD), is simply Th-Tc.

Such idealized constant temperatures on both sides may occur in idealized single-component condensation on one fluid side and idealized single-component evaporation on the other fluid side of the exchanger. However, a number of heat transfer applications have condensation or evaporation of
single-component fluid on one side and single-phase fluid on the other side. In such cases, the idealized temperature distribution is shown in Fig(b) and (c)

The First major step in Thermal design, is to understand the assumptions that are considered while we disign the heat exchanger, but we forgot to understand them, they are:-

1. The heat exchanger operates under steady-state conditions [i.e., constant flowrates and fluid temperatures (at the inlet and within the exchanger) independent of time].

2. Heat losses to or from the surroundings are negligible (i.e. the heat exchanger outside walls are adiabatic).

3. There are no thermal energy sources or sinks in the exchanger walls or fluids, suchas electric heating, chemical reaction, or nuclear processes.

4. The temperature of each fluid is uniform over every cross section in counter flow and parallel flow exchangers (i.e., perfect transverse mixing and no temperature gradient normal to the flow direction). Each fluid is considered mixed or unmixed from the temperature distribution viewpoint at every cross section in single-pass cross flow exchangers, depending on the specifications. For a multi pass exchanger, the foregoing statements apply to each pass depending on the basic flow arrangement of the passes; the fluid is considered mixed or unmixed between passes as specified.

5. Wall thermal resistance is distributed uniformly in the entire exchanger.

6. Either there are no phase changes (condensation or vaporization) in the fluidstreams flowing through the exchanger or the phase change occurs under thefollowing condition. The phase change occurs at a constant temperature as for a single-component fluid at constant pressure.

7. Longitudinal heat conduction in the fluids and in the wall is negligible.

8. The individual and overall heat transfer coefficients are constant (independent of temperature, time, and position) throughout the exchanger, including the case of phase-changing fluids in assumption 6.

9. The specific heat of each fluid is constant throughout the exchanger, so that the heat capacity rate on each side is treated as constant. Note that the other fluid properties are not involved directly in the energy balance and rate equations, but are involved implicitly in NTU and are treated as constant.

10. For an extended surface exchanger, the overall extended surface efficiency is considered uniform and constant.

11. The heat transfer surface area A is distributed uniformly on each fluid side in a single-pass or multi pass exchanger. In a multi pass unit, the heat transfer surface area is distributed uniformly in each pass, although different passes can have different surface areas.

12. For a plate-baffled (1–n) shell-and-tube exchanger, the temperature rise (or drop) per baffle pass (or compartment) is small compared to the total temperature rise (or drop) of the shell fluid in the exchanger, so that the shell fluid can be treated as mixed at any cross section. This implies that the number of baffles is large in the exchanger.

13. The velocity and temperature at the entrance of the heat exchanger on each fluidside are uniform over the flow cross section. There is no gross flow maldistribution at the inlet.

14. The fluid flow rate is uniformly distributed through the exchanger on each fluid side in each pass i.e., no passage-to-passage or viscosity-induced maldistribution occurs in the exchanger core. Also, no flow stratification, flow bypassing, or flow leakages occur in any stream. The flow condition is characterized by the bulk (or mean) velocity at any cross section.

Question??? So many assumption… still we are ‘designing’ a heat exchanger with Guarantee on performance!! sure we are Engineers 😉

Following Table will show the impact of various parameters on design of heat exchanger.
(NTU stands for number of transfer unit, and alternate to LMTD method)

TEMA Learning -Heat Exchanger nomenclature & Designation:

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Heat Exchanger nomenclature & Designation:
The name is splited in three portions e.g 23-192-BEM

1st Position : 23 : Defines, ID of Vessel
2nd Position : 192 : Defines, Tube length
3rd Position : BEM : it further splited in three parts as per fing N-1.2
e.g B-E-M,
the 1st section, defines the Type of Front head (B for Bonnet Integral cover)
the 2nd section, defines the shell type (E – One pass shell)
the 3rd section, defines the type of rear head (M – Fixed tube sheet, stationary head)

See the image below
Typical heat exchanger configuration is shown in below image

Above figure shows typical flow arrangement inside a heat exchanger.

Typically for a heat exchanger, their are two type Tube Bundle exist. see the picture below.

Design of heat exchanger step :3 : Estimate

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The 3rd Step in Heat exchanger design a preliminary estimate of the size of the exchanger is made,using a heat-transfer coefficient appropriate to the fluids, the process,and the equipment.

Generally we can get reference from various resources listed below, either expected heat flux (W/m2) or range of heat transfer co-efficient. based on this information, we can estimate the heat transfer area required.

But if the case is, you are doing for first time, and have no earlier experience in that type of heat exchanger, then better is make your self ready with all information e.g fluid properties which includes

  1. Dynamic viscosity (for the temperature ranges)
  2. Conductivity (for the temperature ranges)
  3. Density (for the temperature ranges)
  4. Boiling temperature
  5. Specific heat

The grate equation (you can see in my earlier post) for heat duty is:
Q = U A LMTD & Q = m. cp. DT

where
Q = heat duty (kcal/hr)
U = over all heat transfer co-efficient (Kcal/hr.m2.oC)
A = heat transfer area (m2)
LMTD = Log mean temperature (degC)
cp = Specific heat of the fluid to be heated / or to be cooled (kcal/kg)
DT = temperature difference of any one of the fluid (degC)
m = mass flow rate of the fluid (kg/hr)

LMTD is calculated based on the temperature on both the side for both the fluid
it’s simple if you understand the following figure

logarithmic arithmetic mean temperature difference lmtd amtd
& LMTD = (dto – dti) / ln(dto / dti)

Just Plot your temperature, take difference on each side, and do the arithmatics…

Some good refrences (Free google search results 🙂

http://www.chemsof.com/lmtd/lmtd.htm
http://en.wikipedia.org/wiki/Log_mean_temperature_difference
http://www.wlv.com/products/databook/databook.pdf