Heat Exchangers Design
Heat Exchanger is an equipment or device used for a Heat
Transfer from one fluid to another, generally in any Chemical Process Industry
for some utilitarian purpose or service. There are a number of types and
classes of these, depending on the application requirements, design &
construction considerations, operating conditions and fluids.
The heat transfer equipment or Heat Exchanger follows the
basic principle of Conduction-Convection process understanding the fluids,
materials : thermal, mechanical, chemical properties,
characteristics & behavior in different conditions of operation.
General classification of the Heat Ex-changers can be done
as below , based on their construction
and required application or use:
Shell& Tube Exchangers Double Pipe Heat Ex-changers Evaporators
Fin
Tube Exchangers Plate
Heat Exchangers
The application areas are in Chemical Process Industry,
Petroleum Refinery, Petrochemical Industry,
Fertilizer & Pesticide Industry, Paper & Pulp Industry,
Conventional Desalination & Water Treatment Plant, Pharmaceutical &
Bulk Drugs Industry, Wood Processing Industry, Home Appliances Industry,
Plastics Processing Industry, Diesel Power Plants, Thermal Power Plants.
Depending the service application, the Heat Exchangers are named
as Heater or Chiller or Cooler or Condenser etc.
In this edition, we shall talk of the Design of Shell &
Tube Heat Exchangers in General.
It all starts with a requirement or necessarily of the Heat
Exchanger & selection. The application, space availability, operating
conditions, process requirements, cost economics guide the selection. National
or International standard codes prescribed are followed as a Guideline form the
Basis of Design, apart from Market Situation of the Materials, Construction Facilities
and a GEP (General Engineering Practice).
Design Basis :
1. Operating
Conditions based on process engineering
2. Selection of
Materials
3. Thermal Engineering
4. Mechanical
Calculations.
5. Design Standards:
TEMA ( Tubular Exchangers Manufacturers Association), ASME – Sec. VIII Div. 1;
Sec. II, Sec. IX (American Society of Mechanical Engineers)
The operating conditions are based on process engineering
unique to individual process application and which is patented / copy righted. Therefore is not
discussed here. Further, based on these operating conditions, certain other
properties and chemical or thermal coefficients / constants are used in the Design and which are
referenced from Process / Properties Table Published and available from deep
research.
The Operating Conditions are :
Shell
Side Fluid?; Working Pressure(In/Out); Working Temperature (In/Out); Fluid State & Concentration ; Specific
Gravity ; Specific Heat Capacity; Operating Heat Flow Rate
Tube Side Fluid?; Working Pressure(In/Out); Working Temperature
(In/Out); Fluid State &
Concentration; Specific Gravity;
Specific Heat Capacity; Operating Heat Flow Rate
As per General Process Engineering Practice it is advisable
to use the shell side for the Service Fluid and Tube side for the Process Fluid
The selection of materials for the heat exchangers is
followed from the design standards & experience. Specific range of materials
are compatible or suitable for specific fluids @ specific operating conditions.
For Example :
Fresh
Water / Steam Applications : Petroleum Oil Applications : Organic
Solvents / Solutions :
Inorganic Chemical Solutions :
Parallel to the selection
of materials the thermal engineering is done. Basic formula is of the Heat
Exchanger heat load calculation. i.e.
Q = m X Cp X ∆T; Where,
Q
= Heat Flow Rate, in Kcal / Hr or BTU/Hr
units; m = mass flow rate, in Kg / Hr or Lb/Hr units; Cp
= Specific Heat Capacity, in 0.998 Kcal / Kg -Deg C or BTU / lb-Deg F
Units ∆T =
Temperature Difference of the Inlet to Outlet,
Deg C or Deg F units
Using the above formula the Heat Load is calculated for one side of the Heat Exchanger,
especially for the Shell Side. The calculated value is applied for the Tube
Side in the same formula to arrive at the Tube Side Outlet temperature. This
helps us to calculate the Log Mean Temperature Difference (LMTD)
LMTD = (∆T1 - ∆T2) / (ln ∆T1/∆T2), such that ∆T1 is
> ∆T2
; Where, ∆T1
is the inlet / outlet temperature difference of one side ( for eg. Shell side)
& ∆T2 is the inlet / outlet
temperature difference of the other side ( for eg. Tube side) Deg C or Deg F.
NOTE: We have what is called parallel flow & cross flow
designs, which are to do with the “PASSESS” of the Process & Service
Fluids. Generally, for better heat transfer rate, it is recommended by
Designers to have cross flow designs.
There can be a number of passes on both Shell & Tube side, which are also
prescribed in the TEMA Standards. Thus the material surface area is reduced meeting
the same performance levels with reduced size of the Equipment and hence
economical, cost-wise.
Now, we know the Heat Load (Q) & the LMTD. Using the
formula,
Q = U X A X ∆T, we can size the heat exchanger in terms of Heat Transfer Area Q = Heat Load, as calculated from above, in Kcal / Hr units.; m = mass flow rate, in Kg / Hr units;
U = Design Overall Heat Transfer Coefficient, BTU / Hr- Sq.Ft- Deg F OR 4.88 Kcal / Hr-Sq. M-Deg C; This is a constant and assumed based on the data on Operating fluid provided and Table 8.
∆T is the LMTD x Ft as per above in Deg C / Deg F Units. Ft= LMTD correction factor for shell side / tube side no. of passes ( refer chart 18-22 of Process Heat Transfer)
Substituting these values in the formula of Q = U X A X ∆T,
we can arrive at the Heat Transfer Area in Sq.Ms.
With this the basic Thermal Engineering of the Heat Exchanger is over.
Mechanical Calculations :
Number of
Tubes: Formula, Area, A = π
X D X L X N
;
A
= Area in Sq. m Units; π = Constant, 3.14 value; D = Tube Diameter in m, units
; L = Length of Tube in m, Units; N = Number of tubes.
A, is available from above calculations. D &
L are based on the market availability of sizes for supply, Space Availability
& Constraints at the Heat Exchanger Installation Site, Material Handling
& Client or End User Preferences & based on Recommendations of the Heat
Exchanger Manufacturer from Capabilities
for different processes in the Manufacture & Good Engineering Practice
(GEP).
We would thus have the Number of Tubes, N of the Heat Exchanger.
Next step is Size of the overall Equipment :
There
is a concept of LLC ( Least Limit Circle). The number of tubes are formed into
a bundle at a constant pitch of tube
centre to centre space, supported by Baffles at constant spacing through the
length of the tubes / Tube Bundle. We have Square & Triangular Pitches for
the Tube Centre to Centre Space, prescribed also by TEMA. However, generally
the Triangular Pitch is recommended for better efficiency of the Heat Exchanger.
The tube bundle form a Circle , sectionally that is limiting the Bundle within its
Diametrical Coverage. This LLC forms the
reference base for sizing the Shell of the Heat Exchanger.
The formula for calculating LLC, LLC =
Pt = Pitch; N = No. of Tubes. The LLC can be calculated in inche or cm or mm
Now we
can calculate the Shell ID ( Inner Diameter), ID = LLC + Shell to Tube Bundle Clearance, in in. / cm / mm, units. The clearance is assumed based on common
sense, understanding the functional operation of a heat exchanger, construction,
supporting arrangements, Material Handling,
experience & GEP.
Pressure
Calculations:
The Shell Side Flow Area in Sq. Ft, as = ID x C’ B / 144 x Pt ID = Shell ID, in in / cm / mm units; C’ = % cut o the baffles for the
shell side fluid flow; B = Baffles spacing in in / cm / mm, units; Pt = the
Tube pitch, in / cm / mm units
The Tube Side Flow Area in Sq. Ft., at = Nt X a’t / 144n, where Nt = No. of Tubes; a’t =
single tube c.s area; n = number of passes on tube side
.......CONTINUED IN NEXT POSTING
