Designing of Air Cooled Heat Exchangers

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Designing of Air Cooled Heat Exchangers

• By
• Mehaboob Basha N.B

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• Purpose
• To provide some general information on air-cooled
  heat exchangers
• Designing a air cooled heat exchanger

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Definition

• An Air Cooled Heat Exchanger is a heat
  transfer device for rejecting heat from a hot
  fluid directly to fan-blowing ambient air.

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The most evident advantages are

• No problem arising for thermal and chemical
  pollution of cooling fluids.
• Flexibility for any plant location and plot plan
  arrangement like installation over other units.

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Fields of application of air-cooled heat
exchangers

• Oil and gas refineries
• Compressor stations for gas pipelines
• Subsurface gas storage facilities
• Plants producing polychlorvinyl, polyethylene,
  glass fibre, biplastic
• Caustic soda plants
• By-product coke plants
• Ammonia transportation and handling plants

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CONFIGURATION

• Arrangement of tube bundles and provision of air
  flow
• Bundles construction and flow configurations
• Finned tube construction

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INDUCED DRAFT UNIT

• The induced draft unit gives a steady and durable
  thermal performance,
• better air distribution,
• less hot air recirculation,
• less fouling,
• lower noise at grade.

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FORCED DRAFT UNIT

• The forced draft unit allows an easy access for
  maintenance to the fans and to the bundles.
  Furthermore,
• the fans remain in the cold ambient air ,
• lower capital cost.

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• Typical heat exchanger

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A typical air cooled heat exchanger
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mechanical components of heat exchanger

• A air cooled heat exchanger is shown in the
  figure 1.
• components may be listed as
• 1. Tubes with fins as basic component which is
  made up of carbon steel thru which process fluid
  at high temperature flow and heat exchange takes
  place.
• 2. Inlet header which distributes the process
  fluid in to tubes
• 3. Outlet header on the other side collects the
  process fluid.

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• Above three are basic components of air cooled
  exchanger and the rest are auxiliary, such as
  side wall which holds the tube bundle structure.
  Tube support which the support the tubes, number
  of tube support varies with length of heat
  exchanger. Tube sheet are found at the inlet and
  outlet of the tubes and tube length ends at the
  tube sheets. Lifting eye is the small grove found
  on the tube support, tube bundles are lifted for
  cleaning by these holes. Gasket is employed in
  order to avoid leakage.

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How are they constructed?

• Typically, an air-cooled exchanger for process
  use consists of a finned-tube bundle with
  rectangular box headers on both ends of the
  tubes. One or more fans provide cooling air.
  Usually, the air blows upwards through a
  horizontal tube bundle. The fans can be either
  forced or induced draft, depending on whether the
  air is pushed or pulled through the tube bundle.
  The space between the fan(s) and the tube bundle
  is enclosed by a plenum chamber, which directs
  the air. The whole assembly is usually mounted on
  legs or a pipe rack.

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•   What standards air used for Air-Cooled
  Exchangers?
• First, almost all air coolers are built to Sect.
  VIII of the ASME Code, since they are pressure
  vessels
• What kinds of finned tubes are used?
• The tubes can be of virtually any material
  available, such as carbon steel, stainless steel,
  Admiralty brass, or more exotic alloys. The
  minimum preferred outside diameter is one inch.
  Some manufacturers sometimes use smaller tubes,
  but most of the process coolers have tubes, which
  are 1.0", 1.25", or 1.5" OD. The minimum tube
  wall thickness vary with the material. In some
  cases the design pressure and design temperature
  of the exchanger govern the minimum thickness.

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Thermal performance calculations

• BASIC EXPERSSION FOR THE TOTAL RATE OF HEAT
  TRANSFER
• is the total external surface are of the tubes
  without fins.
• is the overall heat transfer coefficient
• mean temperature difference.
•  

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calculation of overall heat transfer .

•  
• Enhanced airside heat transfer coefficient
  based on tube outer diameter,
• thermal conductivity of tube
• heat transfer coefficient for fluid being
  cooled based on inner diameter,
• contact thermal resistance between fins and
  tube.
• is external area of the fin and is the
  external surface of the tube between fins
•  

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Calculation of mean temperture difference.

• 1.Calculate R and P
• 2.values R and P read off the values of F from
  the part of the combined chart(F-0-NTU-P).

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• 3. calculate from the expression
• 4. calculate the mean temperature difference from

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Problem description

• tc_i313 air inlet temperature k
• th_i383 process fluid inlet temperature k
• u_a6 air velocitym/s
• m_h7 mass flow rate of process fluidkg/s "
• th_o368 outlet temperaturek"
• process fluid is hydrocarbon
• heat transfer rate218400 w

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Sizing and designing

• Sizing of the heat exchanger tubes
• Assumption are to be made based on
• 1. type of exchanger
• 2. Data available
• 3. physical understanding
• Design criteria

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ASSUMPTION

• Mass flow is equally divided in to number of
  tubes.
• Cross-flow unit ,one pass and unmixed stream
• fins are made up of aluminum
• 4. four tubes in a row

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Sizing

• Outer diameter of the tubem
• Internal diameter of the tubem
• Number of tubes in one passes
• Number of rows
• Number of passes
• Total number of tubes
• Number of tubes in a column
• Length of the tubesm
• Space between the finsm
• Thickness of the finsm
• Thermal resistance of aluminum w/m.k
•  
•  

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Design Criteria

• Heat exchanger was designed in order to perform
  required duty with minimum cost of heat transfer
• Paikert(1983) sugested
• internal htc 200 w/m2.k then TSAFT/TSABT5
• internal htc 1000 w/m2.k then TSAFT/TSABT13
• internal htc 5000 w/m2.k then TSAFT/TSABT23

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procedure

• u_hm_h/(np(pi/4)(di2)rho_h) "velocity of
  hydro corbon"
•  re_hrho_hu_hdi/mhu_h Reynolds
  number of process fluid"
•   nu_h0.023((re_h)0.8)(pr_h)0.4 " nusselt
  number of process fluid"
•  alpha_h(nu_hk_h)/di "heat
  transfer co- effiecient of the process
  fluid"
•   a_tn_tlpidr " total
  area of the tube without fins"
•  aa_t12
  "condition for min cost of heat transfer
  paikert(1983)
•  

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• a_wn_tlpidrs/(sw) " area
  between the fins"
•  a_fa-a_w " area
  between the fins"
•   solve('d_f22d_fw-(a_f2(sw))/(n_tlpi)-dr
  2)0')
• d_f0.042739
•  h(d_f-dr)/2 height of the fin"

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Result

• dr24e-3 outer diameter of the tubem
• di20e-3 internal diameter of the tubem
• np88 number of tubes in one pass"
• nr4 number of rows
• p1 number of passes
• n_t88 total number of tubes
• ntn_t/nr number of tubes in a column
• l2 length of the tubesm
• s1.9e-3 space between the finsm
• w0.4e-3 thickness of the finsm

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• velocity of process fluid 0.3332 m/s
• heat transfer co-efficient on air side
• 115.4027 w/m2.k
• heat transfer co-efficient on tube side
• 535.1891 w/m2.k
• overall heat transfer co-efficient
• 291.5178 w/m2.k
• percentage of error calculation0.07
• Pressure drop 215.5326 N/m2

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• Thank you