Cost of heat exchangers, columns and reactors

How the purchased and installed costs of heat exchangers, columns and reactors are estimated from size, weight, material, pressure and cost-index factors.

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Why it matters

Heat exchangers, columns and reactors usually make up most of the purchased-equipment cost of a chemical plant, and the fixed capital is then built up from that cost with factors. A quick, defensible cost for each item — from its size, material and pressure — is therefore the first step in every study estimate and in comparing design options such as a taller column against more reflux.

Key ideas

Purchased cost correlations. Equipment cost is correlated with a size parameter that reflects how much metal and fabrication is involved:

  • Shell-and-tube heat exchangers — heat-transfer area A (m²). Cost exponent typically 0.6–0.7 for one type over a moderate range. Base correlations are for carbon steel, a standard type (often fixed tube-sheet or floating head) and moderate pressure.
  • Pressure vessels and column shells — either vessel weight (kg of steel) or diameter and height. Weight-based costing is the more reliable: weight = π·D·H·t·ρ for the shell, plus an allowance for heads, nozzles, skirt and supports. Wall thickness t comes from the pressure-vessel code (in India, IS 2825; internationally ASME VIII), so it rises with pressure and diameter.
  • Column internals — trays are costed per tray as a function of diameter and type (sieve, valve, bubble-cap), multiplied by the number of actual trays; packing is costed per m³ of packed volume.
  • Reactors — a jacketed or agitated vessel costed as a vessel (by volume or weight), plus the agitator and drive (by power), the jacket or coils, and internals. Tubular catalytic reactors resemble heat exchangers and are costed by tube area or weight.

Correction factors. A base cost is corrected to the actual design with multiplying factors, all of which are given data from a design text (Peters and Timmerhaus, Coulson and Richardson Vol. 6, Turton, Seider) and vary between sources:

  • Material factor F_M for stainless steel, Monel, titanium, lined vessels; for exchangers it depends on which side (shell or tube) uses the alloy.
  • Pressure factor F_P, rising with design pressure.
  • Type factor F_D for exchanger type (U-tube, kettle reboiler).
  • Cost index ratio to bring the base-year correlation to today.
  • Bare-module factor (Guthrie): converts purchased cost to installed cost including piping, instruments, foundations, insulation, erection and indirect costs for that item. Typical values are about 2–4, larger for small items.

Scaling between sizes. For the same type, material and pressure, the six-tenths-type rule C₂ = C₁·(S₂/S₁)ⁿ applies, with n specific to the equipment (about 0.6 for exchangers and vessels, 0.5–0.6 for agitated reactors, about 0.8–0.9 when cost is based on weight).

How design choices feed costs.

  • Exchangers: higher velocity raises U and cuts area but increases pumping power; a closer temperature approach increases area sharply.
  • Columns: diameter is fixed by vapour load (higher reflux → larger diameter); height by number of trays and spacing (lower reflux → more trays). Higher pressure increases wall thickness.
  • Reactors: volume follows from kinetics (residence time); expensive catalysts or alloys may favour smaller, more intensive designs.

Formulas

Size scaling: C = C_b·(S / S_b)ⁿ Corrected purchased cost: C_p = C_b·(S / S_b)ⁿ·F_M·F_P·F_D·(I / I_b) Shell weight: W = π·D·H·t·ρ_m (plus allowance for heads, nozzles and supports) Column purchased cost: C_col = c_w·W_total + N_act·C_tray Actual trays: N_act = N_theoretical / E_o Installed (bare-module) cost: C_BM = F_BM·C_p

Symbols: C cost (₹); S size parameter (A in m², V in m³, or W in kg); subscript b base value; n exponent (–); F_M, F_P, F_D material, pressure and type factors (–); I cost index (–); D column diameter (m); H shell height (m); t wall thickness (m); ρ_m metal density (≈ 7850 kg/m³ for carbon steel); c_w fabricated cost per kg (₹/kg); N_act actual trays; C_tray installed cost per tray (₹); E_o overall tray efficiency (–); F_BM bare-module factor (–).

Worked examples

Example 1 (standard). A carbon-steel shell-and-tube exchanger of 100 m² cost ₹20 lakh when the cost index was 600. Estimate the purchased cost of a 250 m² exchanger with stainless-steel tubes at a higher design pressure, today (index 780). Given data: n = 0.65, F_M = 1.8, F_P = 1.1.

  1. Size factor: (250/100)^0.65 = 2.5^0.65 = 1.814.
  2. Index factor: 780/600 = 1.30.
  3. C_p = 20 × 1.814 × 1.8 × 1.1 × 1.30 = 93.4.
  4. Purchased cost ≈ ₹93.4 lakh (the carbon-steel, base-pressure equivalent at today's index would be 20 × 1.814 × 1.30 = ₹47.2 lakh).

Example 2 (GATE level). A carbon-steel distillation column has inside diameter 2.0 m, shell height 30 m and wall thickness 12 mm. Allow 15% extra weight for heads, nozzles, skirt and supports. Fabricated vessel cost is ₹250/kg, and each sieve tray costs ₹1.2 lakh installed (given data). The separation needs 28 theoretical stages with an overall efficiency of 70%. Estimate the purchased cost of shell plus trays.

  1. Shell weight: W = π × 2.0 × 30 × 0.012 × 7850 = 17 756 kg.
  2. With allowance: 1.15 × 17 756 = 20 420 kg.
  3. Vessel cost = 20 420 × 250 = ₹51.05 lakh.
  4. Actual trays = 28/0.70 = 40; tray cost = 40 × 1.2 = ₹48.0 lakh.
  5. Column (shell + trays) ≈ ₹99.0 lakh. At 0.6 m tray spacing, 40 trays need about 24 m of tray section, consistent with a 30 m shell once top and bottom spaces are added.

Common mistakes

  • Applying a carbon-steel correlation to an alloy item without a material factor (or applying the factor to the whole cost when only the tubes are alloy).
  • Using theoretical stages instead of actual trays.
  • Forgetting the cost-index correction for an old correlation.
  • Scaling far outside the range of the correlation, or between different exchanger types.
  • Adding a bare-module (installed) cost and then a Lang factor on top, double-counting installation.

For GATE CH

Expect scaling of equipment cost with size and index, combined with given material or pressure factors; vessel weight from geometry; actual trays from efficiency; and conceptual questions on how reflux ratio, pressure or material choice changes the cost of a column, exchanger or reactor. Practise keeping each factor separate and multiplying at the end.

Quick check

  1. A 40 m³ reactor costs ₹30 lakh. Estimate a 80 m³ one with n = 0.6.
  2. 20 theoretical stages, overall efficiency 0.8. Actual trays?
  3. Why is weight a better basis than volume for costing high-pressure vessels?
  4. Shell weight for D = 1 m, H = 10 m, t = 10 mm, ρ = 7850 kg/m³?

Answers: 1. 30 × 2^0.6 = ₹45.5 lakh. 2. 25 trays. 3. Wall thickness, and so metal and fabrication, rises with pressure at the same volume. 4. π × 1 × 10 × 0.01 × 7850 = 2466 kg.

Interview questions

All Plant Design and Economics interview questions

Try answering each one aloud before you open it.

  1. 1.Explain the factors that influence the cost of a heat exchanger.Concept

    The cost of a heat exchanger is influenced by factors such as the type of heat exchanger (e.g., shell-and-tube, plate), the materials of construction (which depend on the fluids being processed), the size and capacity, the design pressure and temperature, and the complexity of the design. Installation and maintenance costs also play a role.

  2. 2.What are the main types of columns used in chemical plants, and what determines their cost?Concept

    The main types are tray columns (sieve, valve or bubble-cap trays) and packed columns, used for distillation, absorption, stripping and extraction. Cost is driven less by the service than by size and construction: the shell is costed by its weight, which depends on diameter, height and wall thickness (set by pressure), and the internals by the number and diameter of trays or the volume and type of packing. Material of construction multiplies all of these. Design choices such as reflux ratio trade column height (more trays) against diameter and energy.

  3. 3.Why is stainless steel often used in the construction of reactors?Application

    Stainless steel is often used in the construction of reactors because it offers excellent corrosion resistance, which is crucial for handling various chemicals. It also has good mechanical properties and can withstand high temperatures and pressures, making it suitable for a wide range of chemical reactions.

  4. 4.How does the choice of material affect the cost and performance of a chemical reactor?Application

    The choice of material affects both the cost and performance of a chemical reactor. Materials that offer high corrosion resistance and mechanical strength, such as stainless steel or special alloys, tend to be more expensive but provide longer service life and reliability. The material must also be compatible with the chemicals being processed to prevent contamination and degradation.

  5. 5.A 50 m² shell-and-tube exchanger is quoted at ₹10,000 per m² of area. What is its cost, and why is a constant cost per m² only a rough approximation?Numerical

    Cost ≈ 50 m² × ₹10,000/m² = ₹5 lakh. Exchanger cost does not rise in proportion to area: with a size exponent of about 0.6–0.7, cost per m² falls as size grows, so a per-m² figure is valid only near the size it came from. Material, pressure rating and exchanger type must also be corrected for with factors.

  6. 6.A distillation column operates at a pressure of 5 bar. If the pressure is increased to 10 bar, how might this affect the cost and design of the column?Application

    Increasing the pressure from 5 bar to 10 bar will likely increase the cost and complexity of the distillation column. Higher pressure requires thicker walls and more robust materials to withstand the increased stress, leading to higher material and fabrication costs. The design may also need to accommodate changes in vapor-liquid equilibrium, affecting the number of stages and column height.

  7. 7.Explain how the scale of operation impacts the cost of reactors in a chemical plant.Concept

    The scale of operation impacts the cost of reactors because larger reactors require more materials and more complex design considerations to ensure uniform mixing and heat transfer. While larger reactors benefit from economies of scale, the initial capital investment is higher. Additionally, larger reactors may require more sophisticated control systems to maintain process stability.

  8. 8.What is the role of insulation in the cost and efficiency of heat exchangers?Application

    Insulation plays a crucial role in the cost and efficiency of heat exchangers by minimizing heat loss to the environment, thereby improving thermal efficiency. While adding insulation increases the initial cost, it reduces operational costs by conserving energy. Proper insulation also helps maintain process temperatures, ensuring consistent product quality and process efficiency.

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