Engineering task and calculation objective
The OPTD module supports the economic optimisation of shell-and-tube heat exchangers following the methodology of the VDI Heat Atlas: it links the thermo-hydraulic design with the investment costs in order to find the optimum between pressure drop, heating surface and cost. Smaller pressure drops mean larger flow cross-sections and thus more area and higher equipment cost – higher pressure drops save area but drive up the pumping or compression energy.
The core of the module is the cost estimate for the equipment: starting from area-specific costs, correction factors capture the design influences – TEMA configurations of shell, front head and rear head, tube pitch, number of tube passes, tube length deviation, expansion joints, material, and the design pressures on the shell and tube sides. Together with the number of exchangers, an addition factor and the dollar exchange rate conversion, this yields the total investment cost of the heat exchanger.
Such a calculation is needed in the concept and proposal phase of plant engineering: anyone who wants to estimate the cost of a shell-and-tube heat exchanger or determine the economically optimal pressure drop can use it to systematically compare variants (configuration, material, tube passes, split across several exchangers) before detailed design begins.
Calculation workflow
- Define the equipment configuration: The number of exchangers, the TEMA configurations (shell, front head, rear head), the tube pitch, the number of tube passes and, if applicable, expansion joints are selected – each decision is linked to a correction factor of the cost correlation.
- Capture pressure and material influences: The design pressures on the shell and tube sides enter via their own correction factors, as does the material via the material correction factor; higher-alloyed materials and high pressures increase the equipment cost disproportionately.
- Determine the area-specific costs: From the base factor for determining the cost coefficient and the correction factors (including tube length deviation and the additional term of the shell cost calculation), the area-specific costs of the equipment are obtained.
- Calculate the total investment: Multiplication by the heat transfer area, the addition factor for auxiliary costs and the number of exchangers, plus conversion via the dollar exchange rate, yields the total investment cost in euros.
- Compare variants and search for the optimum: By varying the pressure drop or the flow arrangement, the area requirement and cost of several variants are compared; the economic optimum lies where the sum of capital and operating costs (pumping energy) is at its minimum.
Input quantities
| Quantity | Symbol | Unit |
|---|---|---|
| Exchanger area Tube pitch | A PT | m² |
| Baffle pitch Equivalent diameter | BAB De | m |
| Specific heat capacity of the fluid | cp | J/(kg·K) |
| Specific heat capacity of the fluid | cp | J/(kg·K) |
| Annualised area-specific costs | CA | EUR/(m²·a) |
| Area-specific costs | CB | EUR/m² |
| Addition factor | CF | – |
| Specific operating costs caused by shell-side-side pressure drop | CM | EUR/kWh |
| Specific operating costs caused by tube-side pressure drop | CR | EUR/kWh |
| Annualised total costs | CT | EUR/a |
| Total invetsment costs | CI = (CB·(1+CF)·A)· N CI | EUR |
| Baffle pitch Equivalent diameter | BAB De | m |
| Tube outside diameter Tube inside diameter | Do Di | m |
| Tube length Shell diameter | L DM | m |
| Tube outside diameter Tube inside diameter | Do Di | m |
| Friction factor of fluid | f | – |
| Friction factor of fluid | f | – |
| Correction factor for logarithmic temperature difference | F | – |
| Annual operating hours | HY | h/a |
| Overall heat transfer coefficient | k | W/(m²·K) |
| Tube length Shell diameter | L DM | m |
| Wall thickness Number of passes | s nW | – |
| Number of tubes Number of baffles | NR NB | – |
| Number of tubes Number of baffles | NR NB | – |
Frequently asked questions
Why is there an optimal pressure drop at all?
Higher flow velocities improve heat transfer, reduce the required area and thus the investment – but they increase the pumping or compression power quadratically to cubically, and with it the operating costs. The sum of both cost components has a minimum; the corresponding pressure drop is the economically optimal design point.
How accurate are such cost estimates?
Factor-based correlations of this kind are of study or pre-project accuracy, typically in the range of about ±30 %. They are excellent for comparing variants under identical assumptions, but they do not replace vendor quotations. For absolute values, the base year of the correlation and the currency conversion (dollar rate) must be updated via cost indices.
What influence do the TEMA configurations have on cost?
A considerable one: a fixed-tubesheet unit (e.g. TEMA BEM) is significantly cheaper than designs with a floating head or removable bundle, which may be necessary for cleaning and thermal expansion compensation. The correction factors for the shell, front head and rear head configurations reflect exactly these price differences – but the cheapest configuration is only permissible if fouling and temperature differences allow it.
When is it worthwhile to split the duty across several exchangers?
Several smaller units generally increase the total investment because of fixed-cost components. Splitting becomes sensible for other reasons: transport and fabrication limits, redundancy, cleaning during operation, or when the required area exceeds the economical size of a single unit. The module makes the cost difference directly visible via the number of exchangers.