CHE3166 Chap.2 Utilities, Economics and System Boundaries
Utilities, Economics and System Boundaries
System boundaries decide which flows, duties, costs and environmental consequences enter a comparison. Define the physical limit, operating case and time basis before collecting numbers. Determine the process need before choosing a utility, and compare only technically equivalent options using consistent hours, tariffs, currency and cost scope. Draw the boundary before writing the balance.
Label each crossing as feed, product, recycle, waste, emission or utility, and decide whether upstream supply or downstream treatment lies inside the analysis. The appropriate limit depends on the decision. An equipment screen may use battery limits, while a site comparison may need electricity generation, cooling-water circulation, treatment and disposal.
State exclusions so the reader can judge whether they could change the result. Separate process need from utility supply. The process defines a temperature, pressure, flow, heat or power requirement. A utility is feasible only if it can meet that requirement with adequate driving force, reliability and controllability. Compare tariffs only after technical feasibility is established. Convert every option to one operating basis.
For energy, show power multiplied by hours. For consumables, show quantity multiplied by unit price. Distinguish installed capital from recurring operation, maintenance and downtime. Keep currency year, location and scope consistent. Simple payback is a useful screen, but it omits time value, equipment life and residual value. Report physical savings separately from prices so assumptions can be updated.
Test utilisation, tariff, efficiency and maintenance cases that could reverse the ranking. A transparent sensitivity is more useful than a precise answer built on one hidden forecast.
What this chapter covers
- 01
Inside and outside the process boundary
- 02
Mass, energy and utility crossings
- 03
Utility temperature and pressure feasibility
- 04
Annual energy and consumable costs
- 05
Installed cost and simple payback
- 06
Sensitivity and boundary consistency
Worked example · free
Annualise an energy saving
- +2Energy reduction = 11 kW × 6,000 h/y = 66,000 kWh/y.
- +2Cost reduction = 66,000 kWh/y × $0.18/kWh = $11,880/y.
- +2Confirm both power figures deliver the same flow and head on the same boundary.
Key terms
- Utility duty
- The process heat, cooling, power or service requirement that a utility system must supply.
- Simple payback
- Installed cost divided by recurring annual saving, without time-value adjustment.
Utilities, Economics and System Boundaries FAQ
Why state the boundary before comparing costs?
A cost can disappear from one option only because its equipment or utility supply has been moved outside the calculation. An explicit common boundary prevents that false saving. It also keeps supply and treatment costs aligned.
Is the shortest payback always the best design?
No. Payback is a screen. Reliability, project life, maintenance, safety, operating flexibility and discounted cash flow may change the final decision. Use a common project life and risk basis as well.
How do I screen a heating or cooling utility?
Start with the process temperature requirement and check for a real thermal driving force. The utility lecture states that a hot utility must be hotter than the process it heats, while a cold utility must be colder than the process it cools. Only after that feasibility check should cost, reliability, controllability and site availability decide between workable services.
Assessment move
For each economic problem, draw a box around the compared system, label every crossing, prove technical equivalence, annualise with visible units and test at least one uncertain input. Begin with the process duty rather than the preferred utility. Write the required temperature, pressure, flow and operating period, then eliminate options that cannot supply them.
Create a cost ledger with installed cost, energy, consumables, maintenance and downtime on separate lines. Carry kW, hours per year, kWh and dollars per kWh through the equation so unit cancellation checks the calculation. Ask whether the same pumps, cooling tower, treatment and waste services are included. Recompute the decision at a lower operating time and a higher tariff. Explain whether the recommendation changes and why.
For project work, record the source date and basis of every price and state which items require current vendor confirmation. Finish with one engineering sentence that relates the economic result back to technical performance, reliability, safety or environmental consequence rather than treating the cheapest number as self-justifying.
Practise boundary selection with three versions of the same case: equipment-only, utility-system and site-wide. List which pumps, treatment units, emissions and waste streams enter each version, then explain which boundary matches a replacement decision and which matches a location decision.
Build a utility feasibility table before costing, with required temperature, pressure, quality, reliability and flow against what each supply can deliver. Rework an annual-cost example using consistent operating hours, tariff units and maintenance scope. Separate physical consumption from price so either assumption can be changed independently.
Add a simple-payback calculation, then name what it omits and identify a life-cycle or reliability consideration that could overturn it. Run high and low cases for utilisation, efficiency and tariff and present the switching condition at which the preferred option changes. For project use, record the source date, currency, location and inclusions for each cost, then mark vendor confirmation items.
End with a recommendation that states both the economic result and the technical reason the options are genuinely comparable. Keep a separate uncertainty list for hours, tariff, efficiency, maintenance and disposal. Rank those inputs by their ability to change the recommendation, then spend verification effort on the variables with the largest decision impact.
Keep the final recommendation tied to the stated basis, and record the observation that would require a revised engineering decision.
