Thermal Utilities and Heat-Recovery Systems

Introduction and Context

Thermal utilities are the systems that generate, distribute, transfer, recover, and control heat in industrial plants. They include boilers, steam and condensate networks, furnaces, kilns, hot-oil systems, heat exchangers, cogeneration plants, and waste-heat-recovery equipment. Although these systems are often treated as separate units, they form an interconnected energy system. A poorly tuned burner increases fuel consumption, raises emissions, and may overload heat-recovery equipment. Poor steam-trap maintenance increases condensate losses and reduces boiler capacity. Inadequate insulation increases surface temperature, energy waste, and personnel risk.

For mechanical engineers, thermal-utility analysis requires more than selecting equipment. It requires energy balances, combustion analysis, equipment-performance evaluation, economic judgment, and strict attention to pressure, temperature, fire, explosion, and chemical hazards. The central engineering objective is to provide the required process heat safely, reliably, and at minimum life-cycle cost.

Learning Objectives

After studying this chapter, a student should be able to:

1. Explain the operating principles and performance measures of boilers, furnaces, kilns, steam systems, and heat exchangers.
2. Perform basic mass, energy, combustion, and heat-recovery calculations.
3. Diagnose losses in steam distribution and condensate-return systems.
4. Select appropriate insulation and evaluate its thermal and safety benefits.
5. Compare heat-recovery, cogeneration, and waste-heat-utilization options.
6. Identify unsafe or inefficient operating conditions and propose practical improvements.
7. Interpret performance data while recognizing measurement uncertainty and equipment limitations.

1. Energy Fundamentals of Thermal Utilities

The first law of thermodynamics provides the basis for utility assessment. For a steady-flow control volume, neglecting changes in kinetic and potential energy,

where Q̇ is heat-transfer rate, Ẇ is shaft-work rate, ṁ is mass-flow rate, and h is specific enthalpy. Units must be consistent, such as kW for energy rates, kg/s for mass flow, and kJ/kg for enthalpy. The equation applies to steady operation; transient storage must be included when equipment is being started, stopped, or rapidly changing load.

For a boiler, a simplified energy balance is:

The useful steam-generation rate is commonly evaluated from

where ṁ_s is steam mass flow, h_s is steam enthalpy at outlet conditions, and h_fw is feedwater enthalpy. Boiler thermal efficiency on a direct fuel-input basis is

where ṁ_f is fuel mass flow and LHV is the lower heating value of the fuel. The lower heating value is used when the latent heat of water-vapor condensation is not recovered. If a higher-heating-value basis is used, it must be stated explicitly.

Important losses include dry flue-gas sensible heat, moisture in fuel and combustion air, incomplete combustion, radiation and convection from surfaces, blowdown, and unaccounted losses. An indirect efficiency calculation estimates each loss and subtracts the total from 100 percent. Direct and indirect methods should agree within the uncertainty of measurements.

2. Boilers and Steam Generation

2.1 Boiler types

Fire-tube boilers pass hot combustion gases through tubes surrounded by water. They are comparatively simple, have relatively large water inventories, and are commonly used for low- and medium-pressure applications. Water-tube boilers pass water through tubes heated externally by combustion gases. They are suitable for higher pressures, higher capacities, and faster load response, but require more sophisticated water treatment and control.

Other important classifications include:

• Packaged boilers: Factory-assembled units with burners, controls, fans, and auxiliaries.
• Waste-heat boilers: Boilers that generate steam from hot process gases rather than direct firing.
• Once-through boilers: Units without a large recirculating water inventory, often used where rapid response is valuable.
• Electric boilers: Units that convert electrical energy into steam; they have no combustion emissions at the site but may have significant upstream emissions and operating cost.

2.2 Boiler performance and controls

A boiler must maintain steam pressure, water level, combustion stability, and safe limits under changing demand. Essential control loops include fuel–air ratio control, drum-level control, steam-pressure control, feedwater control, and furnace-pressure control. Oxygen trim can adjust excess air using flue-gas oxygen measurements, but oxygen alone is not a complete combustion diagnostic because air leakage, carbon monoxide, gas stratification, and analyzer drift can distort the interpretation.

Excess air is necessary to reduce incomplete combustion, but excessive air carries additional nitrogen and unused oxygen through the stack, increasing stack loss. Too little air causes carbon monoxide, smoke, unburned fuel, flame instability, and potentially explosive mixtures. The optimum is therefore a controlled operating range rather than the minimum possible air flow.

Boiler blowdown removes dissolved and suspended solids from the boiler. If the concentration factor is defined as

then, under a simplified steady-state solids balance with negligible solids in steam,

and therefore

where B is blowdown flow and F is feedwater flow. This relation assumes constant dissolved-solids concentration, no other significant solids removal, and consistent units. Excessive blowdown wastes hot water and treatment chemicals; inadequate blowdown promotes scale, corrosion, and carryover.

2.3 Boiler-water safety

Water treatment may include softening, demineralization, deaeration, chemical oxygen scavenging, pH control, and conductivity-based blowdown. Oxygen causes corrosion, while scale reduces heat transfer and can cause tube overheating. Steam carryover may damage turbines, control valves, and heat exchangers. Boiler operation must comply with applicable pressure-vessel legislation, inspection rules, burner-management requirements, and site procedures. Safety valves, low-water cutoffs, flame-failure protection, purge sequences, interlocks, and emergency shutdown systems must never be bypassed during normal operation.

3. Furnaces, Ovens, and Kilns

Furnaces and kilns supply heat directly to a material or process. Furnaces commonly heat metals, glass, ceramics, and chemical feedstocks. Kilns are used for drying, calcination, firing, clinker production, and material transformation. The thermal design depends on temperature, residence time, atmosphere, material movement, heat-transfer mode, and product quality.

Heat transfer occurs by radiation, convection, and conduction. At high furnace temperatures, radiation is often dominant. The net radiative exchange between idealized surfaces can be represented by

where ε is effective emissivity, σ is the Stefan–Boltzmann constant, A is exchange area, and T₁ and T₂ are absolute temperatures in kelvin. This simplified equation applies to idealized direct exchange; real furnaces require view factors, gas emissivity, wall effects, and nonuniform temperatures.

Furnace efficiency may be defined as useful heat absorbed by the product divided by fuel energy input:

A furnace can have good combustion efficiency but poor overall efficiency if much of the heat leaves in exhaust gas or through furnace walls. Practical improvements include reducing air leakage, optimizing burner staging, maintaining correct material loading, improving sealing, recovering exhaust heat, and matching firing rate to production demand.

Kilns require particular attention to residence time and temperature uniformity. Increasing firing temperature may accelerate production but can damage product, increase refractory degradation, and raise nitrogen-oxide emissions. Engineers must evaluate product quality and environmental performance along with fuel consumption.

4. Combustion Analysis

4.1 Stoichiometric combustion

For a general hydrocarbon fuel C_xH_y, complete combustion is

The stoichiometric oxygen requirement is therefore x + y÷4 moles of oxygen per mole of fuel. Since dry air contains approximately 21 percent oxygen by volume, the theoretical dry-air requirement is approximately

This approximation assumes air composition near 21 percent oxygen by volume and neglects argon and variations in humidity. For accurate mass calculations, the complete dry-air composition should be used.

Excess air is commonly defined by

or equivalently using actual and stoichiometric air quantities. The oxygen measured in dry flue gas can be used to estimate excess air only under stated assumptions, including complete combustion, negligible carbon monoxide and unburned hydrocarbons, known fuel composition, and no air infiltration after combustion.

Worked Example 1: Methane combustion

Determine the theoretical oxygen and air requirement for 1 kmol of methane.

The reaction is:

CH₄ + 2O₂ → CO₂ + 2H₂O.

Thus, oxygen required is 2 kmol. Using 21 percent oxygen by volume,

n_air = 2 ÷ 0.21 = 9.52 kmol of air.

If 10.5 kmol of air is supplied, excess air is

% excess air = [(10.5 − 9.52) ÷ 9.52] × 100 ≈ 10.3%.

The result is valid for complete combustion and assumes that the supplied air quantity is accurately known.

4.2 Combustion diagnostics

A practical combustion survey should measure fuel flow, combustion-air flow where possible, flue-gas oxygen, carbon monoxide, carbon dioxide, temperature, draft, and load. A high oxygen reading may indicate excessive air or air leakage. High carbon monoxide indicates incomplete combustion or poor mixing. High stack temperature may indicate fouled heat-transfer surfaces, excessive air, inadequate heat recovery, or a process requiring high exhaust temperature. Measurements should be taken at stable load and at several loads because one operating point cannot represent the whole duty cycle.

Combustion optimization must not sacrifice flame stability or safety. Burner-management systems require pre-purge, ignition proving, flame supervision, fuel shutoff valves, and controlled restart. The hazard of furnace explosions is particularly serious during light-off, flame failure, and accumulation of unburned fuel.

5. Steam Distribution and Condensate Recovery

Steam networks transport energy from boilers to users. A well-designed system minimizes pressure drop, heat loss, water hammer, corrosion, and contamination while providing dry steam at the required pressure.

5.1 Steam quality and pressure reduction

Steam quality affects heat transfer and equipment reliability. Wet steam contains liquid water, which reduces effective enthalpy and can cause erosion and water hammer. Superheated steam contains sensible heat above the saturation temperature and is often used where condensation must be avoided, such as turbine admission. Process heat exchangers commonly use saturated steam because condensation provides a large and relatively constant heat-transfer temperature.

Pressure-reducing valves lower steam pressure but do not recover the pressure energy. Where continuous pressure reduction is required and sufficient flow exists, a back-pressure turbine or expansion turbine may produce useful power, subject to economic and operational feasibility.

5.2 Distribution losses

Steam losses arise from uninsulated pipes, leaking valves and flanges, failed steam traps, open bypasses, poor drainage, and excessive pressure. Pipes should be properly supported, drained through drip legs, and arranged with suitable slopes. Steam traps remove condensate and non-condensable gases while retaining live steam. Common types include mechanical float and inverted-bucket traps, thermostatic traps, and thermodynamic disc traps. Each has appropriate pressure, load, and maintenance ranges.

Water hammer occurs when condensate is accelerated by steam, pressure changes, or poor drainage. It can damage supports, valves, elbows, and heat exchangers. Prevention requires correct pipe slope, adequately sized drip legs, functioning traps, gradual valve opening, and elimination of flooded sections.

5.3 Condensate recovery

Returning hot condensate reduces make-up-water demand, chemical-treatment cost, and boiler fuel consumption. The approximate energy saved by returning condensate is

where ṁ_c is condensate-return flow, h_c is condensate enthalpy at return conditions, and h_makeup is make-up-water enthalpy. The calculation must account for flash steam, return pressure, contamination risk, pumping energy, and heat losses.

Condensate from processes that may contain chemicals, oils, or product must not be returned without reliable monitoring and segregation. A contaminated return can damage the boiler and contaminate the entire water system. Conductivity, pH, hydrocarbon detection, and automatic diversion may be appropriate controls.

6. Insulation and Thermal Protection

Insulation reduces heat transfer from hot equipment to the environment and limits personnel exposure. Common materials include mineral wool, calcium silicate, cellular glass, ceramic fiber, and aerogel-based systems. Selection depends on temperature, moisture, mechanical strength, fire behavior, chemical compatibility, maintenance access, and cost.

For a plane wall under one-dimensional steady conduction, heat loss is approximated by

where k is thermal conductivity, A is area, L is wall thickness, and T₁ − T₂ is the temperature difference. For cylindrical insulation, radial resistance is more appropriate:

where L_z is cylinder length and r₁, r₂ are inner and outer radii. These relations neglect contact resistance and surface convection/radiation; complete design requires all thermal resistances in series.

Insulation must be protected from water ingress, mechanical damage, and ultraviolet exposure where relevant. Wet insulation can have much higher thermal conductivity and may conceal corrosion under insulation. Removable insulation covers are useful for valves, strainers, flanges, and maintenance points. Personnel-protection insulation should be evaluated separately from energy-saving insulation because a surface may be safe to touch yet still lose economically significant heat.

7. Heat Exchangers

Heat exchangers transfer heat between fluids without necessarily mixing them. Common designs include shell-and-tube, plate, air-cooled, double-pipe, scraped-surface, and spiral exchangers.

The fundamental heat-transfer rate is

where U is overall heat-transfer coefficient, A is heat-transfer area, F is a correction factor for non-countercurrent arrangements, and ∆T_lm is the log-mean temperature difference. For a true countercurrent exchanger,

The terminal temperature differences must be defined consistently. For countercurrent flow, ∆T₁ and ∆T₂ are the temperature differences at the two ends. The equation assumes approximately steady conditions, known fluid properties, and a representative overall coefficient.

The overall coefficient includes convection, wall conduction, fouling, and contact resistances. Fouling lowers U, increases pressure drop, and may create hot spots. Performance monitoring should compare measured duty and pressure drop with clean and design conditions. Approaches that are too small may indicate inadequate area, fouling, flow maldistribution, or incorrect temperature measurement.

Worked Example 2: Heat-exchanger duty

Water is heated from 30 °C to 70 °C at 2 kg/s. Taking the specific heat as 4.18 kJ/(kg·K),

This is the heat absorbed by the water. The hot-side duty is approximately 334.4 kW only if heat loss to the surroundings is negligible. If the measured hot-side duty is substantially higher, the discrepancy may arise from heat loss, measurement error, unaccounted phase change, or inconsistent property data.

8. Cogeneration and Waste-Heat Recovery

Cogeneration, or combined heat and power (CHP), produces electricity and useful thermal energy from the same fuel. A simple CHP system may consist of a gas turbine, steam turbine, reciprocating engine, or fuel cell coupled to a heat-recovery unit.

Electrical efficiency is

Useful-thermal efficiency is

and total CHP efficiency is

These efficiencies are additive only when the useful heat and electric output are defined on compatible energy bases and exported energy is not double-counted. CHP is most attractive where electricity and heat demands occur simultaneously and continuously. A system that produces unwanted heat may have high nominal efficiency but poor economic performance.

Waste-heat recovery options include economizers for boiler feedwater, air preheaters, combustion-air preheaters, heat-recovery steam generators, hot-water loops, absorption chillers, and process preheaters. Selection depends on source temperature, flow variability, fouling potential, allowable pressure drop, corrosion, and the temperature required by the heat sink.

A recovery system should follow the heat hierarchy: first reduce unnecessary demand, then recover heat within the process, then export or upgrade heat for other users. Heat recovery should not be installed merely because a hot exhaust stream exists; the recovered heat must have a reliable, technically compatible sink.

9. Applied Case: Illustrative Boiler and Condensate Improvement Project

Background

An illustrative food-processing plant operates a 10 t/h saturated-steam boiler. Steam pressure is 10 bar absolute, condensate return is only 35 percent, and stack temperature is 245 °C. Operators report frequent steam-trap failures and fluctuating boiler-water conductivity.

Problem analysis

The plant survey identifies four principal issues:

1. Several process areas discharge hot condensate to drain because contamination is suspected but not monitored.
2. Failed-open traps create a continuous live-steam loss.
3. Boiler blowdown is controlled manually and is higher than necessary during low-load operation.
4. The economizer is partially fouled, producing high stack temperature.

Fuel and steam meters are checked, and a temporary combustion test confirms that oxygen is higher than the target operating range at part load. The plant therefore has both heat-transfer and control losses.

Approach and solution

The improvement program includes:

• Installing conductivity and hydrocarbon monitoring at selected condensate-return points.
• Returning clean condensate and automatically diverting suspected contamination.
• Surveying traps with temperature and ultrasonic methods, then repairing or replacing failed units.
• Recommissioning the economizer and establishing a cleaning interval based on pressure drop and stack temperature.
• Implementing continuous or modulating blowdown control based on conductivity.
• Tuning the burner across the normal load range while verifying carbon monoxide, flame stability, and safety-interlock operation.
• Insulating exposed valves and sections of steam piping.

If the program raises condensate return from 35 to 70 percent of a 10 t/h steam load, the additional clean condensate is approximately 3.5 t/h, subject to actual steam balance and process losses. The fuel saving must be calculated from the enthalpy difference between hot condensate and make-up water and from the boiler efficiency. A measured result is preferable to assuming a fixed percentage saving.

Outcome and learning points

The project demonstrates that the largest improvement may not come from replacing the boiler. Measurement, trap maintenance, water-quality control, heat-exchanger cleaning, and operating discipline can produce substantial savings with lower capital cost. It also demonstrates that condensate recovery cannot be separated from contamination control and process safety.

10. Performance Assessment and Decision-Making

A thermal-utility audit should establish a reliable baseline before recommending investment. The baseline should include production rate, operating hours, fuel type and quantity, steam generation, steam pressure, condensate return, water consumption, electricity use, emissions, and major process temperatures.

Useful performance indicators include:

• Fuel consumed per tonne of product.
• Steam generated per unit of fuel.
• Condensate-return percentage.
• Boiler efficiency and stack temperature.
• Steam leakage rate and trap-failure rate.
• Heat-exchanger duty and pressure drop.
• CHP electrical, thermal, and total efficiencies.
• Unplanned downtime caused by utility failures.

Investment decisions should include capital cost, energy-price assumptions, maintenance, production effects, reliability, safety, and environmental compliance. Simple payback is useful for screening:

It ignores time value of money, equipment life, taxes, and residual value, so it should not be the only criterion for major projects. Net-present-value analysis is more appropriate when cash flows, project life, and discount rate are available.

11. Safety, Environmental, and Regulatory Considerations

Thermal utilities combine stored pressure, high temperature, combustion, electricity, rotating machinery, and chemicals. A safe design requires hazard identification, pressure relief, guarding, interlocks, controlled access, lockout/tagout, emergency procedures, inspection, and competent operation.

Major hazards include boiler explosion, furnace flashback, fuel leakage, carbon-monoxide exposure, steam burns, water hammer, oxygen deficiency, chemical exposure from treatment systems, and confined-space entry. Risk assessment should address normal operation, start-up, shutdown, abnormal conditions, maintenance, and foreseeable human error.

Environmental controls include nitrogen-oxide and carbon-monoxide management, particulate control, sulfur emissions where applicable, wastewater management, noise control, and prevention of contaminated condensate discharge. Engineers have an ethical responsibility to avoid manipulating measurements or disabling alarms to meet production targets. Energy savings are not acceptable if they increase unacceptable risk or violate emission and pressure-equipment requirements.

12. Contemporary Developments

Digital combustion controls, wireless sensors, thermal imaging, remote condition monitoring, and data historians are increasingly used to improve thermal utilities. Machine-learning tools can identify abnormal patterns in stack temperature, steam demand, trap behavior, and heat-exchanger fouling. However, artificial intelligence should support—not replace—engineering judgment. Models require representative, validated data and should not be permitted to override independent safety systems.

Industrial decarbonization is encouraging electrification, renewable fuels, hydrogen blending, high-temperature heat pumps, thermal storage, and carbon-capture integration. Each option has limitations. Electrification may reduce local combustion emissions but can increase operating cost or upstream emissions. Hydrogen changes flame behavior and materials compatibility. Heat pumps are highly effective for low- and medium-temperature duties but may not supply high-temperature kiln loads economically. Technology selection must therefore be based on the temperature level, load profile, infrastructure, safety, and life-cycle emissions.

Best-Practice Checklist

1. Measure fuel, steam, water, condensate, temperatures, pressures, oxygen, and carbon monoxide at stable loads.
2. Use consistent energy and enthalpy bases when calculating efficiency.
3. Maintain burners, flame safeguards, purge systems, and fuel shutoff devices.
4. Control excess air without compromising combustion safety.
5. Inspect insulation and protect it from moisture and mechanical damage.
6. Maintain steam traps, drip legs, valves, and condensate pumps.
7. Recover clean condensate, but segregate and monitor potentially contaminated streams.
8. Clean heat exchangers according to performance indicators, not only calendar intervals.
9. Match waste heat to a dependable heat sink.
10. Validate savings with post-project measurement and verification.

Chapter Summary

Thermal utilities are integrated energy systems rather than isolated items of equipment. Boilers convert fuel or recovered heat into steam, while furnaces and kilns transfer heat directly to materials. Their performance depends on combustion quality, heat transfer, control, insulation, maintenance, and operating load. Boiler efficiency is evaluated from useful steam enthalpy rise relative to fuel energy, while combustion analysis uses stoichiometric relationships, excess-air assessment, and flue-gas measurements.

Steam-system efficiency depends strongly on distribution design, steam quality, trap performance, drainage, insulation, and condensate recovery. Heat exchangers are evaluated using energy balances and the overall heat-transfer relation based on log-mean temperature difference. Cogeneration can provide high total energy utilization when electrical and thermal demands are well matched. Waste-heat recovery is valuable only when a compatible and dependable heat sink exists.

Safe improvement requires reliable measurement, proper water treatment, functioning safeguards, pressure-equipment compliance, contamination control, and consideration of abnormal operating conditions. Modern digital monitoring and low-carbon technologies offer important opportunities, but they must be implemented with validated data, life-cycle evaluation, and independent safety protection.

Key Terms

Boiler: Pressure vessel that generates steam or hot water by transferring heat to water.

Fire-tube boiler: Boiler in which hot combustion gases flow inside tubes surrounded by water.

Water-tube boiler: Boiler in which water flows inside tubes heated by combustion gases.

Boiler efficiency: Useful energy transferred to steam divided by fuel energy input.

Blowdown: Controlled removal of boiler water to limit dissolved-solids concentration.

Excess air: Combustion air supplied above the theoretical stoichiometric requirement.

Stoichiometric combustion: Combustion with the exact theoretical reactant quantity for complete reaction.

Furnace: Equipment that supplies heat directly to a process or material.

Kiln: High-temperature process equipment used for drying, firing, calcination, or similar treatment.

Steam trap: Automatic device that removes condensate and non-condensable gases while retaining steam.

Water hammer: Pressure shock caused by rapid movement or condensation of liquid in a piping system.

Condensate recovery: Collection and return of condensed steam to the boiler or process system.

Insulation: Material or system that reduces heat transfer and limits surface temperature.

Heat exchanger: Equipment that transfers heat between fluids.

Fouling: Accumulation of deposits that increases thermal resistance and pressure drop.

Cogeneration: Simultaneous production of electricity and useful heat from one fuel source.

Waste-heat recovery: Utilization of heat that would otherwise be rejected to the environment.

Economizer: Heat exchanger that uses flue-gas heat to preheat boiler feedwater.

Lower heating value: Fuel energy basis excluding recovery of water-vapor condensation heat.

Oxygen trim: Control adjustment based on measured flue-gas oxygen concentration.

Review Questions

Short Answer Questions

1. Distinguish between fire-tube and water-tube boilers.
2. Why is excessive combustion air undesirable?
3. State two functions of a steam trap.
4. What is the purpose of boiler blowdown?
5. Define cogeneration.

Long Answer Questions

1. Explain the principal losses in a fuel-fired boiler and describe methods for reducing each loss.
2. Discuss the design and operational causes of water hammer in steam systems.
3. Explain how condensate recovery affects fuel, water, and chemical consumption.
4. Describe the factors that influence heat-exchanger performance and fouling.
5. Compare direct and indirect methods of evaluating boiler efficiency.

Analytical / Application Questions

1. A boiler produces 5,000 kg/h of steam. Steam enthalpy is 2,780 kJ/kg, feedwater enthalpy is 420 kJ/kg, and fuel input is 3,900,000 kJ/h. Calculate the direct boiler efficiency and state the assumptions required.
2. A heat exchanger heats 1.5 kg/s of water from 25 °C to 65 °C. Using c_p = 4.18 kJ/(kg·K), calculate the required heat-transfer rate. Explain two reasons why the actual hot-side duty may differ.
3. A plant has a high stack temperature, low condensate return, and frequent trap failures. Develop a prioritized investigation and improvement plan, including safety checks and measurements.

Multiple Choice Questions with Answers

Question 1

Which expression represents the direct fuel-input boiler efficiency?

A. η_b = ṁ_f ÷ Q̇_steam

B. η_b = Q̇_steam ÷ (ṁ_f × LHV)

C. η_b = Q̇_loss ÷ Q̇_fuel

D. η_b = h_fw ÷ h_s

Correct Answer: B. η_b = Q̇_steam ÷ (ṁ_f × LHV)

Question 2

For complete combustion of 1 kmol of methane, how many kmol of oxygen are theoretically required?

A. 0.5 kmol

B. 1 kmol

C. 2 kmol

D. 4 kmol

Correct Answer: C. 2 kmol

Question 3

What is the principal purpose of boiler blowdown?

A. To increase dissolved-solids concentration

B. To remove accumulated dissolved and suspended solids

C. To raise stack temperature

D. To superheat steam

Correct Answer: B. To remove accumulated dissolved and suspended solids

Question 4

Which condition is most directly associated with excessive combustion air?

A. Increased dry flue-gas heat loss

B. Reduced nitrogen flow through the furnace

C. Guaranteed elimination of carbon monoxide

D. Lower total flue-gas volume

Correct Answer: A. Increased dry flue-gas heat loss

Question 5

A water stream flows at 2 kg/s and is heated from 30 °C to 70 °C. If c_p = 4.18 kJ/(kg·K), what is the heat-transfer rate?

A. 83.6 kW

B. 167.2 kW

C. 334.4 kW

D. 668.8 kW

Correct Answer: C. 334.4 kW

Question 6

Which device removes condensate from a steam line while retaining live steam during normal operation?

A. Economizer

B. Steam trap

C. Air preheater

D. Deaerator

Correct Answer: B. Steam trap

Question 7

What is a major engineering requirement for successful waste-heat recovery?

A. The heat source must always be above 1,000 °C

B. The recovered heat must have a compatible and dependable heat sink

C. The recovery exchanger must have zero pressure drop

D. The process must use only saturated steam

Correct Answer: B. The recovered heat must have a compatible and dependable heat sink

Question 8

In a countercurrent heat exchanger, the log-mean temperature difference is calculated from:

A. The arithmetic mean of mass flow rates

B. The difference between fluid heat capacities only

C. The two terminal temperature differences

D. The ratio of pressure drop to flow rate

Correct Answer: C. The two terminal temperature differences

Question 9

Which practice best reduces the risk of water hammer?

A. Rapidly opening cold steam valves

B. Removing all drip legs

C. Providing proper drainage, slope, and functioning traps

D. Operating every line flooded with condensate

Correct Answer: C. Providing proper drainage, slope, and functioning traps

Question 10

Cogeneration is most attractive when:

A. Useful heat demand is absent

B. Electricity and useful heat demands occur in a compatible time pattern

C. The plant rejects all recovered heat

D. The prime mover operates only during shutdown

Correct Answer: B. Electricity and useful heat demands occur in a compatible time pattern