Control of boilers, distillation columns and heat exchangers

Boiler drum-level, pressure and combustion control, distillation column configurations, and heat-exchanger temperature control with energy and material balances.

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Why it matters

Boilers, distillation columns and heat exchangers are the workhorses of power plants, refineries and chemical units, and their control schemes are the classic applications of everything in this subject — cascade, feedforward, ratio, override and safety systems. Interviewers for core instrumentation jobs almost always ask about three-element drum-level control or column control.

Key ideas

Boiler (drum-type) control.

  • Drum level. Too low risks tube overheating; too high carries water into the turbine. When steam demand rises, drum pressure falls and steam bubbles in the risers expand, so the level first rises (swell) although water inventory is falling; a load drop causes shrink. This inverse response defeats simple level control.
    • Single-element: level controller → feedwater valve. Adequate only for small, steady boilers.
    • Two-element: adds steam-flow feedforward, so feedwater moves with demand.
    • Three-element: the level controller output is added to the steam-flow signal to form the set point of a feedwater flow controller (feedforward + cascade). Steam flow sets the feedwater demand, the flow loop rejects feedwater pressure disturbances, and the level controller only trims. Standard for medium and large boilers.
  • Steam pressure (boiler master). The header pressure controller sets the firing rate demand; it is the measure of balance between steam produced and steam used.
  • Combustion control. Fuel and air must change together. Metered cross-limiting control uses high and low selectors so that on a load increase air rises first and fuel follows, and on a decrease fuel falls first and air follows — the furnace never becomes fuel-rich. Flue-gas O₂ (or CO) trim adjusts the air/fuel ratio for efficiency.
  • Furnace draft (balanced-draft units) is held slightly negative by the induced-draft fan; steam temperature is controlled by spray attemperation, usually as a cascade on attemperator outlet temperature.
  • Burner management system (BMS) — purge, ignition, flame supervision and trips — is a safety instrumented system, separate from combustion control.

Distillation column control.

  • Inventory and pressure first. Column pressure is held by condenser duty, vent or hot-vapour bypass. Reflux-drum level and bottoms level must be controlled, and the choice of which streams do it defines the configuration.
  • Configurations. LV: reflux L and boilup V control compositions; distillate D controls drum level and bottoms B controls sump level. Common and robust to level tuning. DV: distillate and boilup control compositions; reflux controls drum level — good for high-reflux columns. LB and ratio schemes (L/D, V/B) are also used.
  • Composition measurement. Analysers are slow and expensive, so a sensitive tray temperature is used as an inferential composition measurement, often with pressure compensation.
  • Interaction. Reflux affects both top and bottom compositions, as does boilup, so two-point composition control interacts; decoupling or model predictive control (MPC) is common in refineries. Feed-flow feedforward (keeping L/F and V/F constant) reduces upsets.

Heat exchanger control.

  • Steam heater: outlet temperature controller cascaded to a steam flow controller (or steam pressure in the shell); feedforward from process flow and inlet temperature. A condensate trap or level control keeps the shell drained.
  • Liquid–liquid exchanger: manipulate the utility flow, or use a three-way valve to bypass part of the process stream for fast response.
  • Process-to-process exchangers in heat integration often use bypass control because neither stream can be throttled freely.

Formulas

F_fw,sp = F_steam + Δ_LC

  • Three-element drum level: F_fw,sp feedwater flow set point (kg/s); F_steam measured steam flow (kg/s); Δ_LC level controller output scaled to flow (kg/s).

Q = ṁ_p·c_p·(T_out − T_in) = ṁ_s·h_fg

  • Steam heater energy balance: ṁ_p process flow (kg/s); c_p (kJ/kg·K); T in °C or K; ṁ_s steam flow (kg/s); h_fg latent heat at shell pressure (kJ/kg, from steam tables); Q in kW. Basis for feedforward.

F = D + B; F·z_F = D·x_D + B·x_B → D/F = (z_F − x_B)/(x_D − x_B)

  • F, D, B: feed, distillate and bottoms flows (kmol/h); z_F, x_D, x_B: light-key mole fractions.

R = L / D; V = L + D

  • Reflux ratio; vapour to the condenser (total condenser).

EA ≈ O₂ / (21 − O₂) × 100 %

  • Approximate excess air from dry flue-gas O₂ in volume %, for typical hydrocarbon fuels.

Worked examples

Example 1 (standard) — steam heater feedforward and O₂ trim. Water at 10 kg/s (c_p = 4.18 kJ/kg·K) is heated from 30 °C to 80 °C by steam condensing with h_fg = 2100 kJ/kg (take from steam tables in practice). (a) Find the steam flow. (b) The process flow rises to 12 kg/s; what steam flow should feedforward set? (c) The boiler supplying this steam shows 3 % O₂ in dry flue gas. Estimate the excess air.

  1. Q = 10 × 4.18 × (80 − 30) = 2090 kW.
  2. ṁ_s = 2090/2100 = 0.995 kg/s.
  3. (b) Steam/process ratio = 0.995/10 = 0.0995; at 12 kg/s: ṁ_s = 12 × 4.18 × 50/2100 = 1.19 kg/s. The temperature controller only trims this ratio.
  4. (c) EA ≈ 3/(21 − 3) × 100 = 16.7 %.

Example 2 (GATE level) — column material balance. A column receives F = 100 kmol/h of saturated-liquid feed with z_F = 0.40. Specifications: x_D = 0.95, x_B = 0.05. It runs with reflux ratio R = 3, total condenser, constant molal overflow. Find D, B, L, V and the boilup. Then state what an LV control scheme does when the feed flow rises by 10 %.

  1. D/F = (0.40 − 0.05)/(0.95 − 0.05) = 0.35/0.90 = 0.3889 → D = 38.9 kmol/h.
  2. B = 100 − 38.9 = 61.1 kmol/h.
  3. L = R·D = 3 × 38.9 = 116.7 kmol/h.
  4. V = L + D = 116.7 + 38.9 = 155.6 kmol/h; with saturated-liquid feed the boilup equals V = 155.6 kmol/h.
  5. With LV control and feed-forward ratios L/F and V/F, a 10 % feed increase raises L to about 128.3 and V to about 171.1 kmol/h; the level controllers then let D and B rise by 10 % to 42.8 and 67.2 kmol/h, keeping compositions near specification.

Common mistakes

  • Using single-element level control on a large boiler and fighting swell and shrink: the controller cuts feedwater just when more is needed.
  • Letting fuel lead air on a load increase — the reason cross-limiting exists.
  • Trying to control both distillate and bottoms compositions with flows used for level control, or controlling compositions before pressure and levels are stable.
  • Choosing a tray temperature that is insensitive to composition (near the column ends).
  • Taking the latent heat of steam at the wrong pressure when sizing steam-flow feedforward.
  • Treating the BMS as part of the combustion control loop instead of an independent safety system.

For GATE IN

GATE questions in this area are usually conceptual: identify the three-element drum-level block diagram (feedforward plus cascade), explain inverse response (swell and shrink) and its right-half-plane zero, recognise cross-limiting combustion control, and pick appropriate manipulated variables for column or exchanger loops. Short numericals use energy balances for steam heaters and material balances for columns. Practise drawing each scheme as a block diagram.

Quick check

  1. Which three signals are used in three-element drum-level control?
  2. Why does drum level rise at first when steam demand increases?
  3. F = 50 kmol/h, z_F = 0.5, x_D = 0.9, x_B = 0.1. Find D.
  4. In cross-limiting control, which leads on a load increase — fuel or air?

Answers: 1. Drum level, steam flow and feedwater flow. 2. Pressure falls and steam bubbles expand (swell), raising the apparent level although water inventory falls. 3. D = 25 kmol/h. 4. Air leads; fuel follows.

Interview questions

All Process Control and Automation interview questions

Try answering each one aloud before you open it.

  1. 1.What are swell and shrink in a boiler drum?Concept

    When steam demand rises suddenly, drum pressure falls, water flashes and the steam bubbles in the risers expand, so the measured drum level rises temporarily even though water is being lost faster — this is swell. When demand falls, pressure rises, bubbles collapse and the level drops temporarily — shrink. This inverse (non-minimum-phase) response is why drum level needs steam-flow feedforward (two- or three-element control) and cautious level-controller tuning.

  2. 2.Explain the role of a distillation column in a chemical processing plant.Concept

    A distillation column is used to separate mixtures based on differences in their volatilities in a boiling liquid mixture. It is a critical component in chemical processing plants for purifying chemicals, separating crude oil into fractions, and producing high-purity solvents. The column works by heating the mixture, causing components to vaporize at different temperatures, and then condensing them at various levels within the column.

  3. 3.How does a heat exchanger function in an industrial process?Concept

    A heat exchanger transfers heat between two or more fluids without mixing them. It is used in industrial processes to efficiently manage heat, either by heating a fluid using waste heat from another process or by cooling a fluid using a cooler fluid. Common types include shell-and-tube, plate, and air-cooled heat exchangers, each designed for specific applications and efficiency requirements.

  4. 4.Explain three-element drum level control in a boiler and why it is used.Application

    Three-element control uses drum level, steam flow and feedwater flow. Steam flow acts as a feedforward signal that sets the basic feedwater demand, the drum level controller output is added to it as a trim, and the sum is the set point of a feedwater flow controller, which forms a cascade inner loop. It is used because swell and shrink make the level move the wrong way at first after a load change, so a level-only controller would cut feedwater just when more is needed; the feedforward responds to the real mass balance, and the flow loop removes feedwater pressure disturbances.

  5. 5.What happens if the reflux ratio in a distillation column is too high?Application

    If the reflux ratio in a distillation column is too high, it can lead to increased energy consumption and operational costs. While a higher reflux ratio can improve the separation efficiency, it also requires more reboiler heat and condenser cooling, which can be inefficient. Additionally, excessive reflux can lead to flooding in the column, reducing its capacity and potentially causing operational issues.

  6. 6.Describe the impact of fouling on heat exchanger performance.Application

    Fouling in heat exchangers refers to the accumulation of unwanted materials on the heat transfer surfaces, which can significantly reduce the heat exchanger's efficiency. Fouling increases thermal resistance, leading to reduced heat transfer rates and higher energy consumption to achieve the desired temperature change. It can also cause pressure drops and flow restrictions, necessitating more frequent maintenance and cleaning.

  7. 7.Calculate the heat transfer rate in a heat exchanger if the overall heat transfer coefficient is 500 W/m²·K, the heat transfer area is 10 m², and the temperature difference is 20 K.Numerical

    The heat transfer rate (Q) can be calculated using the formula: Q = U·A·ΔT, where U is the overall heat transfer coefficient, A is the heat transfer area, and ΔT is the temperature difference. Substituting the given values: Q = 500 W/m²·K × 10 m² × 20 K = 100,000 W or 100 kW.

  8. 8.What is the effect of increasing the feed temperature on the operation of a distillation column?Application

    Increasing the feed temperature in a distillation column can reduce the energy required by the reboiler, as the feed is closer to its boiling point. This can improve the column's efficiency and reduce operational costs. However, if the feed temperature is too high, it may cause vaporization before entering the column, potentially leading to operational issues such as flooding or reduced separation efficiency.

  9. 9.Explain why redundancy is important in the control systems of boilers.Application

    Redundancy in boiler control systems is important to ensure reliability and safety. By having backup systems or components, the boiler can continue to operate safely even if a primary component fails. This is crucial in preventing unplanned shutdowns, maintaining continuous operation, and ensuring the safety of personnel and equipment. Redundancy can include duplicate sensors, controllers, and communication paths.

  10. 10.A boiler operates at a pressure of 2 MPa and produces steam at a rate of 5 kg/s. Calculate the power output if the enthalpy change of the steam is 2000 kJ/kg.Numerical

    The power output (P) can be calculated using the formula: P = m·Δh, where m is the mass flow rate and Δh is the enthalpy change. Substituting the given values: P = 5 kg/s × 2000 kJ/kg = 10,000 kJ/s or 10 MW.

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