Thermodynamics and Heat Exchangers in the Engine Room

What the rule requires

What sets the rate

A heat exchanger passes heat from one fluid to another through a solid metal wall without letting the two mix, and the rate depends on the temperature difference between them, the surface area of that wall, and the overall heat-transfer coefficient, which is itself a function of the fluids, their flow velocity and the cleanliness of the surfaces DOE-HDBK-1018 Vol.1 §2-1. Fouling — scale, sludge, marine growth or an oil film — adds thermal resistance and is the most common reason a cooler loses capacity over a period of months.

Three mechanisms act at once inside the unit: heat convects from the hot fluid to the tube wall, conducts through the wall and through any scale on it, and convects away into the cold fluid DOE-HDBK-1012 §2-3. The overall coefficient is those resistances combined. Anything that adds one — fouling, an oil film, trapped air, or reduced flow velocity — lowers the coefficient and the heat transferred, which is why clean tubes, full flow and vented air are the whole of cooler maintenance.

Radiation is the odd mechanism out: it travels by electromagnetic waves and needs no medium, crossing a vacuum, and its rate climbs with the fourth power of absolute temperature. That is why it dominates around a furnace, the flame and hot gas, and glowing exhaust components. A dark, dull surface both radiates and absorbs well; a bright, polished surface reflects, which is the basis of reflective shielding around hot components.

The common marine unit is the shell-and-tube exchanger: a bundle of tubes carries one fluid, the other flows through the shell around the outside, and baffles drive the shell-side fluid back and forth across the bundle to raise velocity and turbulence . One tube sheet is fixed and the other floats, or the tubes are U-bent, so the bundle can expand and contract with temperature without buckling. Plate-type units stack thin corrugated plates forming alternating narrow passages, giving high transfer in a compact package that strips down for cleaning, and they are increasingly used on jacket-water and lube-oil duty.

Flow arrangement decides what the outlet temperatures can be

Exchangers are classified by the relative direction of the two streams DOE-HDBK-1018 Vol.1 §2-2. In parallel flow both fluids enter at the same end and run the same way; the gap is large at the inlet and shrinks toward the outlet, and the two outlet temperatures can only approach one another. In counter-flow the fluids run in opposite directions, the temperature difference is more uniform along the length, the unit is thermally more efficient, and the cold fluid can leave hotter than the hot fluid's outlet. Counter-flow is preferred wherever it is practical. Cross-flow, with the streams at right angles, turns up in radiators and charge-air coolers.

Because the gap between the streams changes along the length, the correct average for sizing or assessing any unit is the log-mean temperature difference, which weights that varying gap properly.

Seawater runs through the tubes forward to aft; the lube oil runs through the shell aft to forward. What arrangement is this, and what does it make possible that parallel flow does not?

Counter-flow — the two streams travel in opposite directions. It permits the cold fluid to leave the exchanger hotter than the hot fluid's outlet temperature, which parallel flow can never do because its two outlet temperatures can only converge toward each other.</details>

The condenser and its vacuum

A condenser removes latent heat from a vapour to return it to liquid DOE-HDBK-1018 Vol.1 §2-3. The main condenser takes turbine exhaust steam and cools it with seawater through the tubes; the steam gives up its latent heat of vaporisation, collapses to condensate, collects in the hotwell and goes back to the feed system.

The vacuum is a by-product of that collapse. Condensed water occupies a tiny fraction of the steam's volume, and the resulting low back-pressure lets the turbine take far more work out of each pound of steam, so condenser performance shows up directly in plant efficiency. Non-condensable gases — air in-leakage and gases liberated from the steam — must be drawn off continuously by an air ejector or vacuum pump. Left in place they blanket the tubes, raise back-pressure and cut heat transfer.

Three faults account for most condenser trouble: tube fouling, air in-leakage that spoils the vacuum, and seawater leakage that puts chlorides into the condensate. The last is the serious one, because salt carried into a boiler causes scale and corrosion.

The same principle governs the refrigeration condenser, where high-pressure refrigerant vapour off the compressor is condensed by seawater or air. Elsewhere in the plant the identical hardware does other duty: coolers dumping lube oil, jacket water and charge air to seawater; heaters using steam or hot water to warm fuel oil for pumping and atomisation; and evaporators boiling seawater to make fresh water.

The two laws, as an engineer uses them

The first law is conservation of energy: energy is neither created nor destroyed, only converted or moved, and heat added to a system equals the increase in its stored energy plus the work it does on its surroundings DOE-HDBK-1012 §1-3. Nothing is lost. Energy that appears to vanish has become a less useful form, usually low-temperature heat rejected to the sea or the stack.

In practice that becomes an energy balance drawn around one machine or the whole plant: everything in equals everything out plus any change in storage. Around a heat exchanger, heat given up by the hot stream equals heat gained by the cold stream, plus small losses. Around a turbine, the drop in steam enthalpy from inlet to exhaust equals shaft work plus heat losses. Around a boiler, heat released by the fuel equals heat into feedwater and steam plus stack and radiation losses. That balance is how an unknown flow or temperature is calculated, how instrument readings are checked for sense, and how wasted energy is located.

The second law fixes direction and sets the ceiling DOE-HDBK-1012 §1-4. Heat flows spontaneously only from hotter to colder, and no heat engine converts all the heat it receives into work — some must always go to a lower-temperature sink. Hence every plant needs both a source and a sink, and the exhaust and cooling water always carry off energy that cannot be recovered. Entropy is the property that measures the unavailability of energy to do work, and it increases in every real process; friction, unrestrained expansion and heat transfer across a finite temperature difference all generate it. The theoretical maximum is the Carnot efficiency, which depends on nothing but the two absolute temperatures, and the wider the gap between source and sink the higher it goes. That is the reason for running boilers as hot as the materials stand and condensers as cold as the seawater allows.

Cycles

A thermodynamic cycle returns the working fluid to its starting state while converting heat to work, so it can repeat DOE-HDBK-1012 §1-5. The Carnot cycle — two constant-temperature and two frictionless adiabatic processes — gives the highest efficiency obtainable between two temperatures but cannot be built; it is the yardstick.

The Rankine cycle is the steam plant. Walk the water round the loop and name the four jobs, which is how the cycle is taught and how it is examined:

Pump — raises the feedwater to boiler pressure.
Boiler — adds heat to make and superheat the steam.
Turbine — the steam expands, doing work.
Condenser — the exhaust is condensed back to water for the pump.

Efficiency is raised by superheating, by raising boiler pressure, by lowering condenser pressure with a better vacuum, and by regenerative feedwater heating with bled steam and by reheat.

The Diesel cycle models the compression-ignition engine: air compressed adiabatically, heat added at roughly constant pressure as the fuel burns, expansion doing work, heat rejected with the exhaust. The Otto cycle is the spark-ignition counterpart, with heat added at constant volume. In every one of them the useful work is the heat supplied minus the heat rejected.

The vocabulary the questions are written in

Temperature measures the average kinetic energy of the molecules and is read on the relative Fahrenheit and Celsius scales or the absolute Rankine and Kelvin scales, with absolute zero the point of no molecular motion DOE-HDBK-1012 §1-1. Gas-law and cycle calculations take absolute temperature. Pressure is force per unit area; gauge pressure is referenced to atmospheric, absolute pressure adds atmospheric to gauge, a perfect vacuum is zero absolute, and pressures below atmospheric such as a condenser vacuum are stated in inches of mercury vacuum.

Internal energy is stored in the motion and configuration of the molecules and rises with temperature. Heat is energy in transit under a temperature difference. Work is energy in transit as a force through a distance. Enthalpy combines internal energy with the product of pressure and volume and is what you account with for a flowing fluid. The Btu raises one pound of water one degree Fahrenheit; specific heat is the Btu needed per pound per degree for a given substance, and it differs between substances and between constant-pressure and constant-volume processes.

Telling it apart

Flow arrangement — criterion: the relative direction of the two streams

Parallel flow — same direction, both entering at the same end. The outlet temperatures can only approach one another, never cross.
Counter-flow — opposite directions, with a more uniform gap down the length and higher thermal efficiency. This is the only arrangement in which the cold fluid leaves hotter than the hot fluid's outlet.
Cross-flow — streams at right angles; radiators and charge-air coolers.

Regenerative or not — criterion: whether the recovered heat stays inside the system

Regenerative — takes heat out of the system's own outgoing stream to preheat its incoming stream, turbine exhaust warming feedwater being the standard case, which lifts cycle efficiency because energy that would have gone overboard is recycled DOE-HDBK-1018 Vol.1 §2-2.
Non-regenerative — rejects heat to an independent medium such as seawater, which is not returned to the process.

Pressure and temperature references — criterion: where zero sits

Gauge pressure — zero at atmospheric DOE-HDBK-1012 §1-1.
Absolute pressure — zero at a perfect vacuum; absolute equals gauge plus atmospheric.
Absolute temperature — zero at no molecular motion (0 R, 0 K), and required for gas-law and cycle work.

Working a question

A jacket-water cooler is counter-flow. On its sea trial sheet the fresh water entered at 190 °F and left at 170 °F at 100 lb/min, and the seawater entered at 75 °F and left at 95 °F. Three months later, at the same engine load and the same 75 °F seawater inlet, the fresh water enters at 205 °F and leaves at 185 °F, with the seawater still rising to 95 °F. Decide what has changed.

Draw the boundary and write the first-law balance. Heat given up by the jacket water equals heat picked up by the seawater, plus small losses DOE-HDBK-1012 §1-3.
Compute the duty on the hot side. One Btu raises one pound of water one degree Fahrenheit, so 100 lb/min through a 20 °F drop is 2,000 Btu/min DOE-HDBK-1012 §1-1. The drop is still 20 °F now, so the duty is unchanged — consistent with unchanged engine load.
Check the cold side before blaming the unit. The seawater rise is still 20 °F with the same inlet temperature, so the seawater flow has not fallen off. A choked strainer or a failing pump would show as a larger rise on a smaller flow.
Find the effective temperature difference then and now. On trials the gaps at the two ends were 190 − 95 = 95 °F and 170 − 75 = 95 °F. Equal ends, so no averaging is needed and the mean difference is 95 °F. Today: 205 − 95 = 110 °F and 185 − 75 = 110 °F, a mean of 110 °F.
Isolate the term that moved. Heat transferred equals coefficient times area times temperature difference DOE-HDBK-1012 §2-3. Area is fixed metal. The same 2,000 Btu/min now needs 110 °F of head where 95 °F once sufficed, so the product of coefficient and area has dropped by about 14 percent — the coefficient has fallen.
Name the resistance and act. With flows intact, added resistance means fouling, scale, an oil film or trapped air DOE-HDBK-1018 Vol.1 §2-1. Open the unit and clean the tubes; vent air; confirm velocity through the bundle.

Same cooler, same load, but this time the seawater outlet has climbed to 105 °F while the fresh-water temperatures are unchanged. What does that point to?

Reduced seawater flow. The same heat is still crossing the tubes, so a larger rise on the salt side means fewer pounds per minute are carrying it. Look at the sea suction, strainer and pump, not the tube bundle — and note that lower velocity will degrade the coefficient as well if it is left alone.</details>

Where candidates lose the point

Swapping counter-flow and parallel flow in a single-pass shell-and-tube question. The distractors are written so that only the direction word changes, and a candidate skim-reading picks the first option naming the right unit. Counter-flow is opposite directions; parallel flow enters at the same end and runs the same way.
Answering that the air ejector creates the condenser vacuum. It attracts because vacuum is lost when the ejector is lost. Condensation creates the vacuum by collapsing the steam's volume; the ejector only removes the non-condensables that would otherwise blanket the tubes and raise back-pressure.
Treating a gauge reading as an absolute pressure. Absolute equals gauge plus atmospheric, and zero absolute is a perfect vacuum, not a zero on the gauge board.
Choosing the option that says a well-designed, friction-free engine could convert all its heat into work. No heat engine can; some heat must always be rejected to a lower-temperature sink, and the ceiling is set by the two absolute temperatures alone.
Filing a chloride hit in the condensate as a feedwater-quality nuisance. It is a seawater tube leak, and the salt it carries into the boiler causes scale and corrosion.
Picking the bright, polished surface as the better radiator. Dull dark surfaces radiate and absorb well; polished surfaces reflect, which is what makes them useful as shielding rather than as absorbers.

Check yourself

The main condenser vacuum has been slipping over the watch and the turbine back-pressure is rising. The circulating water inlet temperature and flow are unchanged. What are you looking for?

Air in-leakage, or an air ejector not carrying its load. Non-condensable gases blanket the tubes, cut heat transfer and raise back-pressure, and higher back-pressure means the turbine extracts less work from each pound of steam. Tube fouling produces the same symptom over a longer timescale.</details>

A routine condensate test shows chlorides. Why does this outrank a fouling problem in urgency?

Chlorides mean seawater is leaking into the condensate through the tubes or a tube sheet, and that salt is being pumped into the boiler, where it causes scale and corrosion. Fouling costs you performance; this costs you the boiler.</details>

You are asked to compute a cycle efficiency from a source at 500 °F and a sink at 90 °F. What is the first thing you do with those two numbers?

Convert both to an absolute scale, Rankine or Kelvin. Gas-law and cycle calculations take absolute temperature; the Carnot limit depends only on the two absolute temperatures, and relative-scale figures will not give the right ratio.</details>

Which change lowers a steam plant's efficiency: superheating the steam, raising boiler pressure, raising condenser pressure, or bleeding steam to heat the feedwater?

Raising condenser pressure. Efficiency improves with superheat, with higher boiler pressure, with a better vacuum — that is, lower condenser pressure — and with regenerative feedwater heating and reheat. Losing vacuum runs the change in the wrong direction.</details>

A fuel-oil heater takes steam on the shell and fuel through the tubes. Is it regenerative, and does that matter to anyone on watch?

It is non-regenerative in the sense that matters here: the heat comes from an outside supply rather than being recovered from the heater's own outgoing stream. A regenerative arrangement uses a system's own outgoing stream to preheat its incoming stream, as bled steam does for feedwater, and that is what buys cycle efficiency.</details>

In a Diesel-cycle engine, at what condition is heat added, and how does that differ from the Otto cycle?

Air is compressed adiabatically and heat is added at roughly constant pressure as the fuel burns; the gases then expand doing work and heat is rejected with the exhaust. The Otto cycle, the spark-ignition counterpart, adds its heat at constant volume.</details>

Your lube-oil cooler outlet has crept up 8 °F over two months at the same load, and the seawater inlet temperature and outlet rise are both unchanged. Which of the three rate factors has moved?

The overall heat-transfer coefficient. Surface area is fixed, and the balance shows the same heat still crossing the wall, so the unit is now demanding a bigger temperature head to pass it. Suspect fouling, an oil film on the tubes, or trapped air.</details>