Thermodynamics and heat exchangers

DOE-HDBK-1012 §1-1

DOE-HDBK-1012 §1-1 — Temperature, pressure, and energy fundamentals Thermodynamics is the study of energy, its transformations, and its relation to the states of matter. Temperature measures the average kinetic energy of the molecules in a substance and is read on the Fahrenheit and Celsius relative scales or the Rankine and Kelvin absolute scales; absolute zero (0 R, 0 K) is the point of no molecular motion. Absolute temperature must be used in gas-law and cycle calculations. Pressure is force per unit area; gauge pressure is measured relative to atmospheric pressure, while absolute pressure adds atmospheric pressure to gauge (absolute = gauge + atmospheric). A perfect vacuum is zero absolute pressure. Pressures below atmospheric, such as a condenser vacuum, are expressed in inches of me…

DOE-HDBK-1012 §1-3

DOE-HDBK-1012 §1-3 — First law of thermodynamics and energy balance The first law of thermodynamics is the principle of conservation of energy: energy can be neither created nor destroyed, only converted from one form to another or transferred from one place to another. Applied to a system, it states that the heat added to a system equals the increase in the system's stored energy plus the work the system does on its surroundings. Nothing is ever lost; energy that seems to "disappear" has been converted to a less useful form, most often low-temperature heat rejected to the environment. In practice engineers apply the first law as an energy balance around a piece of equipment or a whole plant: the sum of all energy flows in must equal the sum of all energy flows out plus any change in stor…

DOE-HDBK-1012 §1-4

DOE-HDBK-1012 §1-4 — Second law, entropy, and efficiency The second law of thermodynamics governs the direction in which energy conversions naturally proceed and sets the ceiling on how much heat can be turned into work. It states that heat flows spontaneously only from a hotter body to a colder one, and that no heat engine can convert all the heat it receives into work — some heat must always be rejected to a lower-temperature sink. This is why every real power plant needs both a heat source (boiler) and a heat sink (condenser/seawater), and why the exhaust and cooling water always carry away energy that cannot be recovered as useful work. Entropy is the property that quantifies the second law; it measures the unavailability of a system's energy to do work, and it always increases in any…

DOE-HDBK-1012 §1-5

DOE-HDBK-1012 §1-5 — Thermodynamic cycles: Carnot, Rankine, and Diesel A thermodynamic cycle is a series of processes that returns a working fluid to its starting state while converting heat into work, so the cycle can repeat continuously. The Carnot cycle is the ideal reference cycle — two constant-temperature and two frictionless adiabatic processes — and gives the highest efficiency possible between two temperatures, but it cannot be built in practice; it serves as the yardstick against which real cycles are measured. The Rankine cycle is the practical basis of every steam plant. Feedwater is pumped to boiler pressure, the boiler adds heat to make (and superheat) steam, the steam expands through a turbine doing work, and the exhaust is condensed back to water in the condenser, after wh…

DOE-HDBK-1012 §2-3

DOE-HDBK-1012 §2-3 — Radiation and overall heat-exchanger performance Radiation is the transfer of heat by electromagnetic waves and, unlike conduction and convection, requires no material medium — it crosses a vacuum. Every body above absolute zero radiates energy, and the rate rises very steeply with absolute temperature (with the fourth power of absolute temperature), so radiation dominates heat transfer from very hot surfaces such as a boiler furnace, the flame and hot gases, and glowing exhaust components. A dark, dull surface both radiates and absorbs heat well, while a bright, polished surface reflects it; this is why furnace interiors are lined to absorb and re-radiate heat to the tubes and why reflective shielding protects nearby equipment and crew from radiant heat. In a real he…

DOE-HDBK-1018 Vol.1 §2-1

DOE-HDBK-1018 Vol.1 §2-1 — Heat exchanger purpose and construction A heat exchanger transfers heat from one fluid to another without allowing them to mix, through a solid metal wall (the tube or plate) that separates them. Shipboard examples include lube-oil coolers, jacket-water coolers, fuel heaters, charge-air coolers, condensers, and evaporators. Heat flows from the hotter to the colder fluid at a rate governed by the temperature difference between them, the surface area of the dividing wall, and the overall heat-transfer coefficient (which depends on the fluids, their flow velocity, and the cleanliness of the surfaces). Fouling — scale, sludge, marine growth, or oil films on the surfaces — adds thermal resistance and is the most common reason a cooler gradually loses capacity, so tub…