The
Steam
Power
Cycle
A brief overview
CONDENSATION in the Rankine cycle is the process of extracting latent heat from engine exhaust steam, changing the phase back to
water. As discussed previously, heat energy transferred to a cooling medium is lost to the system and represents a loss of efficiency.
Steam systems operate condensing or non-condensing; the exhausted steam being recycled or discarded. Non-condensing systems are
lighter, cheaper, more compact, simple to operate and reliable; it is easy to throw something away. Condensing benefits often justify
the burden; reusing the same water facilitates long range operation and makes expensive filtration and chemical treatment affordable,
improving machinery life.
At atmospheric pressure, dry steam occupies about 1624 times as much volume as it would if condensed to water, which can be thought
of as one cubic foot of steam condensing into a bit less than one cubic inch of water. If condensation occurs in a closed vessel, a vacuum
occurs since the container walls prevent air from filling the space formerly occupied by the steam. Maintaining condenser vacuum can
improve engine efficiency in a couple of ways. Shorter cutoff is possible since the engine back pressure is reduced and smaller clearance
volumes can be used because the lower remaining steam pressure in the cylinder reduces the problem of over-compression.
Diesel, Otto (gasoline), Stirling, Brayton (jet engines) and steam engines all pressurize their working fluid before heating and then
expanding it to generate work. Water is relatively incompressible and steam systems pressurize feed water with pumps which are much
more efficient than the compressors other cycles must employ to raise pressure by compressing gasses. This is one place the nature of
steam works to our advantage, unfortunately there are always paybacks. Potential engine efficiency is tied to the difference between the
working fluid at its hottest and coolest in the cycle. Internal combustion engines can use working fluids hot enough to damage the metal
from which they are built simply by cooling the metal just enough to protect it without unduly cooling the working fluid. External
combustion engines like steam and Stirling engines must transfer heat into the working fluid through a heat exchanger and these can
never be heated beyond the safe working temperature of the metal.
Ambient temperature and the nature of the cooling medium greatly affect condenser effectiveness. As a hot object is cooled, the rate at
which heat flows into the coolant slows until the temperatures are equal and no heat flows at all. Likewise, not all coolants are created
equal, water at room temperature is about 800 times denser than air, and a fine water mist can provide as much cooling as a large
blast of air.
Condensers come in all varieties, most of which will not work in automobiles. The one universal constant is that the condenser must reject
heat to some other medium, with nothing else available on the road, automobiles must use air for cooling. The most obvious, and almost
universal, automobile condenser design is to fashion the condenser much like an automobile radiator, condensing steam inside the tubes
with the tube exteriors cooled by passing air. Not surprisingly, condensing steam on contact with a cool surface is the defining characteristic
of the surface condenser. Although the condenser and radiator have much in common, automotive radiators operate very well whereas
automotive condensers are a challenging proposition.
The drawing at left contrasts the volumes of one ounce of hot water and
saturated steam, both at atmospheric pressure. Although the steam has
seven times the BTU of the water, it will take far more than seven times
the tubing to encapsulate it. This illustrates one challenge facing surface
condensers, steam applies less heat energy to the same tube wall area,
making for less vigorous heat transfer to the air.
The amount of heat to be disposed by the condenser depends upon the
exhaust temperature and pressure, and the desired condenser pressure.
Engines at short cutoff use less steam and produce cooler exhaust, both
of which impose less work on the condenser. If long cutoff is selected
for climbing hills and drag racing at the light, condenser demand will
increase dramatically. While vacuum condensing improves engine
efficiency by allowing for longer cutoff and reduced clearance volume,
it is also harder to pull off. Lower pressures correspond with lower
saturation temperatures, reducing the temperature difference between
the condenser and the air and reducing the rate of heat flow. The
cooling potential of air is limited by the low thermal conductivity and
specific heat of air which, combined with practical limits on the size of
vehicle condensers, strictly limits the vacuum achievable on all but the
coldest days.
The modern automotive radiator passes coolant through a series of flattened tubes, bracketed on each side by thin shells to distribute
the coolant, fine fins soldered onto the tube exterior provide generous surface area to transfer heat to the air. Radiators are
exceptionally well suited for their task as indicated by the need to install thermostats in the cooling system to prevent the radiator
from excessively cooling the engine and wastefully transferring heat to the atmosphere rather than converting it into work. Since
condensers also reject unwanted heat, the natural question arises, “can a radiator serve as a condenser”? Probably, but it would be a
far from ideal choice. The thin shells mentioned previously will collapse if subjected to any significant vacuum, so the radiator will
not supply the benefits of vacuum condensing. Beyond that, the thermodynamics are wrong…
In the table to the right are the density, thermal conductivity and
specific heat properties for air, steam and water at temperatures
and pressures likely to occur in a radiator or condenser. As we can
see, the values for air and steam are much closer while those for water
differ highly
By selecting one material, air, as a baseline and making relative
comparisons, the chart is simplified and it becomes easier to highlight
similarities and differences
Multiplying the density by the specific heat cancels out the weight
of each substance and creates a term which reveals the heat energy
per given volume for each degree of temperature, for convenience
we can term this the “heat density”.
Since the water contains about 800 times as much heat as steam per cubic inch and 192 times more than air, it is easy to see that
the steam and air must contact with much more surface area to transfer the same amount of heat than is needed by the water.
Thermal conductivity is a measure of how readily heat flows inside a material, water having some 36 times greater thermal
conductivity than steam and 24 times more than air. Since heat flows more readily in water, larger passages present less of an
obstacle in cooling the center of a stream of water than they do for air or steam which would benefit from narrow passages that
minimize the distance heat must flow.
Do these results work out in real life? Let us ponder
home heating. At one time, boilers commonly heated
homes while today baseboard hot water heaters are
more typical. Consider the steam radiator and the
hot water convector finned tube to the right:
The radiator is made of iron cast as a hollow shell;
steam inside the radiator heats the iron, which in turn
heats the air in the room. The thin walls ensure that
the interior surface area is only slightly less than the
exterior. The convector channels hot water through
the round copper tubing; the aluminum fins fitted
outside the tube provide sufficient surface area to
transfer the heat to the air.
It probably doesn’t hurt to remember our description of the automotive radiator, a multitude of fine fins soldered onto tube with
water flowing through the tubes and air through the fins between the tubes. (Please keep in mind that we are referring to exhaust
steam at low pressure and temperature, the engine transforming the majority of the thermal energy into work. Steam coming
directly from the boiler is under pressure, and is thus denser, when combined with high boiler temperatures it makes for an
extremely potent heat source.)
Condenser effectiveness may also directly influence boiler efficiency. The boiler expends heat energy warming cold feed water, when the
water is recycled any heat energy residing in the water is also recycled. The term condensate depression refers to the temperature
difference of the water leaving a condenser and the temperature needed to condense the steam at the engine exhaust pressure.
Ideally, we operate the condenser to minimize condensate depression and thus minimize the amount of heat added back to the
water in the boiler, while taking care not to reduce condenser vacuum in the process. Since nothing is ever simple, minimizing
condensate depression can theoretically somewhat reduce the boiler efficiency and partly offset the efficiency gain. If we imagine two
boilers housing identical burners and producing flames at the same temperature, we would assume the boiler with the cooler exhaust
gasses is more efficient since less heat is being disposed to the atmosphere. The feed water with lower condensate depression is
warmer and cannot extract as much energy from the boiler exhaust, The presence of air and other non-condensable gasses in the feed
water can complicate the task of maintaining low condenser pressure. These gasses come out of solution when converting water
to steam and passing it through the system. The rapid action of the condenser limits the condensate’s ability to reabsorb gases; the
remaining gasses accumulate in the condenser and cause a steady pressure rise. A pump scavenges these accumulating gasses from
the condenser and establishes the initial vacuum when starting the power plant. Some systems employ a single pump to remove both
gasses and condensate from the condenser but do not necessarily remove the gasses from the feed water. Oxygen is of particular
concern, its presence in boiler water at elevated temperatures and pressures can lead to corrosion of the boiler tubes, mechanical
oxygen removal and preventative chemical treatments minimize the potential for damage. thereby reducing the boiler efficiency a bit.
Theory and Stuff, Redux.......