Researching a Heat-Pumped Condensing Steam Locomotive

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Steam locomotives that use condensers have periodically appeared on railway during the heyday of steam operation. The most notable examples were the Class-25C's used on the South Africa Railways during the post WW2 years, to extend their operating range of steam locomotives across the arid Great Karoo region located on the western side of South Africa. The SAR Class-25C's did experience a range of problems, such as fouling by lubrication oil and insufficient condensing capacity that required extra water be frequently added. These problems led the SAR to eventually remove the condensers from their class-25C's. A brief experiment using condensers was undertaken on a steam-turbine-electric locomotive in the USA, except that the condensers froze during cold sub-freezing winter temperatures.

When condensers are used in conjunction with a steam engine, the exhaust heat is usually transferred to a body of water, as was done on steam-powered marine equipment and at steam power stations located next to rivers or lakes. Water is preferred as a heat sink as it has 4-times the heat capacity of air per unit weight at 27-degrees C (80.6-degrees F) and 849-times the density. This gives an equal volume of water over 3,500-times the heat capacity of air at sea level pressure and temperature. A condensing steam locomotive is constrained by being only able to reject heat to the atmosphere.

If the exhaust from a steam (turbine) engine is at 20-psia and 228-degrees F (1.38-bar at 109-degrees C), over 960-BTU/lb or 2228-KJ/Kg of heat has to be removed from the saturated steam to convert it to liquid water. The condenser would need to be able to remove at least 1000-BTU/lb (2300-KJ/Kg) in order to be effective. If the steam mass flow is 5-lb/sec through the engine, the condenser needs to process 5000-BTU/sec ( x 3600/2545) or 7072-Horsepower of thermal energy just to convert vapor into liquid that can be pumped by a water pump.

Condenser Layout:

The South Africa class-25C's used 2 x cross-flow radiators mounted along each side of the tender. Research undertaken by Ranotor of Sweden (http://www.ranotor.se) has shown that counterflow heat exchangers have a higher level of effectiveness than cross-flow radiators. The counterflow heat exchangers may be mounted on the roof of a railway vehicle, or on the sides. However, the size of the air inlet may be restricted in these locations.

To ensure sufficient condensing capacity in a steam locomotive, a garratt-layout may enable the condensers to be optimally located in the lead unit (that is usually a water tender in a garratt). The entire lead unit would be a condenser on wheels, that is, it will have an air intake at the front (6' x 7' = 42-sq.ft or 3.9-sq.m) that will feed air into the heat exchangers. A extractor fan with variable pitch blades (mainly for forward running) would be located at the rear of the roof of the lead unit. The pitch of the blades would be reversed (pushing air through the condensing heat exchangers in parallel-flow mode) for low speed, short distance reverse operations at low power. To allow for multiple-unit operation, the rear sides of the tender may be equipped with louvres to direct air flow from the side and out the rear, directly into the condenser of the trailing locomotive.

Cooling Performance:

Air at 27-degrees C (80-deg F) and a pressure of 1-atm (14.7-psia) has a density of 0.0735-lb/cu.ft. and a heat capacity of 0.24-BTU/lb-deg F. The high temperature is 228-deg F (exhaust steam) and the air temperature is 80-deg F. The following table gives the mass flow rate of air at various rail speeds and 80% heat exchanger effectiveness:

The cooling capacity of the air flowing through the heat-exchanger(s) can be matched with the exhaust steam flow rate (requiring 1000-BTU/lb of steam). The steam turbine has an isentropic efficiency of 80% and uses steam at 1000-deg F at 800-psia, which yields 350-BTU/lb-steam of engine work. The following table illustrates possible steam flow rates at various rail speeds:

The steam turbine horsepower figures have been based on the condenser cooling fan turned off. These power levels are the maximum power that the condensers can allow for. Of interest, the SAR Class-25C's were rated for a maximum of 2,600-Hp which would have taxed the side-mounted, cross-flow heat exchangers to their limit.

Increasing Cooling Capacity:

One way to increase turbine engine power would be to increase the cooling capacity of the condenser, which would be achieved by increasing air mass flow rate through the unit. The condenser cooling fan may be operated so as to increase the volume of air that can flow through the condensing system. Example, while train speed in 40-miles per hour (60-Km/hr), the mass of air flowing through the condenser could be equal to a rail speed of 80-miles/hr (120-Km/hr). This means that 362-lb/sec of air would be entering the condenser However, it may also require the cooling fan to utilize some 1335-Hp if the fan operates an an isentropic efficiency of 85%, yielding a net power output of 3602 - 1335 = 2265-Horsepower

Using a Water-based Heat Pump:

An alternate way to remove heat from the turbine exhaust steam would be to use a heat pump circuit between engine exhaust and the condenser/radiator. The temperature of the saturated exhaust steam is at a level that allows water to be used as a refrigerant in a pressurised heat-pump circuit, which transfers heat from the exhaust steam into the preheater. A diagram of the circuit may appear as follows:

Saturated exhaust steam from the turbine enters the heat pump's evaporator at 228-deg F and 20-psia. A expansion valve located at the exit of the evaporator and on the exhaust steam line, reduces exhaust steam pressure and temperature while enabling more heat to be transferred from the exhaust steam and into the heat pump circuit. The cooling effect in the radiator would cause the water in it to contract, enhancing the performance of the expansion valve as it changes saturated steam into liquid water (at 202-degrees F).

Water is cooled from 202-degrees F to 160-deg F in the radiator. The compressor raises pressure of the saturated water in the closed circuit from 20-psia at 216-degrees F to 100-psia at 328-degrees F. The exhaust steam from the turbine enters the evaporator at 20-psia at 228-degrees F. Heat is transferred in the counterflow heat-exchangers (in the condenser and evaporator) at effectiveness levels of 80%. The circuit compressor operates at an isentropic efficiency of 80%, consumes 150-BTU/lb of energy when running at a coefficient of performance of 5.35 to 1. It can transfer 150 x 5.35 = 802-BTU/lb of heat at 100% effectiveness, or 642-BTU/lb at 80% heat exchanger effectiveness.

This accounts for 66.86% of the thermal energy (960-BTU/lb) that would need to be removed from the exhaust steam to convert it to liquid, leaving 960 - 642 = 318-BTU/lb of heat to be rejected in the radiator. Given that the steam turbine work is 350-BTU/lb and the compressor uses 150/350 = 43% of the turbine power, the net power would be 200-BTU/lb. The heat pump reduces the heating load to the radiator/condenser by two-thirds, allowing turbine total power output and compressor power to both be raised by a factor of up to 3. Total net turbine power output could be increased by up to over two-thirds, making a condensing heat-pumped steam locomotive more able to contend the traction power demands of a modern freight railway operator.

Increased Net Power Output:

A condensing steam locomotive without an exhaust steam heat pump supplying energy to a pre-heater, may be rated at 2700-Hp at 60-miles/hr. Increasing this by two-thirds due to using an exhaust steam heat pump would raise the net power level to 4500-Hp, with the condenser compressor consuming 43% of the total turbine output. This would translate to 3500-Hp for the compressor and 8,000-Hp from the turbine. The heat pump would transfer some two-thirds of the exhaust heat to the pre-heater, leaving the condensing radiators to process the equivalent heat of a 2700-Hp turbine.

The capital cost of the heat-pump steam turbine system would be high, costs that would include a high-powered steam turbine engine, the heat pump system and a higher capacity boiler. However, the higher capital cost could be justified if the locomotive used a low-cost fuel over the long term. It is possible that coal-water fuel could become such a fuel in the future. Other fuels may include biomass and solid fuels, including clean coal technology (gasification).

Pre-heating:

The cooled water from the radiator has its pressure increased by the water pump, which passes the high-pressure water through the hot side of the heat pump, where it can be supplied with up to 642-BTU/lb of heat. Saturated water at 800-psia can be heated to over 500-deg F and still remain in the liquid state. There is scope to use a cascaded heat pump circuit to transfer heat from exhaust steam to the water pre-heater, prior to it being heated/superheated to 1,000-deg F in the boiler.

Superheated steam at 800-psia and 1000-deg F has an enthalpy of 1512-BTU/lb and 42% of this thermal energy can be transferred from the exhaust steam by the heat pump circuit. The percentage of heat transferred into the water at the pre-heater is very close to the percentage of turbine power needed to drive the heat pump compressor. This indicates that the heat-pumped condensing steam locomotive would have an overall thermal energy efficiency level close to that of a non-heat-pumped condensing steam locomotive.

The heat pump can be used to enhance the performance of the condensing system, allowing the locomotive to generate more net horsepower at the drawbar. While a conventional water pre-heater does offer efficiency gains, the use of a heat pump allows more heat to be transferred from exhaust steam to the renewed water supply. Combining the heat pump with the condenser allows for higher power levels plus extended operating range. Boiler washout intervals would be greatly extended due to the continuous recirculation of highly purified water through the boiler, the condenser and the turbine(s).

Increasing efficiency:

The enginion company (http://www.enginion.com - - - "technology" icon) built a steam engine for use in a car (http://www.autofieldguide.com/articles/070102.html), an engine that delivered an efficiency level comparable to a diesel engine. Its low exhaust emissions could barely be measured. Another contemporary high-efficiency steam technology comes from the USA (http://www.cleanenergysystems.com - - - - - the "technology" icon) and can also deliver diesel level efficiency. These technologies can operate using ultra-critical steam at pressures up to 4,000-psia. Enginion's Caloric Porous Structure Cell technology can maintain a temperature of 1200-degrees C (2192-deg F) and generate superheated steam. It may be possible for an upscaled version of Enginion's technology to generate enough ultra-high temperature superheated high-pressure steam (at near 2000-deg F) for use in a high-efficiency condensing steam locomotive.

New engines using ceramic components such as silicon-nitride, boron-nitride and silicon-carbide need to be used as they can withstand the high superheat temperatures (up to 2,000-degrees F or 1100-degrees C). Possible engines for railway operation would be a compound-reheat expansion quasiturbine (http://www.quasiturbine.com) or a 4-stage expansion star rotor (http://www.starrotor.com) using reheat. Both rotary engines could be made from ceramic components and operate without lubrication. Power would be varied by varying steam density/pressure at constant superheat temperature. The engines would use fixed inlet ports and can drive electrical generation gear. They are also rugged enough to withstand the severe shock loadings that are common to locomotive operation.

One conventional steam turbine that may be rugged enough to be longitudinally mounted and drive electrical gear in the locomotive car body, would be the inward radial-flow design from Kuhnle, Kopp & Kausch in Germany. This turbine may be capable of withstanding the longitudinal shock loadings that occurred during coupling maneuvers and broke turbine blades on an earlier generation of steam-turbine-electric locomotives. To ensure competitive engine efficiency levels, a compound-expansion reheat turbine engine may be needed.

Conclusions:

Scope exists to optimize the heat pumped steam condensing system for railway operation. The system presented in this article is a basic concept that was used merely to illustrate that an exhaust steam heat pump can transfer more waste thermal energy into the preheater, while reducing cooling demands imposed on the condensing radiator. New generation steam technology could theoretically raise the thermal efficiency of a a modern condensing steam locomotive to that of a diesel locomotive. The steam locomotive could may actually be able to burn fuels that would other wise be unsuitable for use in internal combustion piston engines.

Evolving and developing modern steam technology could also enable a modern steam locomotive to incur lower maintenance requirements, longer service intervals and comparable availability rates to a diesel locomotive. At present, Enginion technology may be suitable for low-power applications, including smaller railway locomotives. It may be possible to upscale their CPSC technology for use in high-powered railway locomotive applications in the future. Clean Energy Systems technology is presently aimed at large-scale (over 100-Mw) applications such as power station. A scaled-down version of Clean Energy's technology or a scaled-up version of Enginion's technology may be applicable to mainline locomotive operation.

Harry Valentine,
Transportation Researcher.
harrycv@hotmail.com

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