BASIC STEAM CYCLE/BOILERS

INTRODUCTION

Steam is the working substance used for the propulsion of many surface ships, including nuclear and conventionally powered steam ships. Central to understanding the operation of steam propulsion is the basic steam cycle, a process in which we generate steam in a boiler, expand the steam through a turbine to extract work, condense the steam into water, and finally feed the water back to the boiler. This is accomplished by a series of heat transfers and work exchanges throughout system..
We will introduce the basics of naval propulsion and auxiliary boilers, review the basic steam cycle, and examine the major components of this cycle in detail, focusing on required temperatures and pressures and where heat and work are added and removed.

INFORMATION

1. Boilers- boilers are used on almost all naval vessels, either to provide steam for propulsion turbines or to provide steam used in hotel services such as space heaters, hot water heaters, sculleries, laundry, etc. There are many different boilers used in the Navy. They can be classified in several ways.
1. Boilers can be classified according to the location of combustion and water sections.
1. Water-tube boilers are boilers in which the water is contained in generating tubes and hot combustion gases flow around the tubes to heat them. Propulsion boilers are of this type.
2. Fire-tube boilers channel combustion gases through tubes which are surrounded by water. Some auxiliary boilers are of this type.
Boilers can also be classified by the way they circulate water through the boiler. Natural circulation boilers use convection to circulate water in the boiler (figure 1). Relatively cool water enters the steam drum from the economizer and, because of its higher density, circulates down through large diameter downcomers to the water drum and lower headers. From there, the water rises in generating tubes and begins to boil into steam. No pump is required for this process. The difference in fluid densities moves the working fluid.

3. Natural circulation may either be free or accelerated depending upon the steepness of the angle of inclination of the generating tubes and the location of the tubes carrying the cooler water downward. Accelerated natural circulation boilers have very steeply inclined generating tubes.
4. Forced circulation is a boiler configuration in which a pump is used to circulate water through the boiler. Forced circulation is primarily used for auxiliary boilers and land-based power plants where there are few variations in demand.

Figure 1: Principle of Natural Circulation
2. Boilers are most commonly classified according to intended service. Propulsion boilers are boilers which provide steam for propulsion turbines, and propel the ship through the water by way of reduction gears, shafting, and propeller. There are two basic configurations for propulsion boilers used today on naval vessels, the 1200 psig D-type and the 600 psig D-type. The 1200 psig and 600 psig refer to the approximate pressures at which the boilers operate. D-type simply means that the parts of the boiler pressure vessel together form a shape similar to the letter "D."
1. On most ships the propulsion boilers also provide steam at reduced pressure for hotel services mentioned above. On ships which are not propelled by steam, gas turbine and diesel ships, for example, some type of boiler is still used to provide hotel service steam. There are two such boiler types:
1. Auxiliary boilers are usually smaller, lower pressure versions of propulsion boilers, in which fossil fuels are burned to heat the boiler tubes. The LSD-41 and AOE-6 class ships are examples of ships which use auxiliary boilers. These boilers consist essentially of a steam drum and a water drum which are connected by a bank of generating tubes. These boilers are not equipped with superheaters, desuperheaters or economizers. Most of these boilers are not equipped with downcomers. Natural circulation takes place between the water drum and the steam drum via the generating tubes and the screen tubes. The rear most generating tubes act as downcomers to supply water to the remaining generating tubes and water drum. Operation of these boilers varies widely, specific operating manuals from the manufacturer must be used. (Figure 2)

Figure 2: Auxiliary Boiler
2. Waste heat boilers use heat that would otherwise be wasted by exhausting into the atmosphere to heat water and make steam. For example, DD 963, DDG 993, and CG 47 class ships use exhausted combustion gases from gas turbine generators to provide a heat source for waste heat boilers. Steam for ship's services is generated by forced recirculation water tube type boilers. Recirculation of the boiler water is provided by a high head recirculating pump which delivers a minimum of 500 percent excess water under maximum evaporation demand. The boiler tubes are finned and arranged in a coiled bundle configuration. Exhaust gas enters the bottom of the boiler and is discharged out the side of the casing. (Figure 3)



Figure 3: Waste Heat Boiler


3. There are limits to how much steam a boiler can produce. If excessive demand for steam is placed on a boiler, the amount of fuel or air which can be supplied to the boiler may be physically limited. This is the end point of combustion and it will result in improper combustion in the boiler furnace. Beyond this point, increased steam demand can actually cause liquid water to leave the boiler along with steam. This is the end point of moisture carryover and it is destructive to steam pipes, turbines, and other equipment. Eventually, excessive steam demand can even cause a disruption of the natural circulation process discussed previously. This is the end point of natural circulation.
2. The basic steam cycle is a four phased closed, heated cycle. This means that the fluid in the system is reused and heat must be added to the cycle. The heat is added in the boiler firebox or furnace where the chemical energy of fuel is converted to the thermal energy of combustion gases and water is boiled to generate steam. This steam is expanded in the turbines, converting the thermal energy of the steam into mechanical energy of the engines and other turbine driven machinery such as turbine generators and main feed pumps. This steam is exhausted to a condenser which cools the steam and turns it to a fluid which can be pumped through the system again. This condensed steam, or condensate, is deaerated and pre-heated to remove oxygen and stored till needed. When needed, the water, now called feedwater, is raised to the proper pressure so that it can be fed into the cycle again. The basic steam cycle is shown in Figure 4. Understand that the pressures and temperatures in the following text and in Figure 4 are representative of normal parameters in a generic 600 psig steam cycle. Actual parameters vary from specific ship designs and steam plant operating conditions or configurations. Use the given parameters in a comparative manner to gain an understanding of system design and operation.
1. Generation Phase. To generate steam, it is necessary to heat water to its boiling point by adding a sufficient amount of heat to change the boiling water into steam. The heat required to change boiling water into steam, at any given temperature of the boiling water, is called the latent heat of vaporization. When steam condenses back to water and an equal amount of heat is given off, it is called the latent heat of condensation. The amount of heat required to convert boiling water to steam or the amount of heat given off when steam is condensed back to water at its boiling temperature varies with the pressure under which the process takes place.
1. Feedwater enters the boiler steam drum through a perforated internal feed pipe. The feed pipe ensures the incoming feed water (now called boiler water once inside the boiler) is evenly distributed throughout the length of the steam drum. The boiler water then travels down the downcomers between the air casings to the water drum. As the water travels up the generating tubes, the water is heated to its boiling point by the radiant heat from the boiler's furnace. The steam/water mixture reenters the steam drum at 490'F. The water that did not turn to steam repeats the process. The steam is directed to moisture separators in the steam drum to remove any entrained water. The saturated steam is then piped out of the steam drum to the superheater.
2. In order to drive a turbine more efficiently and economically, we need to raise the steam's energy level. We accomplish this by superheating the saturated steam in the superheater. The superheater is usually a four pass heat exchanger closer to the flames of combustion than the generating tubes. As the steam travels through the superheater, its temperature is raised to 800-850'F. Steam exiting the superheater is at 600 psig and referred to as "superheated", or more commonly, "main" steam.
3. Some of the superheated steam will not be used for main steam applications and is directed to a heat exchanger called the desuperheater. Depending upon boiler design, the desuperheater is located either in the water drum or steam drum. A portion of the 800-850'F superheated steam travels through the desuperheater and gives up some (not all) of its superheat to the water in the steam or water drum. Steam exits the desuperheater at approximately 650'F. Although this steam is referred to as "auxiliary" or "600 pound desuperheated" steam, it is still superheated. This steam will be used in smaller, auxiliary turbines (hence the name auxiliary steam), or reduced in pressure for other uses such as 150 psig steam for air ejectors and steam atomization.
2. Expansion Phase. The expansion phase of the basic steam cycle is where steam is expanded in turbines to convert the thermal energy of steam to the mechanical energy of rotation in the turbines. In the main engine turbines, the mechanical energy is used to drive the ship's propulsion shaft and propeller. In the ship's service turbine generators (SSTGs), this mechanical energy of rotation is further converted to electrical energy in the generator. The main steam system is the piping system which leads the steam from the boiler to the turbines which use main steam. These are always the main engines and SSTGs and sometimes the main feed pumps (MFPs).
1. After leaving the superheater, the majority of the main steam is piped to the main engine's high and low pressure turbines (HP and LP turbines). The remaining main steam is used in Ship's Service Turbine Generators (SSTG) and in some designs by the Main Feed Pumps (MFP). In the turbines, the thermal energy (increased by superheating) is converted to mechanical energy which rotates the turbines. As the steam "expands" through the turbines (i.e. its thermal energy is converted to mechanical energy), the steam's pressure and temperature are greatly reduced.
2. After the steam passes through the low pressure turbine (LP turbine) it enters the main condenser. Steam passing through an SSTG enters the auxiliary condenser. Depending upon the amount of steam passing through the HP and LP turbines (a function of engine speed), the steam exits the LP turbine at about 100'F. In the condenser (a heat exchanger), sea water passes through tubes and the steam is directed across the tubes. As the steam comes in contact with the cool tubes, the steam gives up heat (latent heat of condensation) to the sea water and condenses into water called condensate.
3. Condensation Phase.
1. When steam changes phase from vapor to water in the main and auxiliary condensers, that water is called condensate. The main and auxiliary condensers operate under a vacuum in order to lower the steam's condensation temperature. The greater the temperature difference between the heat source (boiler) and heat sink (condenser), the greater the system's efficiency. Simply put, the greater the vacuum in the condenser, the more efficient the system. As the steam condenses and leaves or exhausts from the low pressure turbine, it becomes part of the condensate system. The condensate system is that part of the steam cycle in which the steam condenses to water and is pumped from the main condenser toward the boiler. Before it can be used in the boiler, it must be converted to feedwater which occurs in the feed phase. The three basic components of the condensate system, in sequence, are the main condenser (including the hotwell), the main condensate pumps (usually two) and the main air ejector condenser.
2. The main condenser is a cross flow shell and tube type heat exchanger which receives the steam from the low pressure turbine and condenses it into water. The main condensate pump is a centrifugal pump which takes suction from the main condenser hotwell and delivers the condensate into the deaerating feed tank (DFT). The flow rate of this pump is controlled by the design feature submergence control. This means that the water level in the hotwell controls the pump flow rate and discharge pressure by virtue of the location of the pump in relation to the hotwell and the piping size.
3. Condenser vacuum is measured in inches of mercury where 0"HgVac is atmospheric pressure (no vacuum) and 30"HgVac is a perfect vacuum (Hg is the chemical abbreviation for mercury and Vac = vacuum). A condenser's vacuum is derived from steam condensing. When a large volume of steam is rapidly condensed into a small volume of water, the space formally occupied by the steam is now empty or a vacuum. Unfortunately, mixed with the steam is some air and other non-condensable gases which remain after the steam condenses into water. These gases must be removed from the condenser to preserve the vacuum. For this task, air ejectors take a suction on the condenser to remove the air and non-condensable gases. The main air ejector takes a suction on the main condenser and there is an auxiliary air ejector for each SSTG. In either condenser, a vacuum of 28-29"HgVac (0.5-1.0 psia) is the norm. The outlet temperature of the main and auxiliary air ejector condensers is maintained at 140-165'F by a thermostatic recirculation valve (TRV) located at the air ejector condensate discharge. During low bells (ship speeds), little condensate is produced and the flow rate of condensate passing through the air ejector condensers is low. The condensate temperature at the outlet increases because it stays inside the condenser longer absorbing more heat. When above 140'F, the TRV opens, sending some condensate back to the condenser thereby effectively increasing the condensate's flow rate through the air ejector condenser which reduces the condensate discharge temperature. At higher bells, the condensate flow rate increases (more steam condensed = more condensate), and the TRV recirculates little or no condensate. Although steam condenses in the main condenser around 100'F, condensate temperature in the hot well may vary between 100-130'F due to mixing with the hot condensate recirculated by the air ejector TRV.
4. The condensate collects in a low point in the condenser called the hotwell. From here, the condensate flows to one or both main condensate pumps (MCP) from the main condenser and each auxiliary condenser'sauxiliary condensate pump. These pumps discharge the condensate at 20-25 psig to provide enough pressure head to flow through the condensate system and overcome the 15 psig shell pressure in the deaerating feed tank (DFT).
5. After leaving the main and auxiliary condensate pumps, the condensate enters the DFT .The DFT divides the condensate and feed phases. Its three basic functions are to deaerate the condensate by freeing it of entrained oxygen and air, preheat condensate and store feedwater to accommodate changes in system demand. As the condensate enters the DFT, it is sprayed into the upper dome of the tank by spring-loaded spray nozzles. The condensate is heated to preheat the water before it enters the boiler, and to deaerate the water. The DFT is maintained at 15 psig which raises water's saturation point to approximately 250'F. Heat makes it easier for the oxygen and gases in the condensate to come out of solution. This concept is called "inverse solubility" which means that the hotter a fluid becomes, the easier it is for dissolved gases to come out of solution. Here, the condensate mist is heated by auxiliary exhaust (exhaust steam from turbines which do not have condensers) and high pressure drains (steam). The air rises where it is evacuated from the upper portion of the DFT and oxygen-free condensate falls to the bottom of the DFT.
4. Feed Phase. The condensate that is collected in the storage section of the DFT is now called feedwater and becomes a supply for the steam cycle. It also provides a positive suction head for the main feed booster pumps (MFBPs) or the main feed pumps (MFPs), as applicable.
1. The feedwater in the bottom portion of the DFT is piped to the main feed booster pumps (MFBP). These pumps take a suction on the DFT and raise the feedwater's static (or gravity) head pressure from 15-25 psig to 35-50 psig to ensure a positive suction head for the main feed pump (MFP). Since the MFP is a centrifugal pump, it needs a positive suction head to ensure adequate flow to cool the pump. The MFBPs provide this positive suction head for the MFP preventing cavitation and flashing in the MFP suction. Some ships do not have MFBPs because the DFT is physically located a significant distance above the MFPs. Due to this height difference, there is sufficient static head pressure to ensure a positive suction head for the MFPs.
2. The MFPs operate at variable speeds to maintain a constant feed system pressure to the boiler. The MFP discharges feedwater into the main feed piping system at 150-200 psi greater than the boiler operating pressure. For example, the discharge pressure of the MFP, discharging to a boiler operating at 600-650 psig, will normally be 750-850 psig. This discharge pressure is maintained throughout the main feed piping system, however, the volume of water discharged to the boiler is controlled by the feedwater control valve which opens or closes as needed to maintain proper water level in the boiler. The MFPs are protected from overheating at very low steaming rates by recirculating some discharge back to the DFT or the pump suction.
3. After the feedwater leaves the MFP, it passes through a heat exchanger located in the boiler exhaust stack called the economizer. The economizer is located in the flow of hot exhaust gases exiting the boiler and uses the hot gases to transfer additional heat to the feedwater before it enters the steam drum. By using the otherwise wasted heat in the exhaust gases, less fuel is required (economy) to raise the water's temperature to the boiling point. Feedwater enters the economizer from the feed system at approximately 246-249'F and at 750-800 psig. Depending upon the boiler's firing rate, the economizer transfers approximately 100-200'F from the exhaust gases to the feedwater so that the feedwater enters the boiler around 350-450'F. Because the water was preheated throughout the condensation phase, the boiler need only provide enough energy to raise the water's temperature about 40-14'F to form steam.
4. Although the basic steam cycle is a closed cycle, it is an imperfect system and there are various feedwater losses which must be replaced. Every attempt is made to recover the working fluid through such means as collecting the condensed steam in piping systems (called "drains") into a central location and pumping it back to the system. This central location is known as the freshwater drain collecting tank (FWDCT). Even despite all efforts to recover the working fluid, losses exist through leaks, etc. There also are unrecoverable losses designed into ships such as steam atomization of fuel oil burners and surface and bottom blows from boilers. These losses are made up for by adding more feedwater to the cycle through the condensers. This make-up feed (MUF) is simply feed quality water which is stored in tanks and introduced into the cycle as needed. The DFT water level determines the amount of MUF needed by the system.
5. Steam plant configurations vary greatly in complexity, from destroyer tenders with two boilers and one main engine to aircraft carriers with four main machinery rooms, eight boilers, and four main engines.
1. In ships with multiple propulsion plants, like aircraft carriers and amphibious ships, steam can be produced by one boiler and aligned to one main engine and SSTG(s) in one space, and another boiler in another space can feed a different main engine and SSTG(s) in that space. This is called split plant operation. For ships with more than one propulsion plant, this is the normal alignment. It means that a boiler casualty only affects half of the ship's mobility and auxiliary equipment.
2. In these ships with multiple plants, the boilers in one space can be aligned to provide steam to all operating equipment in both plants. This is called cross connected operation. It allows the use of one boiler to steam two plants, but likewise means that a casualty to one boiler can affect all operating equipment.

REFERENCES

(a) Elements of Applied Thermodynamics, Robert M. Johnson, et al.
(b) Principles of Naval Engineering NAVPERS 10788 series

Bharatiyulam

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