Steam Boiler: Evaporation

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Boiler Blowdown System

Boiler blowdown system is a process of removing unwanted dissolved solids in boiler water. Dissolved solids in boiler water will be left behind during the evaporation process to generate steam. The longer the dissolved solids in boiler water will more and more until the level of solubility exceeds the allowable limit. If the concentration of dissolved solids exceeds a certain limit, then the boiler water tends to form foam that allows water carried along with steam.

In addition the concentration of dissolved solids in water or can be referred to as sediment can cause the formation of crust on the inside of tube, header or drum. This will result in heat transfer process to be disrupted, so that the metal tube or header to be over-heat and the boiler water temperature to be low resulting in less heating in evaporation process.

Therefore, the concentration level of dissolved solids in boiler water must be controlled by the way of doing boiler blowdown system. Boiler water in certain volume is discharged in the process of blowdown system and replaces it with make-up water in the feedwater system.

Boiler blowdown system consists of two types namely:

Continuous blowdown
Intermittent blowdown

Continuous blowdown

Continuous blowdown is discharge process of dissolved solid in boiler water continuously. Therefore, make-up water should be added in the feedwater system continually to compensate amount of discharged boiler water because of this process to maintain the water balance of a steam boiler. Concentration of Total Dissolved Solid (TDS) must be maintained so as not to exceed a certain level so that the purity of steam produced can be maintained.

Blowdown valve setting in this process is only done once on a particular condition, so the operator does not need to set up again at any time. Boiler blowdown system will make the boiler heat energy is wasted, but it can also be used by blowing heat of blowdown into a flash tank to produce flash steam.

Preheating feedwater can utilize the heat from the flash steam. So boiler efficiency can be increased by re-utilize heat energy from blowdown system. Continuous blowdown is generally applied at high pressure boiler.

Intermittent blowdown

Intermittent blowdown is the removal process of dissolved Solid, silica, and other unwanted particles that carried out intermittently. Intermittent blowdown is done manually by opening the valve of blowdown piping system that is usually placed at the lowest level, so that the sediment in the boiler water can be reduced and the expected quality of steam can be maintained properly.

Intermittent blowdown is very effective to remove solids that have been separated from the solution and precipitate in the tube, the header or drum boiler. For example, intermittent blowdown is done in a shift work of operator, performed blowdown within 2 minutes. Intermittent blowdown causes the make-up water must be added in the feedwater system in a short time and large quantities. Therefore its operation requires greater feed water pump than continuous blowdown.

Steam Formation Process in Boiler

Since steam formation process in boiler depends on pressure, so steam formation process is performed at constant pressure. If 1 kg of water at 20C is heated in a closed vessel with constant pressure (1 atm), then during the first heating process, temperature of boiling reach 100C, steam begins to be formed. In this case, this steam is called wet steam (saturated liquid), because the steam still mix with grains of water.

If all steam including grains of water which is mixed in wet steam is heated again, it will get saturated steam (saturated vapor). Saturated steam is condition where the steam form is entirely pure steam, there is not mixture. The amount of heat that required converting 1 kg of boiling water into saturated steam at constant pressure is called latent heat. When the heating is continued so the temperature of saturated steam will rises and this steam is called as superheated steam / superheated vapor.

At steam formation process in boiler, air and fuel are distributed into furnace to perform combustion process. Combustion gases will pass through evaporator, superheater, air heater, and finally to be discharged into atmosphere through chimney / stack. While the feedwater, after doing heating process in deaerator, then to be distributed into evaporator and then saturated steam is heated further in a pressure part which is called superheater and finally heat superheated steam can be obtained.

Figure 1: Graphic T-S on Steam Formation in Boiler

Steam Quality Parameters

The following below are parameters in determining the quality of steam produced by boiler:

- The amount of steam which is generated by boilers in accordance with required amount to drive turbine generator. The amount of steam produced is usually in the unit form of Kg/hour or tons/hour. Therefore the heat transfer should be maintained in order to generate expected amount of steam.

- Steam produced must have required temperature and pressure so as to drive turbine generators. Setting temperature of steam can be done by desuperheater while setting pressure of steam can be performed by relieve valve or safety valve. If the steam pressure and temperature are met then it means that one of the parameters in the determination of steam quality is met.

- The steam which is produced should be free from air and condensed gases because air and moisture can inhibit heat transfer in steam boiler.

- Steam which is produced by boilers must be clean; there should be no crust (e.g., corrosion or sediment carbonate) or impurities that can increase the rate of erosion in the pipe, orifice and valve.

- Steam is produced must be dry or superheated. The presence of water droplets in the steam will reduce the actual enthalpy of evaporation, and will also lead to scaling on the pipe wall and damage to turbine blades.

Phase Steam Diagram

The data provided in the steam table can also be expressed in graph form. Figure 1 illustrates the relationship between enthalpy and temperature at various pressures, and known as the phase steam diagram.

Figure 1: Phase Steam Diagram

When water is heated from 0° C to saturation temperature, its condition follow the line of saturated liquid until receives all of liquid enthalpy, hf, (A - B). If heat is added further, it will change the phase steam diagram to the saturated steam enthalpy and continually improving while remaining at saturation temperature, hfg, (B - C).

If the dryness of mixture of steam and water increase, the condition moves from the saturated liquid line to the saturated steam line. Therefore at the middle point between both conditions, the dryness fraction (x) is 0.5. The same condition occurs on the saturated steam line which has steam 100% dry.

Upon receiving enthalpy of evaporation it will reach the saturated steam line. If heating is continued after this point, the temperature of steam will begin to rise to supersaturated (C - D). The lines of saturated liquid and saturated steam cover areas where there is mixture of steam/water-wet steam. In the area of left side of saturated liquid line, there is only water, and on the right side of the saturated steam line there is only supersaturated steam. The point where the saturated liquid line and saturated steam meet is known as the critical point.

If the pressure raises towards the critical point the enthalpy of evaporation decreases, until it becomes zero at the critical point. This indicates that the water turns directly into saturated steam at the critical point. Gas may just exist above the critical point. Gaseous state is the most diffused situation where the molecule is almost has unlimited movement and the volume increases without limit when the pressure is reduced.

The critical point is the highest temperature where fluid is in liquid form. Giving the pressure at constant temperature below the critical point will not cause phase steam diagram change. Even so, giving the pressure at constant temperature below the critical point will result in melting of steam when passing superheated area to wet steam. Critical point occurs at temperature of 374.15 ° C and 221.2 bars steam pressure. Above this pressure are called supercritical steam and there is not boiling point can be applied.

Circulation Water in Steam Boiler

Circulation of water in the pipes/tubes in steam boiler is a very important thing to be designed. Boiler must be designed in such a way that avoided the formation of steam and out of the water wall tubes from steam drum. In other words, should not be going back-flow.

To get even warming on all parts of the boiler, especially in boiler water tubes, then the perfect water circulation must be maintained to prevent air bubbles and steam in the wall tubes and termination expenses of steam from tubes. The occurrence of bubbles on the wall tubes and the cessation of vaporization can lead to corrosion and salt concentrations which can damage the wall tubes.

The circulation of water and steam in the boiler occurs because:

1. Difference of density between water and steam.

2. The existence of mixture of water and steam.

The type of water circulation in the steam boiler can be classified into two types, namely:

Natural Circulation

In this circulation, the water flowing from upper drums through downcomers which is located in the boiler is relatively cold, flow down to mud drum. From mud drum, water flow back into the steam drum after passing through the evaporator tubes.

Forced circulation

In this forced circulation, fluid is pumped through the evaporator. This causes the boiler can work with very high pressures.

Convection Heat Transfer in Liquid Phase

Convection heat transfer with heat flux (φ) constant is calculated using the following equation:

q_conv = φ . A

Where:

A = Area of the heated surface (m²). In the pipe, cross-sectional area which is heated is π.D.z

φ = Heat flux on the surface of pipe (Watt/m²)

q_conv = Convection heat transfer (Watt)

z = length of pipe (m)

So for a pipe with diameter D, The value of heat transfer that occurs is:

q_conv = φ . π . D . z

While the heat transfer on the fluid inside pipe is:

q_conv = W_f . c_pf . (T_f (z) – T_fi)

Where:

W_f = mass flow rate in the liquid phase (kg/s)

c_pf = coefficient of heat convection in the liquid phase (J/kg⁰C]

T_f(z) = local fluid temperature in the pipe (⁰C)

T_fi = temperature of fluid enter pipe [⁰C ]

So the heat balance on pipe is by combining equations above to be following equation:

φ . π . D . z = W_f . c_pf . (T_f (z) – T_fi)

Mass flow rate (W_f) is often made in mass velocity (G) the relationship between both of them is as following equation:

G = (4. W_f) / (π . D²)

So by rearranging the equation above and combine them can be obtained equation below to calculate distribution the local heat fluid of along pipe.

T_f (z) = T_fi + ((4 . φ . z) / (G . C_pf . D))

Pipe wall surface temperature is the temperature of local fluid coupled with the difference of wall temperature and the local temperature:

T_w = (T_f (z) + ΔT_f )

Where:

ΔT_f = φ / h_fo

So the equation will be:

T_w = (T_f (z) + (φ / h_fo))

h_fo is calculated from Nusselt number as following equation:

Nu_D = (h_fo . D) / k_f

Where:

Nu_D = Nusselt number

h_fo = coefficient of convection fluid (W/m^{2 0}C)

k_f = thermal fluid conductivity (W/m⁰C)

D = pipe diameter (m)

Nusselt number for laminar flow in pipe:

Nu_D = 0.17 Re^0.33 Pr_f^0.43 (Pr_f/Pr_w)^0.25 ((D³ρ_f³gβΔT)/(μ_f²))^0.1

applies to z/D > 50 and Re < 2000, while for turbulent flow in a pipe used Dittus-Boelter equation, which applies to z/D > 10 and Re > 3000.

Nu_D = 0.023 Re^0.28 Pr_f^0.4

Classification of Flow in Evaporation Process

Pattern of flow in the evaporation process can be classified into five sections as follow:

- Single phase liquid flow

- Bubbly flow

- Slag or plug flow

- Annular flow

- Single phase steam flow

Figure 1: Flow Patterns in Evaporation Process

Although it is difficult to know the type of flow inside water wall tubes and the changes position of flow type from one to another, method to explain position of certain flow type is much needed. A method to describe transitional of flow type inside water wall tubes is in the form of mapping type of flow. This type of flow is illustrated in graphical form. The coordinate of graphic is the function of gas phase superficial velocity (j_g) and liquid phase superficial velocity (j_f). The graphic which explain the fluid flow upward in vertical tube has been made by researchers Hewit & Robert in 1969 as figure below.

Figure 2: Mapping of Flow in Vertical Tube

Annular Wispy Flow in Evaporation Proses

Large bubbles occur in slag or plug flow and will be bigger so as to form a cylinder amid tubes and at the inside surface wall tubes still attach liquid. The condition at this region can be called as annular flow. In annular flow, liquid phase inside tube will be increasingly depleted along with increasing quality of steam formed. Thus reach a point where liquid phase at the wall tubes does not exist anymore. This point is called dryout point. But at dryout point the quality of steam is not reach yet saturation steam point.

Figure 1: Annular & Wispy Annular Flow in Evaporation Process

For large heat flux, the liquid in the surface wall tubes will dry out first, while the liquid in the middle of tubes still has not evaporated. Furthermore cylinder flow will be formed with liquid in the middle of tubes. This flow is called as wispy annular flow. This flow generally occurs at condition Departure Nucleate Boiling (DNB) and the quality of steam in this flow is still low.

The limitation of annular flow according the figure above can be seen the equation below:

ρ_f j_f² < 1000 (kg/ms2)

ρ_g j_g² < 168 (kg/ms2)

The limitation of wispy annular is:

ρ_f j_f² < 1000 (kg/ms2)

ρ_g j_g² < 168 (kg/ms2)

Where:

j_g = superficial velocity of gas phase

j_f = superficial velocity of liquid phase

Slag or Plug Flow in Evaporation Process

There are some kinds of flow in evaporation. One of them is slag flow. Along with the increases quality of steam in the water wall tubes, bubbles steam that arises will be greater, thus forming plug or slag inside tubes. The flow in this region is called as plug or slag flow. However, if in the slag flow there are many small bubbles or bubbly, the flow in this region is often called as random or churn flow.

Figure 1: Plug or Slag Flow and Churn Flow in Evaporation Process

Churn flow is formed from the breakage of large bubbles steam in the slag flow. This flow is sometimes referred to as semi-annular flow or slag-annular flow. From the figure above can be known the limitations of slag flow are:

ρ_f j_f² < 5124 (kg/ms2)

ρ_g j_g² < 168 (kg/ms2)

Where:

j_g = superficial velocity of gas phase

j_f = superficial velocity of liquid phase

Bubbly Flow in Evaporation Process

Evaporation is the process to convert water into steam. Evaporation process occurs in steam boiler especially in water wall tubes area. One of evaporation process is bubbly flow. When temperature of water in water wall tubes is equal to saturation temperature of water, the little bubbles will arise especially near metal wall tubes. Saturation temperature of water will be reached faster in area near metal wall tubes because the heat is transferred first from metal wall tubes to water inside. Flow area of little bubbles can be called as Bubbly Flow.

Figure 1: Bubbly Flow in Evaporation Process

In the bubbly flow, bubbles of steam on one side can be small and round and on other side it could be great with a round and flat shape. In this condition the size of the bubbles is not always exactly same, but it is expected have uniform size.

Evaporation in Steam Boiler

Evaporation is the process changing water into steam. In this evaporation process requires heat transfer that be obtained from combustion process in the furnace steam boiler. Evaporation occurs in the water wall tubes of steam boiler. Water from steam drum is flown through downcomer into lower header, and the proceed to water wall tubes either in the left side, right side, front side and rear side steam boiler.

Fig. 1: Evaporation Process in Steam Boiler

Surface water wall tubes get heat transfer by radiation, and then heat is transferred in entire of surface water wall tubes of steam boiler by conduction. When water in the water wall tubes is heated, reduction of density occurs, so water becomes lighter and tends to move upward. Then water which has heavy density will flow downward and replace the heated portion of the water. So there is heat transfer between particle of water which has density lighter and heavier. This process is called convection.

The quantity of water that be evaporated into steam is known as Evaporation Rate and usually expressed in pounds of steam per hour, or pounds of steam per hour per square feet heating surface, or pounds of steam per hour per cubic volume of furnace steam boiler. Enthalpy of evaporation or laten heat evaporation is the amount of heat energy required to convert water into steam.

Steam which be generated in the water wall tubes of steam boiler is saturated steam (still mixture between steam and water where temperature of steam is same with the water from which it was converted). The mixture goes to steam drum, where steam is separated from water. The steam goes from the drum to superheater to be heated again, so steam will be superheated steam. The water is recirculated through downcomer, lower header, and water wall tubes again.