AWT Technical Training Manual: Chapter 3 Boilers
Steam required (lb/hour)=
(T out- T in)x 0.01/10°F
Deaerators typical unit pressure
0.5 psig
Steam required for dearation
1 % of the feedwater flow for every ten degrees of temp rise in the deaerator.
Recommended Deposit Weight Densities for cleaning tubes Operating pressure >2000 psig
10 to 12 (g/ft²)
Recommended Deposit Weight Densities for cleaning tubes Operating pressure 1000 to 2000 psig
12 to 20 (g/ft²)
Condensate must be cooled below _____°F to be used in an ion exchange
140°F
Steam pressure of a tray deaerator
15 psig
Pressure range for hot water heating boilers
15-50 psig
A type water tube boilers
2 mud drums, more susceptible to tube starvation if bottom blows are not properly done, bottom blows cannot exceed 80% of rated steam load
Recommended Deposit Weight Densities for cleaning tubes Operating pressure <1000 psig
20 to 40 (g/ft²)
Remaining oxygen levels in Spray Deaerators
20 to 40 ppb
Temperature of water leaving a deaerator
214 to 216 degrees F at 1atm (101.325 kPa)
Maximum practical operating pressure in fire tube boilers
250 psig
Oxygen scavengers in tandem with DA tanks can achieve oxygen levels as low as
5 ppb
Remaining oxygen levels in Tray Deaerators
5 to 25 ppb
Density of boiling water
59.8 lb/cu. ft
Density of 70 degree F
62.3 lb/cu. ft.
Forced circulation water tube boilers
A circulation pump is used to boost the flow of water and prevent excessive steam buildup in the risers
Mechanical cushion design
A pump and dump valve are used to maintain system pressure in the tank as load changes.
Separators water tube boilers
A series of path changes that expunge excess water during steam generation
As pressure is raised in a system...
A steam line is more efficient at carrying steam
The quantity of the heat of vaporization
About 1000 BTU but changes with temperature
Sub cooled boiling
Boiled bubbles collapse as they are cooled by contact with cooler bulk water
Deposit Weight Density
Boiler tubes are split down the hot cold side boundary. A portion of the tube is cleaned and debris is measured.
One-Pipe System
Circulate hot water from the heater to the air handles, baseboard heaters or radiators then back to the heater.
Water tube boiler Mud Drum
Concentrates solids such as salts formed from hardness and corrosion products carried into the boiler.
Deaerators
Creates a headspace and passes a stripping stream of gas (steam) through the feedwater.
Natural circulation
Derives motive power from the density differences of the supply and return water.
Rifled bore tubing
Enhanced tubing, causes more turbulent water flow
Steam generators
Generate steam indirectly from the transfer of heat to water via a heat exchanger
Superheater
Heat exchanger placed after the steam exit of the boiler. Removes all moisture content from the steam by raising the temperature of steam above its saturation point.
Co-current deaerators (steam and water flow in same direction)
Heating is more efficient because the coldest water contacts the hottest steam but the most oxygen laden steam contacts the most oxygen free water.
Unlike steam generators boilers have
High pressure diffrential across the tubes
triple point of water
Ice, Water and Vapor are in equilibrium. Only possible at a temp of 32°F and .006 atm
Boilers
Implies a direct transfer of heat to the water
Expansion tank
In chilled water systems maintains system pressure, in steam cushion design, water from the boiler is flashed to steam inside this.
Circulation in water tube boilers is a result of
Introduction of colder feedwater creating a pressure differential as steam is created
condensate polishing
Ion exchange and filtration process
Common contamination in condensate returns
Iron, Copper, Hardness and organic chemicals
Main consequences of superheated steam
Lack of moisture prevents corrosion in steam lines, larger specific volume as heat increases, require larger diameter pipelines.
Steam drum
Main purpose is to permit efficient steam disengagement from the boiler water.
Steam stores _________ latent energy than most other liquids
More
Heat transfer is more efficient with....
More turbulent water
D type water tube boilers
Most flexible design, single steam drum, generally more tube surface exposed to radiant heat
The bank of tubes closest to the heat source
Most susceptible to failure due to flow problems or corrosion/deposition problems.
Mass of the attemperation water=
Ms⁰ (hs⁰-hs)/(hs-hw)
For every thousand BTUs lost
One pound of water will condense
Steam Blanketing
Over generation of steam that prevents water from reaching tube walls. causes deposit formation and/or corrosion
Most common reason for poor deaerator performance
Oversizing vent for feedwater flow.
Ideal Gas Law
PV=nRT
Dew point of steam is dependent on its...
Partial pressure
High steaming rate water tube boilers
Pressure drop causes steam bubbles to swell and drum level initially rises.
Low steaming rate water tube boilers
Pressure drop causes the drum level to initially decrease
Carbon steel
Primary material of construction for hot water piping systems
Attemperators
Primary means for controlling the degree of superheat in a superheated boiler.
Nucleation
Process by which the first microscopic bubbles form. Indicates the hottest points in the system.
Coalesence
Process that occurs when micro-bubbles begin to merge as they grow and their boundaries begin to collide.
Natural circulation water tube boilers
Produce a pressure differential between the down-comer tubes and riser tubes sufficient to overcome frictional resistance to flow in the risers creating steam flows.
What is the main operating cost in a boiler system?
Purifying raw water sources
The heat transfer equation
Q=UA ΔT
Flash Tanks
Recover heat value from condensate or blowdown and produce a cleaner condensate. Reduces pressure below saturation pressure and a portion of the water is converted to steam.
Spray deaerators
Rely on direct contact of steam with fine droplets of feedwater ran through a "shower head"
Deaerator storage tank
Serves as a level control device to prevent starvation of boiler feed water pump.
Feedwater heaters
Shell and tube heat exchangers (1-4 depending on system size) designed to heat water to within 50-100 degrees of the boiler temperature.
Economizers
Simple finned tube heat exchangers prone to corrosion due to their high surface area. Common area to remove residual oxygen in the feedwater.
O type water tube boilers
Single steam and single mud drum, larger mud drum design renders boiler less prone to starvation due to flow blockage
Cast iron boiler
Small low pressure devices designed to generate heating steam and or hot water for apartment building and small commercial installations.
Deaerator with higher available surface area for oxygen transfer
Spray deaerators
Critical heat flux
Steam insulates tube walls, heat transfer to the bulk fluid is greatly restricted and the tube wall temp rises rapidly. AKA: Departure from nucleate boiling, burnout or dryout
Forced circulation system
System that uses booster pumps, satellite pumps, ect to circulate water
enthalpy of vaporization
The amount of energy needed to convert water from the liquid state to a gaseous state at the boiling point without changing the temperature.
Countercurrent deaerators (steam and water flow in different directions.)
The entering steam contacts the hottest and most oxygen free water and passes successively to the cooler more oxygenated water. more efficient at stripping oxygen but not heat transfer.
Nucleate Boiling
The entire bulk fluid reaches the boiling point for the operating pressure, bubbles form everywhere.
Corrosion and precipitation occur more frequently here
The hot side of tubes (side facing the heat source)
Vapor pressure of water
The pressure at which water vapor is in thermodynamic equilibrium with its condensed state.
Field erected water tube boilers
Up to millions pounds per hour of saturated steam
Two-pipe system
Use the same air handlers and fan coils for heating and cooling. Use the same piping for hot and chilled water.
Flash tank
Used with a continuous blowdown system to recover residual heat
Gas cushion design
Uses inert gas to pressurize expansion tanks.
Indirect use of steam
Using a steam-heated heat exchanger to boil deionized water for humidity
Heat recovery steam generators
Usually found in combined cycle electric power generation, waste heat from gas turbine exhaust is used to generate steam
Tray deaerators
Water is introduced into a vertical stack in the tank. Water overflows from one side to the next below until it enters the storage removing oxygen trough steam contact.
For a given steam rate (pounds per hour) water velocity is higher in....
Water tube Boilers
Deposit Weight Density Formula
Weight of removed deposits (g)/Surface area from which they were obtained (ft²)
Disengage
When a bubble grows to a certain size and breaks free from the tube surface
Typical boiler water level
about 1/2 the diameter of the cylinder.
Phase Equilibria formula
f=c-p+2
Shell strength
limiting factor for fire tube boiler maximum pressure
Steam separators
minimize physical carryover of water droplets in the drum.
Attemperation mechanics
process of partially desuperheating the steam by controlled injection of water into the steam flow
Makeup water
required to replace lost condensate or process steam
Ideal recirculation in water tube boilers
ten times the steam flow rate
Package water tube boilers
up to 250,000 pounds per hour of saturated steam
turbulent water flow
water flow where water at any point is moving in many different directions at once and at differing velocities.