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.