DESIGN of FLUID SYSTEMS STEAM UTILIZATION Copyright© 1991 by Spirax Sarco, Inc. $19.95 All rights reserved. Printed In the United States of America, No part of this book may be used or reproduced In any manner whatsoever without written permission. 7514 Rev 4/91PREFACE Recognizing the on-going need for education as it relates to the fundamentals of steam including the most efficient use Of its heat content, Spirax Sarco has developed the Steam Utilization Course. This handbook represents over 80 years of steam experience in the proper selection, sizing and ap- plication of steam traps, pressure and temperature controls, and condensate recovery systems in major industrial plants throughout the world. ‘The Steam Utilization Course can be used in conjunction with “Design of Fluid Systems-Hook Ups” for a complete and concise knowledge of the use of steam for heat. For adcitional information or technical assistance talk directly to a Spirax Sarco Application Engineer toll-free: 800-833-3246 In Penna. 800-522-2384Design of Fluid Systems Steam Utilization Contents Basic steam engineering principles Steam traps and the removal of condensate Correct steam trapping Practical energy conservation in steam systems SPM7sarco Spirax Sarco Inc. P.O. Box 119 Allentown, PA 18105 Phone (610) 797-5830, Fax (610) 433-1346Basic steam engineering principles Introduction 3 What is Steam? 3 Why Use Steam? 3 ‘The Formation of Steam 3 Terminology and Units 4 Enthalpy 4 Specific Enthalpy 4 Specific Heat Capacity 4 Absolute and Gauge Pressure 4 Heat and Heat Transfer 4 Sensible Heat 5 Latent Heat 5 Total Heat of Steam 5 Steam Pressure 8 Steam Quality 6 Dry Steam and Wet Steam 6 Superheated Steam 7 Steam Generation 7 Condensation of Steam 9 ‘The Heating Surface 10 Barriers to Effective Heat Transfer 10 The Steam Circuit " Steam Tables 2 Questions 16 Answers 80Introduction The Spirax Sarco steam course is intended to ‘cover the characteristics and use of steam, as a conveyor of energy to space heating or process heating equipment. The use of steam for Power is @ specialized subject, already well documented, and is outside the scope of this course, What is steam? Like other substances water can exist in the form of a solid (ice), asa liquid, (water), or as 2 gas (steam), In this course our attention will largely be concentrated on the liquid and gas phases, and on the change from one phase to the other. If heat energy is added to water, its tem- perature rises until a value is reached at which the water can no longer exist as @ liquid. We call this the “saturation” point and with eny Why use steam? Steam has been used as a conveyor of energy since the Industrial Revolution. After its first use for cooking foodstuffs, it hes continued to be a flexible and versatile tool for industry wherever heating is needed. It is produced by the evaporation of water which is a relatively Inexpensive and plentiful commodity in most parts of the world. Its The formation of steam Perhaps the best way of explaining the formation of steam is by considering an imaginary, idealized, experiment (see Fig. 1). Suppose we took a cylinder with its bottom end closed, and surrounded it with insulation which was 100% efficient so that there was no heat loss from it. If we poured into the cylinder 1lb of water at the temperature of melting ice, 320F, we could use this asa datum point and say that for our purposes its heat content, or enthalpy, was zero, Any addition ‘of heat to the water would raise its tempera: ture, until this reached 2120F [the cylinder being open at the top so that only atmospheric pressure is applied ta the water). The course is aimed at those people engaged in the design, operation, maintenance or ‘general care of a steam system. Some moderate knowledge of physics is assumed, but the first, part of the course is an attempt to define the basic terminology and principles involved in steam engineering. further addition of energy, some of the weter will boil off as steam. This evaporation re. quires relatively large amounts of energy, and while it is being added, the water and the steam released are both at the same ‘temperature. Equally, if we can encourage the steam to release the energy that was added to evaporate it, then the steam will condense and water at the same temperature will be formed. temperature can be adjusted very accurately by the control of its pressure, using simple valves; it carries relatively large amounts of energy in a small mass, and when it is en- couraged to condense back to water, high rates of energy flow {into the material being heated) are obtained, so that the heat using equipment does not have to be unduly lerge. With any further addition of enthalpy, the water cannot exist as a liquid and some of it will boil off as steam, The total enthalpy held by each pound of liquid water at the boiling temperature is called the sensible heat of water and is shown by the symbol “ hf", The extra heat which has to be added to each pound of water to turn it into steam is called the latent heat of evaporation shown by the symbol “hig. The total heat in cath pound of steam clearly is the sum of these two. It is called ‘the total heat of steam and can be shown by: hf + hfg = hg.Fig. 1 Steam formation experiment When the whole of the latent heat of evaporation has been added to the pound of water in our cylinder, then all the water will exist as steam at atmospheric pressure. {ts volume will be very much more than the volume of liquid water, by a factor of over 1,650 times. Clearly the molecules of water in the liquid condition are held together much more closely than are the molecules of steam. The proeess of evaporation can be thought of as one of adding sufficient energy to each molecule that it can break the bonds holding it to its neighbors so that it can leave the liquid in the cylinder and move freely in the gas phase. Terminology and units Enthalpy This is the term given for the total energy, ue to both the pressure and temperature, of a fluid or vapor (such as water or steam} at any given time and condition, ‘The basic unit of measurement for all types of energy is the British Thermal Unit (BTU) Specific Enthalpy Is the enthalpy (total energy) of a unit mass (1 Ib). The units generally used are BTU/Ib, Specific Heat Capacity ‘A measure of the ability of @ substance to absorb heat. It is the amount of energy (BTU's) required to raise 1lb by 10F. Thus specific heatcapacity is expressed in BTU/IbOF The specific heat capacity of water is 1 BTU/IbOF. This simply means that an increase in enthalpy of 1 BTU will raise the temperature of 1 Ib of water by 19F. Now it is to be expected that if the pressure above the liquid were increased, the molecules would find itmore difficult to leave. We would have to give them more energy before they could break the bonds and move into the gas phase, which means that the temperature of ‘the water would Increase to over 2120F before boiling occurred, This is, indeed, exactly what is found in practica. If our imaginary cylinder were fitted with a frictionless piston, and a weight placed on top of the piston so as to apply pressure to the water, then the tempera- ture of the water could be increased above the normal 2120F before any evaporation commenced. However, at any given pressure there is a corresponding temperature above which water cannot exist as a liquid, and any heat above the ‘sensible heat’ will ‘evaporate some of the liquid, Equally, if the pressure of the water is lowered below the normal atmospheric pressure, then it is easier for the molecules to break free. They require a lower energy level, so the temperature at which boiling accurs, and the corresponding sensible heats are reduced. Each schoo! child learns the difficulty of boiling eggs at the top of a mountain where the air pressure is low! Fortunately for all of us, Engineers and Physicists have already carefully measured and recorded the temperatures and energy amounts. Their results appear in the “Steam Tables” which we will look at later. Absolute Pressure & Gauge Pressure ‘The theoretical pressureless state of a perfect vacuum is ‘absolute zero’’. Absolute pressure is, therefore, the pressure above absolute zero. For instance, the pressure exerted by the atmosphere Is 14.7 psi abs. at sea level, Gauge pressure is the pressure shown on a standard pressure gauge fitted to a steam system. Since gauge pressure is the pressure above atmospheric pressure, the zero on the dial of such @ gauge ls equivalent to approx. 14.7 psi abs. So a pressure of 45 psi abs. would be made up of 30.3 psi gauge pressure (psig) plus 14.7 psi absolute atmospheric pressure. Pressures below zero gauge are often expressed in inches of mercury. Heat and Heat Transfer Heat is a form of energy and as such is part of the enthalpy of a liquid or gas.