August 1996 Volume 2 Issue 14
THERMAL EFFICIENCY OPPORTUNITIES
Many recipients of this newsletter are involved with process heat. Our recent articles on “Boiler Efficiency” (Issue 11) and “Energy Recovery from Hot Liquids” (Issue 12) focused on some key opportunities for saving process-heat dollars. In this issue we discuss several additional opportunities not discussed in those articles.
For a helpful overview of process heat from an energy perspective — as well as more complete coverage of the subjects discussed in this issue of $mart Energy User — we recommend our new booklet, Energy-Efficient Process Heat. See the final paragraph of this issue for information on how to get your free copy.
ELIMINATE STEAM LEAKS
Steam leaks would seem to be such an obvious energy loss that they would be corrected immediately, but that is not always the case. It is possible to estimate the magnitude of the loss by noting the length of the plume that the steam creates. A plume 1-metre long indicates a loss of 16 kg of steam per hour, for instance. How big a dollar loss does that represent? At $22 per 1000 kg (a typical P.E.I. steam cost), a continuous (24 hr/day) leak of that magnitude would cost $3084 per year.
KEEP STEAM TRAPS WORKING PROPERLY
At various points in a steam system, steam and liquid water are present together. Steam traps are used to separate this mixture into “dry” steam going down one pipe, and liquid condensate going down another. Steam traps typically open and close millions of times per year, and this continual activity makes them prone to failure. It has been estimated that up to 25 per cent of steam traps fail each year, and that steam leakage through a defective trap can range from 5 to as much as 50 per cent of the trap capacity. Unfortunately, it is often difficult to spot a defective trap because the steam leaking through it is piped away with the condensate, totally out of sight. Abnormally high temperature and pressure in the condensate return line are one indication of steam trap problems.
INSULATE BARE PIPES AND TANKS
Radiation from bare pipes, tanks, and other steam-fed devices raises the fuel bill. Adding insulation to these bare surfaces reduces it. For example, adding 1-1/2 inches of insulation to a 2-inch pipe carrying 121°C steam will save about $2.20 in fuel every 1000 hours of operation. The insulation costs about $8.90 per foot installed. Thus, the fuel saved during 4045 hours of operation is enough to pay for the insulation.
DISCONNECT UNUSED STEAM PIPING
Many plants are periodically revised and refitted. New equipment is installed, and old equipment is taken out of service. Unfortunately, changes in the steam distribution system do not always keep up with these equipment changes. Yes, if a new steam line is needed, it will be installed right away. But old, unused parts of the steam distribution system are not always disconnected. Steam often continues to flow through pipe networks that no longer serve any useful function. All unused steam piping should be disconnected.
REDUCE HEAT AND EVAPORATIVE LOSSES FROM OPEN LIQUIDS
Where cooking involves uncovered containers of hot water or cooking oil, heat energy is lost through evaporation. For example, the heat loss from an 80°C water surface having an area of 10 square metres is 200 megajoules per hour. In dollars spent for fuel, this amounts to $1600 for each 1000 hours of operation. Simple covers can reduce these losses considerably.
USE FLASH STEAM FOR PREHEATING
Both condensate and blowdown water exist first at boiler (steam) temperature and pressure. When these liquids pass out of that high pressure environment through a steam trap or blowdown control valve, part of the water is instantly converted to steam. This steam is called flash steam and, if captured, can be used for various heating tasks. One of these involves the preheating of boiler feedwater, and is illustrated in the figure at the bottom of the page. Here boiler blowdown water enters a flash steam tank in which the flash steam and liquid water separate. The steam escapes from the top of the vessel and is sparged into the feedwater storage tank where it preheats the feedwater. The mineral-laden blowdown water goes to drain.
REDUCE STEAM PRESSURE IF POSSIBLE
The higher the steam pressure, the higher the steam temperature, and the higher the thermal losses. To minimize fuel expense, generate steam at the lowest pressure which meets the needs of the processes that use it. A periodic review of the situation is a good idea. It’s not unusual for a plant to replace old equipment with new equipment capable of working at lower steam pressure, yet fail to reset the boiler controls and actually reduce the pressure.
OPERATE EQUIPMENT NEAR CAPACITY
Because many heat energy losses are independent of load, efficiency tends to be highest when equipment is run at or near its rated capacity. It does not, for example, make energy sense to run two units at half capacity if the job can be done by one unit running at full capacity. Naturally, production requirements come first, but it is sometimes possible to save energy and energy dollars and still meet production needs.
The booklet mentioned earlier, Energy-Efficient Process Heat, is available free of charge from the Energy and Minerals Section. Call Mike Proud or Ron Estabrooks at 368-5010 (toll free). Excellent literature on steam traps and their problems is available from Spirax-Sarco Canada Limited, phone 905-660-5510.
Flash steam recovered from blowdown preheats boiler feedwater