Conserving energy through thermal options

A brief survey of some options that may work in your plant.

By Russ Kratowicz, Executive Editor

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It is philosophically untenable to attempt to conserve energy by cutting back on production. The only viable option we have for successful energy conservation is reducing the latent energy content in each unit of output. For industries that rely on molecular reactions, such as chemicals and petroleum, thermodynamics imposes a lower limit on unit energy content. Some purely physical processes, such as compressing air and other gases, also must succumb to the strict dictates of thermodynamic limits.

Still attendant on nearly every manufacturing process is a great waste of energy. Steam systems develop leaks. A steam trap fails with no outward indication of its impaired condition. Boilers and burner systems can be maladjusted and remain that way through neglect. Bare runs of uninsulated pipe can be, in effect, terrific heat exchangers that move energy where it is not supposed to be going. Because the human eye is not sensitive to infrared radiation, it is impossible to easily determine the integrity of thermal insulation.

These are just some of the possible sources of incidental process-related energy loss that a plant may be suffering. The list could be extended to include losses attributable to problems with the building envelope, plant infrastructure and other aspects of the manufacturing business.

Steam leaks and condensate return lines

Operating a boiler on plain tap water is out of the question. Minerals present in the water, even potable water, will form deposits that quickly give the boiler the mechanical equivalent of a thrombosis, reducing both flow through the unit and the boiler’s ability to heat water. Treating raw water with additives alters its chemistry to a point that minimizes the natural propensity of minerals to deposit on the inside of the hot boiler surfaces and interconnecting pipe. Water may be relatively inexpensive, but the chemicals used to treat it are definitely not. In addition, the make-up water that must replace the treated boiler water lost over time must also be treated if protection against scaling is to be maintained.

The only viable option we have for successful energy conservation is reducing the latent energy content in each unit of output.

Steam is used as a heat transfer medium because it can carry a lot of thermal energy and because it is generally abundant and represents a well understood, old technology. Steam acquires its thermal energy when the boiler transforms the heating value contained in the fuel into something more useful. In the process, the steam acquires an economic value that exceeds the purchase price of the water and fuel from which it is formed. At the other end of the pipe, the steam gives up its load of valuable BTUs when it condenses back to the still rather hot liquid state. How steam is handled after it condenses is the issue.

The water throughput in a boiler rated at 500 hp is more than 17,000 pounds per hour, a quantity equivalent to a water flow rate of more than 34 gallons per minute. If the steam simply passes through the system, only to be vented to the atmosphere, it wastes an enormous amount of water and residual heat. Returning condensate to the boilers conserves money. Venting spent steam and steam system leaks represent equivalent forms of waste.

Flue gas analysis

By definition, a stoichiometric combustion process consumes exactly enough oxygen to convert the primary components present in a fuel — hydrogen and carbon — into carbon dioxide and water vapor, the major products of combustion. The source of oxygen, however, is atmospheric air, which introduces about four times as much nitrogen as oxygen to the flame. The nitrogen, of course, is the source of the oxides of nitrogen that everyone is talking about.

Instead of running a burner stoichiometrically, it is more practical to operate it with excess air to achieve better control. It is possible to have too lean a combustion process if too much excess air is present. Also, the inescapable, concomitant heating of excess air can consume a significant part of the caloric value of the fuel, thus reducing the burner efficiency.

One way to control a combustion process is by monitoring, in real time, the concentration of oxygen flowing up the stack. The signal can be used to control the speed of the combustion air fan or a damper to hold excess air at a constant optimum value.

Steam traps

The purpose of a steam trap is to ensure that valuable steam yields its full measure of heat as it passes through a piece of process equipment. The steam condenses to a liquid when it gives us as much heat as it can. To make room for more steam, the condensed water needs to be removed from the process vessel or heat exchanger. Because the steam behind the condensate is at a pressure above atmospheric, simply opening a valve to drain the condensate is not a viable solution.

A steam trap holds back the pressure of the steam while allowing the condensate to pass unimpeded. When properly sized, the steam traps will not be overwhelmed with condensate upon equipment startup, when steam is being used at an extraordinarily high rate. When undersized or failed in a closed position, however, the vessel or heat exchanger will be flooded with condensate constantly, which prevents proper heat transfer. If it fails open, it wastes steam.

Pipe sweat

Cold operations can also be a source of energy loss. If the temperature of chilled material flowing through a pipe, duct or conduit is below the dewpoint of air in the space through which the pipe passes, humidity will condense on the surface of the pipe, a phenomenon called sweating. If the fluid is sufficiently cold, the condensed moisture will coat the pipe in an ever-thickening layer of ice.

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