Energy Management

Conserving energy through thermal options

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

By Russ Kratowicz, Executive Editor

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.

During the condensation process, the ambient air is giving up its heat to the pipe, hence to the cold material inside the pipe. If the plant is intentionally chilling the material for use in a cryogenic process, the condensation represents a waste of energy.

Even small amounts of insulation on the pipe will reduce heat transfer and nuisance condensation. But, if the low temperature of the fluid is capable of cooling the outer surface of the insulation to below the dewpoint of the ambient air, the result is wet insulation, which offers no insulating value. Similarly, exposure to rain also degrades the thermal value of the insulation. That is why most insulated lines are covered with a waterproof jacket. A sufficiently thick layer of thermal insulation, however, minimizes this heat transfer to insignificant levels. Finding the optimum balance between the installed cost of the insulation and the energy saved is a topic that is beyond the scope of this article.

Thermal insulation

Many industrial manufacturing processes function best at only steady state operation, where variables such as pressure, temperature and flow rate are held at reasonably constant values. Some processes require material to be held at a fixed temperature for an extended time to exploit optimized reaction kinetics.

Moving the temperature of anything away from ambient temperature initiates thermal transport between the object and the environment. If presented with the opportunity, both heated and chilled materials will seek to return to ambient temperature as a result of heat transfer through the walls of the vessel and piping that contains them. Preventing the possibility of heat transfer prevents the temperature from drifting away from the desired value.

Covering the outside surfaces of equipment that holds hot or cold material with thermal insulation reduces the flow of heat across the container wall. Increasing the thickness decreases the heat transfer, but increases the installed cost of insulation. The benefits derived by adding more insulation quickly reaches the point of diminishing returns. The optimum solution is to tolerate some level of heat transfer by balancing its associated energy cost against the cost of the insulation.

Tools

There is a tool for evaluating the efficacy of thermal insulation. Non-contact monitoring of a surface temperature is the essence of thermographic analysis. The measuring instrument — a thermographic camera — captures and converts the thermal information present in the scene visible through the unit’s viewfinder into a false color image. The spectrum and distribution of colors observed is directly related to the temperatures present. The thermal data is also stored as a digital file that can be analyzed in detail at a later time.

Thermography is a tool that finds use in many areas of the plant, not just those dealing with thermal processing. The major advantage of thermography is its ability to identify positive and negative temperature anomalies that are not obvious to the naked eye. For example, the technology can just as easily locate areas of defective insulation on a hot process vessel or under the jacketing of insulated pipe carrying chilled brine. It can indicate a defective steam trap by monitoring the extent to which condensate is backed up in a heat exchanger.

Another tool for evaluating steam traps is the ultrasonic detector that responds to the high frequency sound a badly operating unit makes. The device heterodynes the raw sound signal and outputs a lower-frequency signal in the audible range.

Waste heat boilers and preheated combustion air

The driving force behind any heat transfer operation is the magnitude of the effective temperature differential between the heating and the heated mediums. The temperature of flue gas is always high and represents a source of inefficiency in the combustion process.

The triangle of fire requires the presence of three things before anything will burn — fuel, oxygen and a source of ignition. At some point in the process, air that is consumed during combustion gets heated to the flame temperature, a fact of life that decreases the amount of energy available from the fuel. If some way could be found to present the burner with air that is already at the combustion temperature, then more of the caloric content of the fuel would go to useful purposes.

One way to increase combustion efficiency is to capture some of the heat going up the stack for use elsewhere in the plant or for preheating the combustion air. A waste heat boiler can preheat the water being fed to the boiler. A gas-gas heat exchanger in the stack can heat the combustion air. Both measures increase energy efficiency.

Waste incineration

Every plant has a waste stream it is paying someone to haul away. But many scrap materials have a significant heat value and are routinely burned for the energy content. Among these are refinery waste streams, coke oven gas in steel mills, sawdust, broken pallets, paper waste — the list goes on. Unfortunately, there is generally a capital cost involved with the material handling systems needed to take advantage of the heating value of waste materials and increase energy efficiency.

If possible, it may be better to sell waste materials that have an economic value and apply the proceeds toward defraying the overall cost of energy. In general, waste that is segregated has a higher value than mixed waste. But, once again, there may be a capital cost involved in the segregation process. A rigorous economic analysis will clearly indicate whether paying a waste hauler, recycling or burning is the most advantageous move for a plant. In any case, the available options for conserving energy while increasing production are many. General guidelines on how to do so are only that; it’s up to the plant professional to ferret out the plant-specific methods that will make energy conservation a winning strategy.