PRACTICAL ELECTRICAL ENERGY CONSERVATION ON INDUSTRIAL SITES - ARTICLE 3: HEATING

Electrical heating systems are common in industrial environments. They may be assumed to operate at very high efficiency levels i.e.  close to 100% of the electrical energy supplied to heating elements is converted to heat. Of course, generation of electricity is not a process that operates at 100% efficiency, and this has to be considered when assessing the overall emission impacts of using electrical heating systems.They also can have significant demand impacts, and as resistive devices with power factor close to unity, demand reduction through power factor correction is not possible.

Typical applications are for the heating of process fluids (e.g. solutions of chemicals used for washing, or for air used in drying) or sometimes for the heating of materials (e.g. heating of plastics during extrusion). Electrical heating may also be used for producing hot water and even steam.

Given the fact that heating is itself a very efficient process, energy savings associated with heating are unlocked by taking a systems perspective. Hence if you are analysing a steam system, you would need to consider the distribution and usage of steam and the recovery of condensate in order to increase the efficiency of the steam system, thereby reducing the electrical energy input.

      

In smaller operations I often see manual control being employed, particularly in batch processing environments, and generally I always recommend some type of feedback temperature control system, even if in the form of a simple thermostat. Any overheating represents a waste of energy, and also accelerates heat losses to the environment due to an increase in driving force.

An obvious opportunity with heated systems is the use of insulation to limit losses to the environment. Basically you want to close the system as far as possible, and this includes any open surfaces, as would be found in things like heated process baths and the like. The use of covers helps to limit losses to the environment. Heat losses from agitated liquid surfaces tend to be significantly higher than from still surfaces, and hence any agitation should be limited as far as possible. Obviously many processes require agitation for legitimate reasons, and it’s about finding a happy medium between your process goals and heat losses.

Heat loss occurs not only through direct heat transfer, but can also happen as a result of carryover of hot process fluids into downstream processes. This can happen easily in process such as electroplating, or washing. It’s clear that such carryover should be limited as far as possible, since not only is heat lost with the fluid leaving, this fluid is generally replenished with fluid at room temperature which has to be heated to the process temperature.

Depending on the heat load and when the heating is done, the rate of heating could impact on maximum demand charges, and you may want to look at how heat-ups of individual process vessels are scheduled at start-up.

Lastly, heated processes should be located in areas free from drafts (either natural or induced) since these increase the rate of heat transfer from hot surfaces and increase heat losses.

Let’s briefly explore what I mean by taking a "systems approach" by considering a simple process in which a tank cleaning solution is heated on a batch basis using an in-line heating element, with the cleaning solution circulated through a sprayball in order to clean the tank. Water is added to the tank, cleaning chemicals are added and the resultant cleaning fluid is circulated through an in-line heater and back into the tank to be cleaned. After cleaning, the fluid, which becomes soiled, is drained. The system is shown in the diagram below. 

Important basic issues to consider would be:

Is there a temperature probe used for feedback control of the heating operation? Without control of the temperature, the fluid could be heated excessively, wasting energy. If a probe is installed, what is the temperature used for cleaning? This should be as low as is practically possible in order to reduce energy requirements for heating up the cold cleaning fluid. Lower temperatures will also translate into reduced heat losses to the environment due to a reduced driving force for heat transfer;  Is the system well insulated? This includes the vessel as well as the piping used to circulate the cleaning fluid;  What volume of fluid is used for each cleaning operation? The larger this volume, the greater the sensible energy requirement for heating the fluid up;  How often is the cleaning operation performed? – clearly the less frequently this can be done (while still meeting all other process goals, such as sterility for example) the lower the energy requirements for cleaning; Instead of a once-through cleaning system with the cleaning fluid discarded after each wash, is it possible to collect and reuse the fluid, if not for subsequent cleaning, then for other purposes in the facility? Integrating this washing process into other processes in the factory would lead to reduced facility-wide energy consumption.

    

     While heating elements are essentially simple devices, the systems within which they operate can be quite complex, and optimising these systems in an integrated fashion is the challenge when seeking to reduce their energy consumption.