Energy Analysis Guide

After the process flow diagram has been developed to the point that a material and energy balance has been calculated, there are analysis that will reveal additional ways to reduce energy consumption but will require modifications of the flow diagram.

The Energy Balance

The basic data needed for an energy analysis is an energy balance of each process section. The study should be done by an engineer who is thoroughly familiar with the process flow diagram. The objective is to define in detail the energy input, energy utilized, and the energy dissipated or wasted. In some areas this will necessitate additional equipment.

The additional costs associated with this improvement in energy efficiency must be weighed against the potential reduction in energy requirements. Having identified the individual energy uses, the engineer can then determine methods for reducing or using these energy wastes. The next step is to evaluate the alternate methods and recommend the best one.

Energy Surplus

After the energy balances have been completed, some coordination between the engineers responsible for the various areas is required. The energy analyst should look at the areas or sections of the proposed process that wastes energy and determine if these energy wastes could be recovered economically. Possibly there is no use for the steam in the area that it is generated, but could be used in some other section of the plant.

Let's say that there is potential for recovering waste heat from furnace flue gases by using it to Suppose further that you determine that it is impractical to preheat combustion air and there is no use for low pressure steam within this section. The process engineer in charge of this section should check with the engineers in charge of designing other sections of the plant if they can perhaps find a use for low pressure steam.

Steam System

Use Lower Pressure Steam

Search for situations where use of high pressure steam can be switched over feasibly to lower pressure steam. It is advantageous to use the lowest pressure when high pressure steam is not required for say providing high temperatures.

This is particularly true when the lower pressure steam is being supplied from extraction or back-pressure turbines or a low pressure boiler separate from the high pressure boiler. Of course, lowering pressure by a pressure reducing valve offers no savings in energy.

Consider Cascading

Possibly the condensate from one heating operation can be used used as the heating media for another process step before returning the condensate to the boiler. Possibly the feed stream to a reactor can be pre-headed by exchanging heat with the reactor effluent stream for exothermic reactions.

Be sure verify that the heat duties match. If there is an excess duty on one side, possibly you will need to add a trim heater/cooler to meet these needs. Make sure that you are not having heat flow from lower to higher temperatures. This is particularly a problem trap when matching vaporization and condensation operations. Look at the costs of these duties to see how important it is to effectively integrate heating/cooling loads by proper cascading.

Increase Condensate Return to Boilers

Loss of condensate is a waste of heat and of valuable high purity water. Identify all sources of condensate and evaluate economic feasibility of installing pumps and insulated piping to return condensate to the boiler feedwater tank.

If the condensate is contaminated, evaluate possible clean_up.

Consider Cogeneration

Consider generating the steam at the highest steam pressure and dropping the pressure through a steam turbine. This will allow the highly efficient generation of electrical power for use in the process.

Electric Motors and Equipment

Electric motors and equipment, such as centrifugal pumps, operate with best efficiency at rated load. If they are operating at reduced load, efficiency suffers.

Compressed Air

Survey all uses of plant air to find the minimum pressure levels required. Lowering compressor discharge pressure saves energy. If all but one or two users can be satisfied with a lower pressure, an evaluation of the feasibility of installing a separate compressor or a booster to supply these higher pressure users should be made.

Insulation

Check potential energy losses for equipment operating at high temperatures. Make a determination if energy losses can be reduced economically by specifying better thermal insulation.

Worksheets

The same worksheets as are presented in the material and energy balance chapter are used here for material and energy balance presentations. Also, several special worksheets have been developed to provide a convenient/systematic way for collecting, summarizing, and presenting the energy analysis. They also provide a checklist for identifying information which is necessary.