Equipment Sizing

Preliminary Equipment Sizing for Capital Cost Estimation

The Need for Equipment Sizing

Before one can carry out capital cost estimates on process equipment, the process engineer must carry out a preliminary sizing of the equipment.Although each process will have some unique pieces of equipment there are some general categories applicable to most designs

Equipment Types

TowersHeat ExchangersInstrumentationTanksVesselsPumpsCompressorsFired HeatersOthers

Towers

Size (height and diameter),Pressure,TemperatureNumber and Types of TraysHeight and Type of PackingMaterials of ConstructionUnique Characteristics, Swaged Columns

Heat Exchangers

Type;Vapour-Vapour,Vapour-Liquid,Condenser,Vapourizer,EvaporatorsProcess Duty, Area, Temperature and Pressure for all streamsType, Shell and Tube, Air-Cooler, Tube and Tube etc. Cooling TowersNumber of Shells and and Tube passesMaterials of Construction

Instrumentation

Pressure, Temperature, Flow and LevelAnalytical, Gc, IR etcComputer Control, Central or Distributed

Tanks

Type of Tank and Service, Standard API, Floating roof, Blanketed etc.Materials of Construction

Vessels including Reactors

Height/Length,Diameter,OrientationPressure, TemperatureMaterials of Construction

Pumps

Flow,Discharge Pressure,TemperatureΔP,Driver Type, Shaft PowerType of pump, Centrifugal, Reciprocating, number of stagesMaterials of Construction

Compressors

Actual Inlet Flow Rate,Temperature,Pressure,Driver Type,Shaft PowerType of Compressor, Centrifugal, Single or Multi-Stage,ReciprocatorMaterials of Construction

Fired Heaters

Type,Tube Pressure,Tube Temperature,Duty,FuelMaterials of Construction

Other Equipment

Filter,Blenders,Mixers, KneadersCentrifugal Separators,CrystallizersCrushers and Grinders,Dust CollectorsElectrostatic PrecipitatorsConveyors

(This is by no means an exhaustive list)

Sizing Procedures - Towers

Flow Sheet Simulators such as PRO/II ® and HYSYS ® have built in sizing routines for most type of mass transfer devices such as Fractionation Towers and Liquid-Liquid Extraction Towers. Unique devices such as Rotating Disk Contactor would require a specific procedure.

Towers Continued

For Fractionations, the designer must determine the number of theoretical stages required for a certain reflux and boil-up rate in order to achieve the required separation. The appropriate type of Mass Transfer device must be chosen, I.e. Trayed Tower, Packed Tower etc.

Towers Continued

If a trayed tower is selected, the type of trays must be specified, bubble-cap, seive or valve trays. For packed towers the type of packing must be selected. There are many types of towers, Simple Absorbers, Rectified Absorbers, Reboiled Strippers, Steam Strippers etc.

Heat Exchangers

The most common type of heat exchanger is the shell and tube. Shell and tube exchangers come in many configurations, however as in the case with fractionators most simulators such as HYSYS ® can carry out a rigourous exchanger design. Condensers and Thermosyphon Reboilers are tedious calculations best done by a computer.

Heat Exchangers - Continued

For unique types of heat exchangers separate sizing procedures may be required. Often the designer must rely on a vendor for a good preliminary design sizing. Realistic sizing and easy of maintenance are two important considerations.

Instrumentation

For the purpose of preliminary estimates instrumentation is usually costed simply as a factor of major capital.Instrumentation (Process Control) has become increasingly more sophisticated and costly.

Instrumentation continued

Although a factor approach may be suitable for a preliminary estimate, a better definition is required for a definitive estimate. Some units have a very sophisticated plant-wide supervisory control which is based on complex computer technology.

Tanks

It is no longer satisfactory to only consider the amount of material that must be stored and the appropriate materials of construction, environmental considerations have a significant effect on the design of tanks. Inventory instrumentation has become more sophisticated as well. Often one will have to rely on a vendor.

Vessels

This category covers a wide range of equipment from reactors to flash drum, overhead accumulators, flare drums etc. Some of this equipment can be readily sized by heuristic residence time considerations. Reactor design can be a simple residence time consideration, or a very complex design problem.

Vessels continued

CSTRs are usually a relatively simple vessel with an agitator, although heat removal problems may complicate the design. Other reactors are considerably more complex, for example catalytic fluid bed cracker reactors and regenerators, and trickle phase reactors.

Pumps

The major concern with pump sizing is selecting the particular type of pump for the service. Although centrifugal pumps are the most common, other types are often required, reciprocating, gear pumps and so on. You will be provided with a spread-sheet for the sizing of pumps.

Compressors Expanders

Flowsheet simulators such as HYSYS ® have routines that are to be used for sizing compressors (and expanders). One common error that students in the design course make is specifying a compressor to move liquid. Compressors are used for vapour, pumps for liquids. Compressors can be destroyed by slugs of liquid in the feed.

Fired Heaters

This is a relatively complex sizing problem, and fortunately we don’t have to deal with it in the design exercise. Direct fired reboilers are not uncommon, particularly in the petroleum industry, however we have avoided the need for such a unit in the design project

Others

There are a great deal of unit operations that fall into this category, all with their own unique sizing procedures. Solids handling is a fairly broad field, as is the equipment used for bioseparations.