What problems would occur if an inquiry were issued without a job specification and simply called up the applicable design code? After the purchase order was placed, the contractor would have to negotiate, typically, the price and delivery of spares, number of copies of drawings, guarantees, extent of supply, special tools and test equipment, and so on.

The code covers the detailed mechanical design methods to be used and refers to a large number of allowable materials of construction with their associated design stresses. It also details fabrication and inspection methods. The code requirements are legally mandatory in many countries.

A specification will add the contractor's and client’s technical requirements and clarify the code (or standard) when choices are available. It should contain a design and fabrication section based on experience in fabrication and problems in the field.

The specification, when accepted by the vendor, will be contractually binding. It will not deal with commercial considerations. Table 1 gives a list of items that should be included in most specifications. Table 2 lists typical requirements for shell-and-tube heat exchangers. Table 3 lists typical requirements for air-cooled heat exchangers. Table 4 lists typical requirements for plate and frame type heat exchangers. Table 5 considers a package unit (a cold box), which has much broader scope and requires liaison with other engineering disciplines. Table 6 lists typical requirements for electrical process heaters.