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2.1.1 Vacuum pressure The Hg maximum level is the maximum vacuum level (measured in inches of mercury) rec- ommended for the pump. This vacuum level translates to the maximum amount of work ef- fect or clamping pressure that can be generated. Two inches of mercury (2" Hg) equals about one pound per square inch (1 psi) of air pressure. (Remember that 1 atmosphere = 29.92 inches Hg = 14.7 psi) If you are vacuum bagging a one square foot laminate, a 20" Hg vacuum will yield 10 psi clamping force or a total of 1440 pounds of clamping force over the entire laminate. If you are laminating a 4' × 8' panel, the same 20" Hg (10 psi) will yield over 46,000 pounds of clamping force spread evenly over the entire panel. 2.1.2 Displacement The volume of air a pump can move (rated in cubic feet per minute or CFM) is also an im- portant consideration in the selection of a pump. If the vacuum system (the mold, bag, plumbing and all seams and joints) were absolutely airtight, any size pump should be able to eventually pull its rated Hg maximum vacuum regardless of the size of the system. How- ever, creating a perfectly airtight vacuum bagging system is nearly impossible, especially as the system gets larger or more complex. The greater the CFM rating, the closer the pump can come to reaching its Hg maximum and maintaining an adequate clamping force against the cumulative leaks in the system. A vacuum pump with a high CFM rating will also achieve an effective clamping force more quickly. This is an important consideration if the working life of the adhesive is limited or if the laminate will not hold its position until the clamping force is applied. 2.1.3 Horsepower and performance The horsepower requirement of the pump is an indication of how efficient the pump is and is not in itself an indication of how well a pump is suited to vacuum bagging. When select- ing a pump, use the “Hg maximum” and CFM ratings as a guide rather than horsepower. Smaller pumps designed for specific applications may trade off either vacuum rating or air displacement to suit a particular job. Generally, to get both higher “Hg maximum” and CFM ratings, more horsepower is necessary. Pumps that are useful for moderate boat yard vacuum bagging may range from 1/4 hp to 2 hp Pumps for large production operations may be as big as 20 hp or 30 hp. Vacuum Bagging Equipment 4 Figure 2-2 A typical vacuum pump capacity vs vacuum rating diagram. Note that the free air flow decreases as the vacuum pressure level increases. 2PDF Image | Vacuum Bagging Techniques 002150
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