Gravity Feed vs Conventional Feed Spray Guns


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When selecting a spray gun you have a choice between gravity feed (cup on the top) and conventional feed (cup on the bottom). It is really a misnomer to call a conventional feed spray gun “siphon feed” because “siphon feed” has a specific meaning. However, using the term “siphon feed” often communicates “cup on the bottom” to many technicians. This article discusses the differences between the two systems.

 


Conventional
Feed

 


Gravity Feed

 

On the left are samples of gravity feed and conventional feed spray guns.

Conventional feed guns were the first to be widely used in autobody repair. Until the advent of HVLP (high volume low pressure), all conventional feed spray guns were siphon feed.

In the 1980’s gravity feed spray guns came into wide use and their popularity has overtaken the use of conventional feed. Painter’s have begun to favor them over conventional feed guns and the introduction of HVLP as a requirement by environmental laws made gravity feed the natural choice.

Spray guns all have one thing in common - the atomization of fluid at the tip where the paint leaves the fluid nozzle. There are two passageways in a spray gun - one for air and one for fluid. Atomization is achieved by a vacuum formed by the air rushing past the fluid passage while drawing the fluid into the air stream. At this point, air is mixed with fluid and a spray mist ejects from the nozzle.

In order to achieve sufficient atomization, paint has to be either drawn up from below the gun (conventional feed) or fed from the top down into the nozzle (gravity feed).
In order to achieve sufficient atomization with siphon feed, greater air pressure must be used in order to attain a strong enough vacuum to pull the fluid up the feeding tube from below. The requirement for greater air pressure in siphon feed guns is the main limitation for their use in HVLP systems. It is much more difficult to keep low air pressure and achieve adequate vacuum.

It is obvious that gravity feed overcomes this limitation and less air pressure is required to atomize the paint.

 

Spraying with less air pressure has the advantages of less overspray, less waste and greater control for the painter. These are the reasons why painters have been switching from conventional feed to gravity feed guns.

Although HVLP is a requirement in most areas, whether or not you use an HVLP gun, gravity feed offers the advantage over conventional feed because of the lower air pressure requirement.

For those users who prefer conventional feed, it is possible to achieve atomization at lower air pressures by pressurizing the cup. This assists the siphon process by forcing the fluid up the feed tube. You can identify a pressurized cup by the presence of an air tube that extends from the air supply to the cup.

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