The Washdown Pan | Wash-out Pans | Syphonic Pans

Introduction

Lavatory pans are usually made of vitreous china, but used to be made of fireclay or earthenware. Manufactured in one piece, they are complex pieces of ceramic and took many years to perfect. The first lavatory pans were formed of two or more pieces, with the bowl cast as a separate unit to the trap. Hopper closets took the form of a conical pan and an s-trap, and evolved to form the washdown closet, whilst the wash-out closet evolved from the highly complex monkey closet of 1854.

1: The washdown closet.

The washdown closet is so called because the contents are washed down by the force of the flush. The pan consists of a cone-like bowl with a flushing rim connected to the water inlet, with a water trap at the base before the outlet, all cast in one piece.

Whilst simple in action, much thought has gone into the design of the washdown pan. You will note from the vertical cross section (Figure 2) that the back of the pan is nearly vertical whilst the front gently curves. The sharply sloping back minimises soiling of the porcelain, and the gently sloping front is necessary to allow the outlet to be at the back of the oval pan (see Figure 1). Years of research have determined the optimum placement of the outlet so that solids drop straight into the water rather than onto the porcelain. Even the trap has been optimised to provide a water seal against sewer gas without dissipating the energy of the water.

2: The wash-out closet.

The wash-out closet has been condemned in Britain since the beginning of the twentieth century, but is common in some European countries. Water enters through the inlet (Figure 3, A), and is directed through the rim into the bowl. The bowl (B) has a self-contained pool of water trapped by a lip (C) at the front of the pan, and the flush washes the contents of the bowl over the lip, down the vertical pipe into the trap (D) and through the outlet.

The wash-out fell out of favour in Britain because the bowl into which waste falls has to be shallow to allow the contents to be swept out, and so tends to be easily soiled. The trap situated down a narrow tube is also more prone to become blocked, and more difficult to clear. The pans also tend to be larger than washdown closets.

3: The syphonic closet.

The syphonic closet is rather more complex than washdown or wash-out closets, since it does not rely on the force of the flush to wash the waste out of the pan. Instead water flowing into the pan sets up a syphon which sucks the contents of the pan through the traps and through the outlet.

Figure 4 shows a cross-section through a double-trap syphonic pan. As with a washdown pan there is an inlet (A), a rim to distribute the flush water around the pan (B), and a water trap (C) at the base of the pan. In addition there is a diversion tube (D) between the inlet and the vertical leg of a second trap (E). The second trap leads to the outlet (F).

When the closet is flushed, water enters the pan via the rim, but some is diverted straight into the vertical leg of the second trap, as shown in Figure 5. This compresses and picks up (entrains) air as it falls, and sweeps this air out of the second trap. This lowers the air pressure in the vertical leg, causing the contents of the pan to be sucked through the first trap. Syphonic action having been set up, the syphon is maintained by the water flowing into the pan and through the traps, flushing the closet completely. When the cistern empties the flow of water from the cistern drops, air enters the syphon leg, the syphonic action is broken and water ceases to flow out of the pan.

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