Sieve Tray

A sieve tray is one of the perforated plates stacked up inside a tray column, a plate column being the alternative to a tower filled with packing. Each tray is a horizontal metal deck drilled with a regular field of small holes, the sieve. The column works tray by tray. Liquid arrives on the tray from the one above, flows across it in a shallow pool whose depth is set by a low dam, the outlet weir, and then tips over the weir into a downcomer, a walled channel that carries it down to the next tray. Meanwhile the vapour rising from below cannot get past except through the holes, so it jets up through them and bubbles through the pool of liquid, whipping gas and liquid into a froth. In that froth the two phases exchange material, the lighter components moving into the vapour and the heavier into the liquid, which is the whole purpose: a stack of trays performs the distillation, absorption or stripping. Each tray is a single stage of that separation, and the more trays, the sharper the split.

There are three classic tray types, and they trade cost against flexibility. A sieve tray is the simplest and cheapest: just a punched plate. It gives a high capacity and a low pressure drop and is easy to make and clean, but it has a limited turndown, since running the vapour too slowly lets the liquid rain straight down through the holes, which is called weeping, so the tray only works well over a fairly narrow range of rates. A valve tray puts a movable cap over each hole that opens and closes with the vapour flow, which widens the operating range a lot while keeping cost moderate; it is the common modern default. A bubble-cap tray puts a fixed, slotted riser and cap over each hole so that liquid always stays on the tray even at very low vapour rates, giving the widest turndown of all, but it is the most expensive and has the highest pressure drop. So: sieve for simple, steady, cost-sensitive duty; valve for a wide operating range; bubble-cap where the rates swing very low. Tell us your vapour and liquid rates and we will recommend the tray.

Trays and packing are the two ways to build a mass-transfer column, and each suits different service. Trays, whether sieve, valve or bubble-cap, are usually preferred for high liquid loads, for large-diameter columns, for dirty or fouling service because they are easy to clean, and where the design needs side-draws, cooling coils or many stages in one shell. Packing, random or structured, sitting on support plates and fed by liquid distributors, is preferred for a low pressure drop, for vacuum columns where that matters most, for corrosive service where ceramic or plastic packing is cheaper than exotic-alloy trays, and for small columns. Plenty of towers use both, packing in some sections and trays in others. If you are choosing between them, the liquid rate, the operating pressure, the fouling tendency and the column size usually decide it. Send us the duty and we will advise trays or packing, and size whichever you need.

A sieve tray is made to suit the fluids and the column. The usual material is stainless steel, 304 or 316L for corrosive streams, or carbon steel for mild duty; more resistant alloys or even plastic are used for special chemistry. It is cut to the inside diameter of the column, and for anything but a small tower it is supplied as sectioned panels that pass through the manway and are assembled and sealed on the support ring inside. Sieve trays are used throughout distillation, absorption and stripping, in oil refining, petrochemicals, gas processing, and general chemical plants, wherever a plate column is the right choice. Because the listing supplied gives only the name and model, with no material, dimensions or hole pattern, the details here are the usual ones for a sieve tray; the design has to be set to the column and the duty, so please confirm the specification before ordering. Send us the column diameter, the vapour and liquid rates and the service and we will design the tray.