Chamber capacity is quoted in sterilisation units, and the arithmetic that gets you from a theatre list to a chamber size is short. Why two smaller chambers usually beat one large one, and why drying is the real constraint.
Chamber capacity for large steam sterilisers is expressed in sterilisation units, and EN 285 defines one sterilisation unit as a volume of 300 mm by 300 mm by 600 mm. A chamber described as six units holds six of those volumes. That is the currency to work in, because litres tell you nothing about how many trays fit.
Two chambers totalling the same capacity almost always serve a hospital better than one, for three reasons. A steriliser under service or under investigation after a failed test does not stop the department. Smaller loads turn round faster, which matters more than total capacity when theatre needs a set back. And smaller, less dense loads dry properly, which is the single most common quality problem in a Bangladeshi CSSD.
The costs are real: two chambers cost more to buy, more to service, more to validate and they take more floor space and more utility connections. For a small hospital with one theatre, one chamber plus a genuine contingency arrangement with a neighbouring facility may be the honest answer. For anything with two or more theatres, buy two.
Small bench top sterilisers fall under EN 13060, not EN 285, and they are a different class of machine for a different job. A tabletop unit with a removable tray rack is right for a dental surgery, a clinic, a small theatre's immediate use instruments or a ward procedure room. It is not a CSSD steriliser, and using one as the department's main capacity means running it continuously, which is how they fail.
Where they are genuinely useful is as backup and as local capacity that takes small urgent items off the main machines. Specify the cycle type carefully, because not all small sterilisers can process wrapped or hollow loads, and a machine that cannot handle a wrapped pack cannot produce anything you can store.
Chamber size is a decision about the plant room as much as about the machine. A larger chamber needs more steam, which means either a bigger generator or a connection to a house boiler with clean steam quality that has been specified. It needs feed water treated to the manufacturer's requirement, because hardness will scale the chamber and the pipework and shorten the life of everything. It needs a drain that can take the condensate and blowdown, and it needs ventilation, because a steriliser puts a substantial heat load into the room. It needs floor loading checked. And a double door machine passing through a barrier wall needs that opening built, which is a decision made with the building rather than with the equipment.
Sizing to today's theatre list is the first. If the hospital's plan includes a third theatre in four years, the CSSD serves it, and adding a chamber later means a plant room shutdown. Size the room, the utilities and the barrier wall openings for the final configuration even if you install fewer machines now.
Sizing to sterilising time rather than drying time is the second. The load that comes out warm and damp has to go back through, and a department that solves this by opening the door early to let packs dry in the room has stopped producing sterile packs. If a chamber cannot dry the loads the department actually runs, its rated capacity is fiction, and that is a question to settle during the performance qualification with your own heaviest load rather than with a demonstration load.