Because of their very different treatment capacities, your need to use one over the other is pre-determined, the best example I can think of is that you wouldn’t use 10’000 little handheld fans to keep your office cool in summer, you would buy an air conditioning unit. Likewise, you wouldn’t carry an air conditioning unit around with you if out shopping (or wherever else you might use a handheld fan). This is relatively comparable to low and medium pressure applications.
While looking for an appropriate UV lamp and unit for your home, you wouldn’t need something capable of treating a small river, you need a unit that runs a low pressure UV lamp. Low pressure lamps run at lower temperatures (around 45°C ) and operate far more efficiently than their medium pressure counterparts (around 35% output at 254nm). The effect this has is that although similar in size, they’re cheaper, cost less to run and are far safer to be around (Your unlikely to get burned or scalded if there is no flow through the chamber for a prolonged period). The UV lamp life for low pressure lamps is usually 9000 hours with 18000 hour low pressure UV lamps becoming more available more recently.
Medium pressure UV lamps run at higher levels in every respect, they operate hotter (around 600-900°C), are far less efficient (8-10% at 254nm), cost more to run because of the power input they need, have less than half the lifespan (around 1000 – 4000 hrs) and they have long start-up and cool down times. Now although this seems to be heavily one sided in Low pressure’s favour, there’s a few other things to consider. Even though medium pressure UV lamps are only up to 10% efficient, due to them operating at around 10kW you achieve around 1000W of useful UVC output at 254nm.
Now, our more efficient low pressure UV lamps although more efficient, run on around the same power as a regular light bulb (around 20 -100w) making their useful UVC output for a 40W lamp around 24W, a difference of 976W between the 1000W from the medium pressure lamp and this one.
It’s obvious from looking at these numbers that if a regular low pressure system is adequate to treat the supply for a block of flats or even a shopping centre, it’s unlikely that anyone would need a medium pressure system for anything other than the most massive of industrial level projects. I hope that explains it all for you.