"We haven't got a cooling tower, it's just the condenser for the cold store." We hear some version of this fairly often. And it's an understandable mix-up, because from the outside an evaporative condenser can look like a smallish cooling tower, and in day-to-day conversation the names get used loosely.
The difference matters less than people hope, though. Both use evaporation to reject heat, both create spray, and both are covered by the same law and guidance on Legionella. Here's how they differ, and what that means for keeping them under control.
How a cooling tower works
A wet cooling tower cools water. Warm water comes back from whatever it's been cooling (usually a chiller's condenser or a process heat exchanger), is sprayed or distributed over fill inside the tower, and a fan pulls air through it. A small amount of the water evaporates, which takes heat with it, and the cooler water collects in the pond at the bottom before being pumped back round.
The key point: the water in the tower circulates through pipework to equipment elsewhere on site. So the "system" isn't just the tower. It's the pond, the pumps, the pipework, the heat exchangers and any dead legs along the way.
How an evaporative condenser works
An evaporative condenser cuts out the middle step. Instead of cooling water and sending it off to a separate condenser, the refrigerant itself runs through a coil inside the unit. Water is sprayed over the outside of the coil, a fan pulls air across, some water evaporates, and the refrigerant inside condenses. The spray water drops into a sump and is pumped back up to the sprays.
You'll often find evaporative condensers on industrial refrigeration, cold stores, food and drink sites and some larger air conditioning plant. Closed-circuit coolers work on a similar idea, with a process fluid in the coil instead of refrigerant.
What's the same
From a Legionella point of view, quite a lot:
- Warm water and spray. Both have recirculating water at temperatures that can suit bacterial growth, and both throw out aerosol that people nearby could breathe in.
- Same law and guidance. The Health and Safety at Work etc. Act 1974, COSHH, ACOP L8 and HSG274 Part 1 all apply. HSG274 Part 1 covers both types of system.
- Same notification duty. The Notification of Cooling Towers and Evaporative Condensers Regulations 1992 cover both, as the name suggests. We explain how that works in do you need to notify the council?
- Same building blocks. A risk assessment, a written scheme, a treatment programme, regular monitoring, cleaning and disinfection, and records.
What's different in practice
The differences are mostly about how the control measures work day to day:
- Smaller water volume. Evaporative condensers often hold much less water than a cooling tower system. Chemistry can change quickly, so a dosing fault or a stuck bleed shows up faster, and dosing needs to be well matched to the system.
- The coil is the heat transfer surface. Scale or biofilm on the coil goes straight to refrigeration performance and energy use. Good inhibitor control and bleed management pay for themselves.
- Cleaning access. Getting to the coil, sprays and sump for a proper clean can be fiddlier than on an open tower, and some units need panels removed or specific access arrangements. That needs planning, especially on sites that can't lose refrigeration.
- Cleaning frequency. HSG274 Part 1 expects cooling towers to be cleaned and disinfected at least twice a year. For evaporative condensers, the frequency should be set by your risk assessment, taking account of the guidance. Either way, it goes in the written scheme.
- Different people looking after it. Evaporative condensers are often maintained by refrigeration engineers rather than the building services team. Make sure someone's clearly responsible for the water side, not just the refrigerant side.
Imagine a food distribution site with an evaporative condenser serving its cold rooms. The refrigeration contractor services the plant carefully, but water treatment has never been formally set up because everyone thought of it as "part of the fridge". There's no risk assessment covering it and it's never been notified. Getting it covered is simple: assess it, notify it, set up treatment and monitoring, and agree who does what between the refrigeration and water treatment contractors.
And dry or adiabatic coolers?
A dry cooler with no water involved isn't a cooling tower or an evaporative condenser, so it's outside the notification regulations. Adiabatic and hybrid coolers sit in between. Some spray water onto pads or coils in hot weather, which can create aerosol in some designs. HSG274 Part 1 expects the Legionella risk from any unit that can produce spray to be assessed. If you're not sure where a unit sits, get competent advice rather than assuming.
How to tell what you've got
If you're not sure, a few quick questions usually settle it:
- Does water leave the unit and travel to equipment elsewhere on site? That's a cooling tower.
- Is there a coil inside with refrigerant or process fluid in it, and water sprayed over it? That's an evaporative condenser or closed-circuit cooler.
- Is there no water involved at all? That's a dry cooler.
The nameplate and the manufacturer's literature will usually confirm it.
The bottom line
Whatever it's called, if it recirculates water and makes spray, it needs a Legionella risk assessment, a treatment programme, monitoring and regular cleaning and disinfection. We look after evaporative condensers as well as cooling towers, and we're happy to work alongside your refrigeration contractor. If you've got a unit you're not sure about, give us a ring.
Got an evaporative condenser nobody's quite sure about?
Tell us what's on site. We'll help you work out what it is, what it needs and who should be looking after the water.