Last night onboard Theatreship we had an event with some brilliant musicians, including pedal-steel guitarist BJ Cole. BJ is a genius - his own work is deeply careful and experimental, and over a musical career that’s stretched back 60 years he’s played with everyone from Elton John to Brian Eno. In short he’s a living legend. …but last night it was 37 degrees in Canary Wharf, and Theatreship is fundamentally a metal tube. You’ll appreciate the urgent need for air-conditioning so that BJ retained both parts of that moniker.

We were quoted over £40,000 for a same-day professional install which was well beyond our means. Our means in this instance being about £200, 24 hours, and whatever we could find in nearby shops. We've only really been able to deliver a project of Theatreship’s scale by thinking hard about ways to repurpose what we have (that and the enormous generosity of the people who put in their time. In this particular case, a huge thanks to Trev, who managed to cut 6 perfect 150mm holes while up on a baking aluminium cargo roof. He's a hero with a hole saw).

A couple of years ago we’d bought three mobile air-conditioning units from Amazon - rated at 14,000 BTUs each. These had proven wildly ineffective and had been sitting in our stock cave ever since, but I wondered if they could be modified to resolve the issues. This was our analysis of why they were so ineffective, and the steps we took to resolve it. I’ve written this up because these are incredibly easy modifications to make that made an enormous difference to the efficiency of cheap units, so I hope it might be helpful to other people similarly underwhelmed by mobile aircon units, given the heatwave for the rest of this week.

Problem 1: waste heat removal. An aircon unit works a bit like a fridge. It compresses a refrigerant in one place, runs this compressed hot refrigerant through coils so it condenses and forms a liquid, pumps it to another place, and then lets it expand again. As it expands it changes phase (back into a gas) absorbing a huge amount of ambient heat to do so. What this means is that like a fridge, it has a hot side and a cold side. Unlike a fridge though, where the cold side is in an insulated box (the fridge), both sides are in the same space, the room. The aircon relies on ducting to move the heat away. Unless this heat is moved out of the room, the overall effect is simply to warm the room by exactly the power consumed by the unit (in our case around 1.5kW). Much cleverer men than me have explained why this is elsewhere, so I won’t repeat their work.

The factory supplied ducting is a plastic tube - as the waste heat moves through this tube it radiates heat back into the room. Spot readings on the tube showed nearly 60 degrees. The tube had a 6” diameter, so over the supplied 2m length of the tube that’s just under 1m^2 of surface area at 60 degrees.

This is compounded by the fact that the inlet vents sucking air into the unit are right next to this outlet pipe, meaning room air is consistently sucked over this duct. The overall transfer coefficient here looks to be about 28W/m^2K, so at a delta-T of 35 degrees we’re getting almost 1kW of heat going back into the room per unit.

Solution - I reduced the exhaust run to 1m, and insulated the exhaust. I used superfoil SF6, partly because that’s what I could get in Screwfix that morning, and partly because that provides a particularly good protection for radiative heat, and also isn’t itchy. With this fitted the lagged exterior read 27 degrees - reducing the delta-T to 3 degrees, and by halving the length we halved the heat transfer area, so the heat going back into the room reduced to around 40W.

Problem 2 - negative air pressure. The air that’s being thrown out through the exhaust in problem 1 has to be drawn from somewhere. On these mobile units, the air is drawn from the room. 500m^3 is exhausted from the units - that means that you’re throwing away 500 cubic metres of air that you’ve just put a lot of effort into cooling straight out the exhaust every hour (that’s all the air in the room - twice. For each unit). That creates a partial vacuum (which nature, anthropomorphised, abhors) - air travels along that pressure differential, sucked in from outdoors - so we’re replacing our chilled air with 37 degree air. At 500m^3/hr, we’re bringing in 580kg of warm air each hour. At a 12 degree delta-T, that’s approx 2kWh of heat - the equivalent of having an additional 2kW space heater on per unit.

Solution 2 - we ran a second duct from outside into the compressor cooling inlet vents. This means the vacuum drawn by the compressor cooling fans draws the air from outside rather than inside, keeping our cooled air in the room and reducing the vacuum (and therefore influx of warm air) to 0. I was lucky in that our units happened to have a big gap between the casing and the compressor coils, so it acted as its own plenum. We could tape up the vents and just cut through the plastic to fit the hose in, with that void space providing space for the air to expand back out so there weren’t local cold spots on the coils just where the duct entered. If your unit isn’t built like this you could also fairly easily make a box to tape around the inlet vents to provide this expansion void.

Problem 3 - air circulation - cold air is denser than hot air (which is why we get the wind that Theatreship used to be powered by). Mobile aircon units sit on the floor and rely on fans to move the cold air around the room, but in a room the size of Theatreship, they’re not sufficient. Rather than simply put more power (and therefore more heat and noise) into the room in the form of large fans, we just moved the aircon units up to the ceiling to allow convection to do the job for us. This was also a happy side-effect of shortening the hoses. The other benefit of this was taking advantage of the temperature difference in the room. Due to heat rising, the cooling effect of the dock water on the hull, and the heating effect of the sun on the roof, with no other air circulation there’s about a 5 degree temperature difference between the top of the room by the ceiling and the bottom of the room by the floor. Moving them from the bottom to the top meant the units had to work less hard, because the hotter air increases the temperature difference between the air and the coils, meaning the heat transfers faster. It also meant they weren’t sitting in their own cold spots. The combination of the removal of cold spots that would trigger the thermostat cut off and the reduced pressure on the machine, which prevented them from overheating, meant the machines didn’t cycle so often (or even at all - they remained on continuously, where previously they were on at most 80% of the time).

14,000 btu is approximately 4.1kW. With 1kW lost to problem 1, 2kW lost to problem 2, and 20% lost to cycle time in problem 3, we were originally running at just around 20% of their rated capacity. They were only removing a net of under 1kW heat from the space per unit. These alterations took less than a day, and allowed us to quintuple the efficiency of the units. The three of them together went from providing 2.6kW of cooling to 12kW of cooling - and all for the price of some hose, some insulation, and some time.

I’m delighted to say BJ survived the night.