Wind Tunnel Heating Coil | University of Surrey

SPC has supplied a bespoke wind tunnel heating coil to the University of Surrey’s Environmental Flow (EnFlo) Laboratory, where it now sits at the inlet of the laboratory’s meteorological wind tunnel. The coil preheats the incoming air using hot water from a heat pump, which recovers heat rejected at the downstream end of the tunnel, helping to reduce the energy needed to simulate stable and unstable atmospheric boundary layer conditions. Built as a single 3.5m wide water-to-air heat exchanger with six independently piped parallel circuits, it lets researchers shape the temperature profile through the height of the airflow while adding very little resistance.

Background: Recreating Real Atmospheric Conditions

The EnFlo Laboratory was established in 1993 in the University of Surrey’s School of Engineering and is recognised for its research into atmospheric boundary layer flows, air quality and pollutant dispersion. Its main wind tunnel is part of the National Wind Tunnel Facility and has a 20m long working section, 3.5m wide and 1.5m high, with air speeds from 0.5 to 5m/s. The facility describes it as the only tunnel of its kind in the UK.

Most wind tunnels run with air at a uniform temperature, but the real atmosphere rarely does. On a clear, frosty night the air is warmest at height and coldest at the ground, and on a hot, sunny day the pattern reverses. To reproduce these non-neutral boundary layers, the tunnel uses controlled temperature differences to create stable or unstable conditions, which makes accurate control of the incoming air temperature central to the work. Further information is available on the University of Surrey EnFlo Laboratory page.

The Challenge

The EnFlo team approached SPC in July 2026 looking for a bank of water-to-air coils to sit across the tunnel inlet. The brief had three main requirements. Each level of the air stream needed to be heated independently, so the profile could be set hottest at the top or hottest at the bottom. The pressure loss across the coil had to be as low as possible, since any resistance at the inlet limits the wind speed the tunnel can reach. And the coil needed to suit a hot water supply from a heat pump, as part of a wider heat recovery system designed to cut the tunnel’s running energy.

The initial enquiry was for six separate coils, each delivering around 50kW at a face velocity of 1.5m/s, with hot water at around 70°C. The depth of the coil in the direction of flow was not a concern, which gave SPC room to optimise for low air resistance.

Our Heating Solution

SPC’s Technical Manager, Richard Meskimmon, led the design. Six coils stacked one above the other would have been easier to handle and install, but Dr Hayden confirmed that the dead space between them would disturb the uniformity of the flow entering the tunnel. Richard therefore proposed a single coil split into six circuits, each with its own flow and return connections. The airflow stays continuous across the full inlet, and each height band can still be controlled on its own.

The coil was selected at a face velocity of 1.47m/s, close to the 1.5m/s target, with an air pressure drop of just 6.6Pa. Three rows of 12mm copper tube carry rippled aluminium fins, and an enclosed galvanised steel casing allows it to be fitted straight into the tunnel inlet. Like all SPC water coils, it was pressure tested to 22 barg.

Because it is fed by a heat pump rather than a boiler, the coil supports a low-carbon approach to running the tunnel: heat that would otherwise be rejected at the tunnel exit is put back to work at the inlet.

Key Specifications

Tunnel Inlet: Bespoke Water Heating Coil (x1)

Overall size: 3,500mm wide x 1,528mm high x 230mm deep
Construction: 3 rows of 12mm copper tube, 40 tubes high, rippled aluminium fins (0.25mm), copper phosphorus braze
Casing: enclosed, 16g galvanised steel with 50mm flanges
Connections: six sets of 1″ flow and return connections, with vent and drain
Weight: 213kg, with an internal volume of 49.5 litres
Air side: 7.875m³/s at 1.47m/s face velocity, 20°C to 41°C, 6.6Pa pressure drop
Water side: 70/50°C, 2.483kg/s total flow, 7.9kPa pressure drop
Total coil duty: 203.9kW across all six circuits
Tested to 22 barg

Project Outcome

The coil is now installed at the inlet of the EnFlo wind tunnel, and the tunnel has been run with it in place. Once connected to the heat pump circuit, it will preheat the incoming airflow using recovered heat, reducing the energy needed to run the tunnel when simulating non-neutral atmospheric boundary layers. The single-coil, six-circuit design gives the laboratory the controlled temperature profiles it needs without compromising flow uniformity or tunnel speed.

SPC is pleased to have supported the University of Surrey’s research with a coil engineered specifically for the application. For other bespoke projects, see SPC’s coil heat exchangers.