The problem of dirty air heat recovery and a solution

The Problem With Dirty Air Heat Recovery.

As energy becomes more expensive and the reasons for recovering heat become more pressing it is difficult to be a bystander watching industrial processes discharge hot dirty air into the atmosphere. It seems to be bonkers that heat recovery is not standard on these dirty air exhausts. So much wasted energy and wasted money.                

Closer examination of this energy source and the available technology reveals some background as to why heat recovery on these types of air streams is not more common. The first thing to consider is the increased levels of droplet and particulate contamination on the airstream. This will inevitably present issues to the heat exchanger and will likely blind its narrow airways as the deposited contaminants build up. The instinctive reaction is some sort of filtration but there can be isues with this;

• Oily contaminants can react with synthetyic filters and can cause filter failure
• Dust loadings can be very high very quickly giving rise to eyewatering maintainance and spares costs
• Not all filters are suitable fur running at higher than ambient temperatures and many would struggle or even fail in the face of temperatures above 50°c. 

This is why dirty air heat recovery is widely regarded as being just too difficult, even when the amount of heat available to be recovered is potentially very large and could massively reduce users energy costs. In spite of very large potential energy savings, heat carried by dirty exhaust air is routinely wasted. 

When you coinsider all the industrial and commercial hot dirty air discharges that are thought to be simply too complecated and expensive to recover heat from you will realise that this is a sector that if there was a solution, it could make a very large contribution to reducing energy use across the World. 

The Solution to Dirty Air Heat Recovery.

Like so many great ideas, the solution comes from looking at the problem from a different angle.

In the case of dirty air heat recovery, instead of trying to clean the air before the heat exchanger, why not redesign the heat exchanger so that the contaminants don't blind it. This was the light bulb moment that engineers in Sweden had when they were faced with the problem. Their subsequent research and testing following this idea resulted in a heat exchanger design that used 'particle repellent geometry'.

The Swedish engineers noticed that when dirty air flows past a flat surface that is parallel to the airflow there is a tendancy for the particles to diverge from the air flow and stick to the surface. This is because of the coanda effect;

Basically as air passes a parrallel flat surface the friction that the surface has on the moving air casues it to spin in eddys. It is this spinning effect that makes the particles and droplets spin and hit the flat surface. The overall effect is that dirt and contaminants build of on the surface. When the surface is a heat exchanger, the buildup of contaminants reduces the rate of heat transfer. Worse still the accumulation of contaminants will restrict the air flow causing increased running costs for the system fan and will in time cause blockage preventing flow through the heat exchanger. 

The engineers were curious what would happen if instead of having flat heat exchange surfaces that seemed to blind with dirt, the heat exchanger was designed to force the air to always change direction, turning round heat exchange pipes. On test rigs they noticed that the heat exchanger pipes at the inlet end got some limited build up of larger particles from their direct impingement on the pipes. More interesting, however, was that the particulates did not build up on the heat exchanger pipes inside the heat recovery unit. What little build up did occur was such that it could be easily removed by cleaning.  It proved that the idea of keeping the direction of the air changing as it passsed through the heat exchanger actually works, and with this particle repellent geometry or PRG was born.  

The next stage of the development of PRG based Dirty Air Heat Recovery. 

The Swedish engineers, enthused by proving thier theory, continued to develop a marketable dirty air heat recovery system that had particle repellent geometry as its basis. Adding a tough housing, complete with headers for the pipes, and a drain for any condensates, they came up with a heat exchanger which by necessity was larger than an equivalent duty heat exchanger that might be typical for clean air application. 

The engineers realised that thier heat dirty air heat exchanger  was not so much a product but a design capability, and as it developed further, so did the software they used to design the system to make it suitable for each application they were presented with. No two applications are the same and by putting the parameters into the design programme they can ensure that the efficiency of the design is specific to the application and optimised for heat recovery efficiency. 

Enjay Dirty Air Heat Recovery Unit 

Because they are designed to suit thier specific application it follows that no two Enjay  are exactly the same although they do have a number of shared characterises irrespective of the application. The key to the Enjay design is the geometry of the pipe array, and the overall size of the unit for a given air flow, a function of optimised velocity through the unit. Other factors include on air temperature, heat recovery medium, duct size and other physical site constraints.   

Summary 

The particle repellent geomtry of the Enjay heat revovery units make them the ideal basis for recovering heat from dirty exhaust air streams. Their inherent resitance to clogging means that they do not need expensive prefiltration and can go for prolonged periods between maintainance, whilst recovering heat energy that can be reused to best economic effect elsewhere in the process or indeed in space heating the building. Numerious installations are proving that the system is realaible and cost effective with rapid payback on investment in suitable applications. 

More detail on the Enjay dirty air heat recovery units can be found here.

If you have an application and you can see the benefit of recovering the waste heat then let us know more about it so we can answer the questions you may have. 01729 824108