Heat Recovery Coil

The Dynamic Duo: Run-Around Coils Vs. Heat Pipe Heat Exchangers

In terms of heat recovery coils there are two heavyweights that enter the ring: the run-around coil and the heat pipe heat exchanger. Each one has its strengths, as if you were picking the right tool for the job.

Run-Around Coil Loops: Keeping Things Apart (And Clean)

Think of two coils, like siblings — except they are in different rooms. One is getting cold in your exhaust airflow, the other is getting hot in your supply airflow. They’re linked by a closed loop of piping, and a fluid — typically water or a glycol solution — is pumped between them.

• How They Work: A Closed-Loop Circuit The voodoo here is pretty simple: The fluid grabs heat from the warmer air in the exhaust coil, then zips over to the supply coil and drops that heat off to the cooler incoming air. This cycle continues, and the heat is continually passed around. A pump flows the fluid along and a control valve adjusts the heat transfer rate.
• The Real Perks: Zero Cross-Contamination Here’s the big flex for run-around coils: the airflows never mix. No mixing. Zip, nada. So if your exhaust air is scented like a number of times gone by, or packed full of particles, (I’m looking at you lab, hospital, some funky industrial fumes) it’s not sneaking into the fresh supply air. Hygiene and safety? Locked in.
• What’s the Catch? Efficiency & Freeze Concerns: For all that great separation, run-around coils also tend to have a bit lower efficiency than other air-to-air heat recovery means, somewhere in the range of 40% to 70%. And that circulation pump requires electricity, which can gnaw at overall efficiency. In cold climates, you’ll want antifreeze (such as glycol), though it can thicken the fluid and reduce the heat capacity of the mixture, increasing the power needed to pump it.
• Who Needs ‘Em? Perfect Applications: These coils are perfect when your air streams can’t sit side by side, or if total air separation is a necessity. Hospitals, research labs, some industrial facilities, or applications with comfort by comfort where remote ductwork is used are all good ones. Swegon, for example, provides full run-around coil units such as their GOLD CX and GOLD SD which offer solutions for hygienic and location- flexible air heating and cooling.

Heat Pipe Heat Exchangers: Engines Without Engines

For something a little different, now. Heat pipe coils appear as though they are still standard finned coils but each tube is a sealed, independent unit. Inside? A volatile refrigerant.

• The Inner Game: Evaporation & Condensation Hot air travels over one side of the heat pipes ( the evaporator side ). The refrigerant within warms, vaporizes and zooms off to the cold end (the side of the condenser). It gives up its latent heat as it reverts to the water state, which is released into the cold air stream. The liquid refrigerant can then return (flow back) gently to the evaporator side and the process can start again evenly. It’s a continuous, self-driving show.
• Why They’re a Game-Changer: No Moving Parts, No Freeze Risk This is partly where heat pipes really power up: they’re low-tech, passive devices. No mechanical parts, no pumps, no external tubing. That translates to less maintenance, fewer components to break, and no cross-contamination, because the air streams never intersect. And since they rely on volatile refrigerants, there’s less risk of annoying freeze damage that you’d get with water or glycol loops. 
• More Than Just Your Basic Heat Recovery: Wrap-Around Wonders Heat pipes are not only for basic air-to-air heat recovery. For example, Telawell manufactures “U”-shaped wrap-around heat pipes. These bad boys will pre-cool air before it gets to your main cooling coil and then re-heat it after. The result? Much-improved dehumidification without eating up additional energy. It’s a great thing to improve IAQ.
• Special Ops: Hermetic Heat Rejection & Dirty Air Streams Have an air contamination free environment? Consider high-voltage drives or delicate electronic gear. Heat pipe coils can contain hermetic heat rejection, containing those volumes. They’re also much more resilient in dirty or smoky air streams, like casinos, for example (where other forms of heat exchangers, such as energy wheels, would instantly foul up).
• Where Do They Flex? Applications: From regular HVAC applications to challenging industrial processing, heat pipes are flexible. And they can pre-cool in summer and pre-heat in winter, all in the same unit.

Why Your Business Can’t Afford to Miss out on Heat Recovery Coils (Seriously)

Listen, about being “green” — well, that’s a big plus here. The installation of heat recovery coils as an integral part of your system delivers real benefits that impact on your finishing line and enable your building to perform even better.

• Energy & Cost Savings Your Wallet’s Best Friend: This is the big one. Heat recovery systems can reduce your energy usage by 20-30% depending on your building. By reusing that waste heat, you’re relying less on your boilers or electric heaters — and saving big on utility bills. Heating: Heat recovery ventilators (HRVs) Studies have shown that HRVs can achieve savings of 4-5x their energy use, so you can expect a reduction in heating load of 15-18% even in the coldest climates.
• Breath Easy: Improved Indoor Air Quality (IAQ) As everyone knows, fresh air is good, but it’s very expensive to condition it. Heat-recovery systems do allow you to bring in a lot more fresh outdoor air while pre-conditioning it, without the huge energy penalty. For systems like run-around loops and heat pipes, with their different air streams, you also avoid concerns about cross-contamination that can cut mold, mildew and bacteria. Better IAQ equals healthier, more productive occupants.
• Future-Proofing Your Building: Sustainability & Compliance Stringent laws and regulations. In some places, such as New York City, Local Law 97 requires huge reductions in emissions of greenhouse gases. Here, heat recovery systems are effectively a cheat code: By decreasing the consumption of fossil fuels and not emitting CO2, they help your building hit those all-important compliance targets, while keeping sustainable. This isn’t merely expected; it’s turning into required.
• Fewer Headaches, More Life: Equipment Longevity When your heating and cooling equipment isn’t fighting huge temperature swings, it’s in use less. That’s a lesser workload requiring less wear and tear, for longer equipment life and fewer maintenance calls. It’s win-win: Save money on energy and on repairs.

Diving Deeper: Sizing, Performance, and the Nitty-Gritty

Getting a heat recovery coil right isn’t just guesswork. It involves some solid engineering to ensure you’re getting the best bang for your buck.

• The Math Behind the Magic: Key Equations Understanding heat transfer is fundamental.
  • Sensible Heat Transfer: Temperature is the Game Sensible heat is the kind that changes a substance’s temperature without changing its state. When air or water warms up or cools down, that’s sensible heat.
    • For air: Qair = 1.085 SCFM (LAT – EAT), where Q is heat transferred (BTU/hr), SCFM is standard cubic feet per minute, LAT is leaving air temperature, and EAT is entering air temperature.
    • For water: Qwater = 500 GPM (LWT – EWT), where GPM is gallons per minute, LWT is leaving water temperature, and EWT is entering water temperature. Remember, the heat gained or lost by the water side equals the heat lost or gained by the air side.
  • Total Heat Transfer (Enthalpy): When Moisture Joins the Party Most cooling coils aren’t just doing sensible cooling; they’re also handling latent heat. Latent heat is about changing state (like water vapor condensing into liquid) without changing temperature. Enthalpy is the total heat, combining both sensible and latent heat.
    • For air (total heat): Qair = 4.5 SCFM (LAH – EAH), where LAH is leaving air enthalpy and EAH is entering air enthalpy. This equation is your go-to for accurate total heat calculations.
• What Makes a Coil Perform? Critical Factors A coil’s performance isn’t just about its type. It’s about its DNA.
  • Rows, Fins, and Flow, Oh My! Things like the number of rows, fins per inch (FPI), how it’s circuited, and even the fin type (waffle vs. flat) all play a huge role in how much heat gets transferred and how much air pressure drop you get. More rows generally mean more heat transfer, but also higher pressure drop.
  • Don’t Forget Pressure Drop Air pressure drop is critical. If it’s too high, your fans have to work overtime, chewing up more energy. You’ve got to find that sweet spot between heat recovery efficiency and acceptable pressure drop. Typical face velocities for run-around coils are between 300-600 FPM to prevent water carry-over.
• Glycol? Freezing? We’ve Got You Covered In cold conditions, the fluid in run-around coils needs freeze protection, often with glycol. It’s a necessary step, but it does impact the fluid’s properties and the system’s overall efficiency. Heat pipe coils, thankfully, sidestep this issue with their volatile refrigerants.
• Your Digital Wingman: Sizing Software Trying to eyeball these calculations is a rookie mistake. Pros use software like Colmac’s “CoilPRO” to accurately estimate thermal performance, configure various setups, and get precise specifications. It lets you play with variables like airflow, temperatures, glycol concentration, and desired temperature differences to nail the design.

Keep ‘Em Running: Maintenance for the Long Haul

The best tech could always use a bit of love.

• Run-Around Loops: What to Look For These systems have pumps, valves and fluid that require routine inspection. Consider changing hydraulic fluids periodically (especially the antifreeze), lubrication of pumps and checking electrical connections. It’s more complicated than passive systems, I admit, because it has more moving parts.
• Heat Pipes: The Low-Maintenance Champs That ‘passive’ stuff from before? Heat pipes are largely hands-off. No parts to wear out, no moving parts that need to be maintained or replaced, and the refrigerant media never “loses its cool”. Just keep the air paths free and they will keep trucking.
• A Brief Comparison to the Other Guys For context, other energy recovery devices, like energy wheels, must also clean their media in order to avoid fouling, which has a longterm effect on latent efficiency. With heat pipes, you avoid that headache, even in dirty airstreams.

Coil Heat Exchangers: Providing the Energy for Various Industries

Nor are these coils limited to office buildings. They are everywhere, helping to change things, across the spectrum of industry.

• Commercial & Industrial Powerhouses They are a commercial HVAC industry staple for comfort conditioning and pre-treatment of supply air. At the industrial level, they recover waste heat from high-temperature exhaust, facilitating process cooling and heating. You are talking thermal oxidizers, boilers, turbines, and industrial ovens.
• Food, Power, and More With custom coils that are designed to resist corrosion in food processing applications and waste heat recovery units (WHRUs) in power generation, these coils are designed to withstand the toughest conditions. Special stainless steel tubes and material for high temperature and / or corrosive environment protection. Even buildings that require heavy cooling, like data centers, will work for heat recovery.

Conclusion: Next Steps for Smart and Efficient HVAC

So, there you have it. Heat recovery coils are not a ‘nice to have’ item, they are a smart investment that will save you money, make you more energy efficient and also more environmentally friendly. Whether you go with the pure separation of a run-around loop or the passive energy of a heat pipe, the right system can provide a massive performance boost for your building.

It’s all about working smarter, not harder when it comes to your HVAC. Excited to be rid of those sky-high utility bills and walk into a greener future? Contact the specialists and learn how heat recovery can make your building soar.