AHU Coil: The Ultimate Guide to Types, Function & Maintenance

Ok, enough of the gibber-jabber, let’s move onto something very important to your inside comfort the AHU Coil. Have you ever entered somewhere that is the right temperature? Not too hot, not too cold, and the air simply breathes well?’ The likelihood is, an AHU coil is doing all the hard work behind the scenes. So what are these things and why should you care? We’re going to the bottom, no fluff, just the stuff you need.

Understanding AHU Coils: Your HVAC System’s Silent Workhorse

Just consider an coiling and AHU act as a major part of your Air Handling Unit (AHU). Its charge is a simple one, though it is the most important of all: to cool or heat the air in a space and keep you comfortable. Most of those coils are fabricated out of copper or aluminum tubing, chosen because they’re the kings of the heat transfer. It’s about getting that thermal energy where it needs to go, and doing it quickly.

The Science of AHU Coils: What They Do and How They Do It

The heat exchange process is the name of the game for an AHU coil. It really is a silent transaction that’s occurring all the time.

• When in the cooling mode, pretend we have refrigerant or chilled water passing through this coil. As the air from your room flows over this cold coil, the coil actually pulls the heat out of the air. But here’s the fun part: This process also dehumidifies the air, so your space feels less sticky and much more comfortable. It’s like a two-for-one for better air quality.
• In reverse, it’s the reverse. The hot water AHU heating coil pumps warm water through its tubes, heating the air inside to warm your space. Nothing fancy, works, and doesn’t make you shiver.

The making of the AHU Coils, All coils are NOT the same

If you wouldn’t use a screwdriver for every task, then you shouldn’t use an AHU coil for all kinds for use either. What you really need depends on how your system is designed, and what your building needs to be able to do.

Here’s the lowdown on the most prevalent ones:

• Chilled Water Coils: The workhouse of cooling. These carry chilled water, doing the heavy lifting in air-conditioning systems, particularly when it’s sizzling outside.
• Hot Water Coils: Whenever you need some heat, these coils deliver. They pump hot water to heaters and even throw cold water in as a part of a dual-coil system (one for heating and one for cooling).
• DX Coils: DX coils are also greeted for cooling as they cool air by means of refrigerant. You’ll also see them in smaller HVAC systems.
• Evaporator Coils (Cooling Coils): These bad boys are extremely important in the world of air conditioning; they suck up all of the heat in the air to cool it down. They are paired with condenser coils in refrigeration systems to cool things down.
• Steam Coils: Coils using steam for heat exchange. They’re especially handy in applications where individual tubes can accommodate greater thermal expansion, providing reliable heating.
• Runaround Coils: You have supply and exhaust AHUs in different locations, but you want to save energy. There are the runaround coils — the “cheat code,” as one architect put it. Whatever they have coils in both the supply and return air handling units, and the coils are interconnected with piping. A pump moves water between them, grabbing waste heat from the exhaust air and dumping it into the fresh air intake. This greatly reduces the amount you need to fire the heating coil, particularly on a cool day, when it’s chilly outside but your return air is warm. It’s smart, energy-saving design.
• Industrial Coils: Available in numerous materials, geometries and sizes for almost any industrial or commercial fluid cooling application, these are all finned tube heat exchangers with the exception of a large portion of the CT models. These contain water heating and cooling coils, evaporators, refrigerant condensers, diathermic oil heating coils and steam condensers.
• Refrigeration coils: These are commonly used for HVCR applications and are usually box-mountable and can be made of a variety of material depending on the application.
• Heater Batteries: Some types include electric heater batts for best heating range.

Where AHU Coils Strut Their Stuff: They’re Everywhere

AHU coils are more than for just keeping it cool at your office, too – they’re essential in a variety of industries where controlling the temperature is critical. You’ll find them in:

• Hotels: Maintaining comfortable guest spaces with premium air-conditioning and dehumidification.
• Hospitals: Necessary means of ensuring required humidity levels and cleanliness: even in operation theatres.
• Government: Maintaining comfortable temperatures for an efficient work environment.
• Petrochemicals: Ensuring essential cooling and heating for intricate manufacturing processes.
• Building owners: Ensuring students and employees are comfortable inside, working to implement reduced energy use, efficient heating and cooling.
• Pharmaceutical: Critical for controlling the temperature of drugs and preserving aseptic conditions (particularly in clean rooms).
• Data Centers: Protecting Against Electronic Overheating.
• Food Processing: Keeping held food safe and in good condition, such as Dairy plants.
• Malls, Exhibition Halls, Auditoriums, Factories, Offices, Sports Halls: Essentially, anywhere it is necessary to maintain certain specified environmental conditions.

AHU vs. FCU: Knowing the Difference

It’s easy to get these two mixed up, but they’ve got distinct roles.

The AHU is your building’s central lung system, pushing conditioned air everywhere. An FCU? That’s more like a room-specific air conditioner, handling just that local spot.

The insides of an AHU Coil – construction and materials

Have you ever wondered what makes these heat-transfer behemoths tick? Let’s break it down:

Tubes: The workhorses. They’re usually copper, and you can get them in all the usual outside diameters (O.D.), such as 5/8”, 1/2”, or 3/8”. The thickness is generally between 0.3 and 0.6 mm or so, about. 020” to. 035” for durability. Some brands employ “belled ends” to further decrease pressure drop, which is a good move.

Fins: They are the enhancers for your heat transfer. IIRC stock fins are.100” aluminum this stock. 0075” in thickness and had a FPI ranging from 5 to 12. They increase the surface area for the most heat exchange.

Casing: The frame that keeps it all together. Hot water or steam coils: are generally fabricated from Galvanized steel. For cold-water coils (which handle a lot of moisture and condensation), however, you’ll want 16 gauge 304 stainless steel. Why stainless? It laughs at water corrosion, after all. Most casings have a 1-1/2″ leg, but can be wormed as tight as 0.75″ or 1″ if space is tight.

Headers: These are the manifolds that deliver fluid to the tubes. They may be carbon steel or copper. In cooling coils, especially when dealing with heavy condensation we frequently see “non-ferrous headers” to prevent rusting. Why are ends called “Connections”?Connections have Male Pipe Threads (MPT).

Coil Sizing:

• Fin Height: Typical maximum for heating coils, a FH of 60” is normally the largest practical size that can be handled. Cooling coils (especially those that generate a lot of condensate and have a drain pan ) can achieve ratings up to 42” FH. Why the limit? Condensation up top can screw up air flow down below.
• Fin Length: Without glycol fin lengths can be very long up to 180.” But if you are running glycol, the water pressure drop (WPD) increases astronomically with longer lengths, and you might need to run your coil in two or four lengths half the size of the larger one.

Coatings: If your AHU is being installed chilling (or heating) on the ocean, you need a coating. Electrofi n is frequently a recommended choice for a fl exible, durable product that doesn’t chip and can work well in multiple rows. It’s, like, putting a suit of armor on your coil.

AHU Coil Selection Criteria: Comfort!

Choosing the correct AHU coil is not a stab in the dark. It’s all about matching the capabilities of the coil to what your system requires for best performance and greatest energy savings. Here’s what the pros look at:

Face Velocities: This is the speed with which air passes across the coil face. There are different types of coils that have optimum velocity limits:

• Hot Water (HW) & Steam Coils: 200 – 1200 FPM. Your average HW coils run at 600-700 FPM.
• Chilled Water (CW) & DX Coils: 200 – 550 FPM. A standard CW drum is ~500 FPM. Why is there a floor for cooling?’ Go over 500 FPM, and you risk moisture carry-over (or having the velocity be too high for water droplets to hit them and get pulled into the system) where water droplets actually might get divorced from the fins and into your ductwork – not a good thing.

Air Pressure Loss: You’d prefer to have the air travel through this coil with little resistance. Designers typically want a pressure drop of less than 1 inch, and around 1/2 inch is even better.

Water Coil Pressure Drop (WPD): This is the resistance of the water as it moves through the coil. The point of diminishing returns in most cases is around 25 feet WPD. HW coils generally need no more than 10 WPD, while CHW coils will have 15-20 WPD. This can be reduced by fitting bell ends to the tubes.

Tube Velocity: The speed with which the fluid flows through the tubes. 3 to 6 feet per second is normal, and certainly not above 8 ft/sec. Glycol will affect this.

Reynolds Flow number: This is a nerdy but significant one. If you go much below 2100, you get laminar flow, and that’s no good for heat transfer. The fix? You’re best off re-circuiting the coil, or you could use “turbulators,” (which also introduce a pressure drop).

Antifreeze Considerations (Glycol): Add propylene glycol (PG) as an antifreeze and the game is radically altered. It’s sucks at facilitating heat transfer, so think more GPM, bigger coils, bigger pipe, etc. So, if you get a request for 40% glycol, turn around and question it!

Fouling Factors: Use 0 where possible. This is the accumulation of matter that can interfere with heat exchange.

Pro-Tip: If you have the space, adding some footprint to the coil section to drop air velocity to 300-400 FPM can save you a lot of fan energy throughout the life of the unit, even though it is a little more upfront. It’s a play for long-term energy savings.

Maintaining Your AHU Coils in Top Condition: It All Comes Down to Maintenance

Just as your car requires a regular tune-up, so does your AHU coils to ensure they continue to run smoothly and without expensive performance failures. Ignore them, and you risk lower efficiency, worse air quality and higher energy costs.

Here’s your maintenance playbook:

• Frequent Cleaning And Check Up: In time, your coils will accumulate dirt, mildew, grime, or even mold. This buildup isn’t just unsightly; it forces your system to work harder and can even reduce air quality. Regular cleaning is a must. If you don’t want to do it yourself, then hire a pro – these guys will know what they’re doing and also check for blockages in your drain line.
• Inspect for Leaks and Corrosion: Inspect your coils for leaks and inspect for damage to the outside of the coils. Leaks result in lost refrigerant or water, and corrosion causes the coil to deteriorate. You can head these off early to avoid big headaches.
• Check Coil Fins: Despite how tough it is for them to be, those thin, delicate fins are little heat-super-transfer heroes! Ensure they are straight and in good condition. The fins being bent, clog up in addition to deaths the heat transfer.
• Clean and Switch Out Air Filters: Obviously. Dirty air filters strangle your airflow and allow junk to get to your coils, and that makes them dirty sooner. Replace as necessary to maintain the free-flow of air and extend the life of your coils.
• Examine Fluids: If your AHU coils are refrigerant or heating fluid coils observe the condition of these and the level should be monitored. If levels are too low or if there is a leak, the coil will not be able heat or cool as it should.
• Regular Maintenance: Think of it as an investment, not a cost. A routine maintenance agreement with an expert can help you steer clear of massive, expensive repairs and system breakdowns. They’ll catch problems you may not notice and extend the life of your system.

Is Your AHU Coil Acting Up? Troubleshooting 101

As good as maintenance can be, things can go sideways. Normal wear and tear from a bad AHU coil can cause a lot of base nuisance in your comfort and your energy bills, depending on where you live in the world. Here are the red flags to look for:

• Decreased Heating or Cooling Effectiveness: Are you no longer reaching the temperature you want to be? If it’s having trouble keeping you warm or cool, a bad coil may be to blame.
• Unusual Noises: Do you hear hissing, gurgling, or other noises emitting from your AHU? That’s usually a symptom of a coil issue.
• Water Leaks: Find water collected around your AHU? This is a fairly clear sign of a problem, such as a broken cooling coil.
• Frozen Coils: When you notice ice developing in your cooling coils, you have a real issue. If the coils are frozen it greatly compromises the efficiency of the coil and the flow of air.
• Rising Energy Costs: If your bills are skyrocketing without an increase in usage, a battered coil can cause your system to work overtime and consume more power.

Optimizing Your AHU’s Brain: Heating and Cooling Control

This is potentially where you can gain some efficiency. It’s smart control on your AHU coil operations to not waste energy.

Avoid SimHAC (simultaneous heating and cooling) Yes, this goes without saying: you do not want cooling and heating coils to be simultaneously active. “Which is like hitting the gas and the brakes at the same time — a huge waste of energy.” Cross contamination may be an unavoidable combination of climates which need dehumidification but the objective is to reduce it.

Know Your OAT Lockout Set Points: This is a “game cheat code” as far as overall efficiency goes.

• Heating Lockouts: Until about 50°F outdoor temperature, heating should be”locked out” in many buildings. If the building is able to hold the discharge-air temperature without heating, then why heat?.
• Cooling Lockout: Similarly, cooling should be locked out when the outdoor temperature falls below around 55°F (12.8°C). And again, if the building can be kept at a comfortable temperature without running down the temperature of the building all the way to 72 degrees, shut off that cooling.
• Nonoverlap: These set points must not overlap. If you have a cooling lockout that is lower than the heating lockout, that’s a red light — it could indicate a problem with your leaky valves. The objective is plain: we shouldn’t be cooling in winter and shouldn’t be heating in summer.

Finding Valve Leaks for Savings: Youd have to listen carefully to hear a leaking heating or cooling coil valve sip energy 24 hours a day. This is something you can sometimes see by monitoring your coil valve signals and OA temperature. That’s “bad operation” if your cooling coil valve is spilling wide open, as it might on a sub-zero winter day. Quick and dirty test; if the pipe burrowing through the AHU is colder than the room should be when you don’t have any cooling going on, that valve is probably sneaking. The same is true for a heating coil that is warm when it should be off. Fan, heating and cooling energy use is higher when valves are leaky.

When it’s Time to Bring in the Heavy Hitters: AHU Coil Replacement

At times you just need to toss a coil and get a new one. If your AHU coil is super dirty, rusted, or inefficient, it’s time to do a replacement. It’s not just a replacement part; it’s an extension of the life of the entire AHU.

Replacement services typically involve:

• Complete Inspection: Pros will investigate all aspects of your current coil to locate any problems and accurately replace the device.
• High-Quality Coils: They’ll install high-end quality coils made to your AHU’s specifications which are durable and high performing.
• Efficient: The new coils are engineered to maximize heat transfer leading to less energy used and lower energy cost.mk
• It fits like a glove: The Mantis is not just a typical drop-in coil. Rather, it is a completely integrated product that works seamlessly with your existing AHU.
• Deep Cleaned: Surrogate will not work at optimal performance unless all surrounding parts are meticulously cleaned during the replacement.
• Less Downtime: First class service will always be about minimizing downtime and disruption to get your system up and running good as new.
• Compliance: All labor will be in accordance with local safety regulations and standard HVAC practices.
• After Replacement Performance Testing: The AHU is thoroughly tested and adjusted after installation to reach optimum performance levels.

One intelligent choice also is Removable Coil Modules (RCMs). These are one off sections we manufacture to make replacing whole coil housing sections on your existing AHUs simple. You can have those to fit your AHU size, with new coil supports, even with stainless steel drain pans- to in the long run a good quality AHU”’s life can add many years and big bang for your buck$! Call it a precision surgical strike to reboot your system without a full overhaul.

FAQs for AHU Coils

Got more questions? Here is some quick-fire responses to various questions:

Is there refrigerant in air handling units? They can, absolutely. The heat exchange fluid in the AHU coils can be refrigerant. But it’s a generic way; you could be using hot water, or chilled water or depending on the heating or cooling requirements for the building.

Can an AHU Operate With No Chiller? Yes, it can. Though a chiller is the most widespread option for cooling via a circulation of cold water, an AHU can even be used without that. Alternate methods, such as the use of DX cooling coils, enables the AHU to operate without the use of a chiller.

There were disambiguate coils this is so you can know more about your AHU coils, as they are the lynchpin of a comfortable, efficient, and affordable environment. Keep ‘em clean, monitor ‘em, and get the pros involved if things get dicey. It’s a modest investment that rewards comfort and saves money.