How Chilled Water Coils Work
If you’ve browsed our HVAC Coils page, you likely saw all the different types of coils we offer. With all of these coils to choose from, you’re probably wondering how they differ. To help provide some clarity, we’ve selected one coil type and decided to dive deeper into it.
Chilled water coils are one of the most common mediums for cooling air, and are typically used to cool or remove moisture from air streams. These coils are most commonly used to provide comfort cooling to industrial and commercial buildings, but can also be used for dehumidification purposes.
A chilled water coil is a fin-and-tube heat exchanger: chilled water runs through the tubes while air passes across the fins, giving up heat and, below dew point, moisture. Sizing one correctly is what decides whether the coil hits its rated capacity at the airflow and pressure drop the system can actually live with.
1) Chilled water coils are constructed the same as hot water coils.
Whether hot water or chilled water, these are both still water coils. The design of a chilled water coil is not too different than a hot water coil. The biggest difference is that chilled water coils can range anywhere from 3 to 12 rows deep, while hot water coils are usually 1 or 2 rows.
Chilled Water Coil Sizing
2) Most chilled water coils are constructed from either 1/2″ OD tubes or 5/8″ OD tubes.
We’ve learned throughout our practice that sizes ½” OD or ⅝” OD can typically be substituted for each other. It depends on the original equipment manufacturer, and whichever option would be more cost-effective, but these sizing hacks help make coil replacement easier!
3) You can usually determine the quality of a coil by its tube thickness.
A great way to ensure the longevity of your coil is to increase the tube wall thickness. Factors like untreated water and high tube velocities can lead to failure over time. A thicker tube wall will help combat and prevent some of the most common coil failures.
Materials & Fin Options
4) Fins can act as filters.
Although they weren’t designed that way, fins act as a good filter. Facility managers opting for a cheaper option will sometimes choose 14 fins/inch with fewer rows rather than 8-10 fins/inch. If you’re considering this option, PLEASE USE CAUTION: deep coils are often more difficult to clean, and cheaper is not always better. You get what you pay for!
5) Aluminum fin for cost-effectiveness.
Fins are responsible for nearly twice the amount of heat transfer as tubes. You’ll typically see aluminum fins and copper fins. Aluminum offers effective heat transfer at a lower cost than copper, so it’s often a popular choice. You’ll need a good reason to use copper fins since copper is very expensive (and prices are only increasing).
6) Casing matters
Most chilled water coils are built with grade 304 stainless steel casings. This is the most common stainless steel and is strong, lasts long, is easily stackable, and is reasonably priced.
7) Fins are designed for maximum heat transfer.
When it comes to coils, fins are the second surface, while the tubes are the primary surface. Many of the coils you’ll see have copper tubes and aluminum fins.
They may not seem like it, but the fins have a complicated design that helps to maximize heat transfer. You must have a terrific tube/fin bond for the best coil performance.
Chilled Water Coil Replacement
8) It’s easy to replace a chilled water coil.
Need a chilled water coil replaced? No problem. The biggest concern when replacing a coil is the surrounding space and making sure the new coil can fit into the allotted space. Other than that, you’ll never have to worry about the performance of a replacement coil. A replacement coil operates at 100% efficiency – which is far greater than whatever your old one was operating at! Need professional chilled water coil replacement? Mainstream field-measures every job to guarantee an exact match, with fast turnaround. When the coil is part of a larger heat exchanger, we also handle tube bundle replacement.
Coil Questions? We Can Help!
The world of coils is more complex than it seems, but we hope this guide to chilled water coils helped simplify things a little bit! If you’re still unsure about coils, head to our FAQ page to learn more. If you would like some guidance, our team of experts will help to answer any of your questions.
Sizing a Chilled Water Coil
Four inputs decide the coil. Cooling capacity — the load the coil has to remove, in tons or BTU/h — sets how much surface area you need. Entering and leaving water temperature fixes the water-side delta T: a wider delta moves the same heat with less flow, but pushes the coil deeper. Airflow in CFM, against the coil face area, gives face velocity, which drives heat transfer and also the risk of carrying condensate off the fins. Available static pressure is the ceiling — rows and fin spacing raise capacity and air-side pressure drop together, so the fan has to absorb whatever the coil costs.
Get those four right and rows, fin spacing and face area fall out of them. Get one wrong — usually the static pressure budget — and the coil performs on paper but not in the unit.
Chilled Water Coil FAQs
How long do chilled water coils last?
A chilled water coil typically lasts 15 to 20 years with proper water treatment and routine maintenance. Life varies with material and application: copper tube with aluminum fin in a clean commercial building outlasts the same coil in a coastal or chemically aggressive environment, where fin corrosion sets the limit.
How do you size a chilled water coil?
Sizing starts with four inputs: required cooling capacity in tons or BTU/h, entering and leaving water temperature, airflow in CFM, and the static pressure available for the coil. Those four determine the rows, fin spacing and face area needed to hit the load without exceeding the fan pressure budget.