Chilled Water Coil: Overview, Design & Selection Guide

In a chilled water system, the cooling coil used in air handling units (AHUs) and fan coil units (FCUs) is known as chilled water coils. As the name suggests, chilled water coils use the chilled water produced by chillers to cool and dehumidify the air. There are many things involved in the design and selection of chilled water coils.

In this post, I’ll introduce and explain what is a chilled water coil and how it differs from direct expansion (DX) coils. Then, I’ll guide you through all the parameters you need to know when designing the chilled water coil for AHUs and FCUs. I’ll also explain some of the parameters you can change based on your application.

If you have seen the AHU selection report, you may find the information in the coil section is overwhelming. There are at least 10 parameters that you need to review and alter based on the requirements. No worry, I’ll break it down for you. Let’s get started.

What are Chilled Water Coils?

Let’s start with some basic information about chilled water coils. The chiller produces chilled water and supplies it to various AHUs and FCUs in the building. Inside each AHU and FCU, there is a chilled water coil and chilled water circulates in and out of it.

The chilled water entering the chilled water coil is known as chilled water supply (CHWS). The chilled water leaving the chilled water coil is known as chilled water return (CHWR). In simple terms, they can also be called entering water temperature (EWT) and leaving water temperature (LWT).

Note: CHWS, CHWR, EWT and LWT are commonly used acronyms in chilled water related systems. They can be found on layout drawings, schematic drawings and project documents. See all HVAC acronyms here.

Chilled water coils are basically finned-tube heat exchangers. They look exactly like DX coils with aluminium fins and copper tubes. Their fins can also be coated with hydrophilic, giving them a blue color look.

In terms of construction, chilled water coils are made up of rows. You might have heard of 4-row, 6-row or even 8-row coil. The row refers to the number of horizontal plates in a coil. See the below diagram for clarity:

In an AHU or FCU, the blower fan draws/pushes air through the chilled water coil. The air passes through the chilled water coil and is cooled, and water vapor within the air condenses and drips down along the coil and is collected by the drain pan underneath the chilled water coil.

Similar to DX coils, the fins can easily accumulate dust and affect the heat transfer efficiency. So, air filters are necessary to dust away and maintain the performance of the coil.

Chilled Water Coil vs DX Coil

The main difference between a chilled water coil and a direct expansion (DX) coil is the medium of heat transfer. Chilled water coils use low-temperature water (known as chilled water) for cooling while DX coils use refrigerant.

That’s where the name direct expansion comes from. It means the expanded refrigerant (expanded by the expansion valve) is directly sent to the cooling coil. Whereas chilled water coils are sometimes known as indirect expansion coils because the expansion of refrigerant happens within the chiller, not at the coil.

Now that you have an overview of chilled water coils, let’s get into the main content of this post – the design and selection guide. I’ll split them into two sections, starting with the design guide.

Chilled Water Coil Design

The design of chilled water coils is partially based on your inputs and partially depends on manufacturing capability. Basically, you only control half of the parameters. The other half comes from the manufacturer. Here is a quick glance at everything we’ll be going through:

For those that you can control (decided by you and given to the manufacturer for coil selection), capacity and temperature are the two primary parameters. Some other parameters you can control as well but they are normally based on certain industry standards. Let’s go through them one by one.

1. Chilled Water Coil Capacity

The capacity of chilled water coils is the total cooling output. It is mostly expressed in terms of kilowatt (kW) because China uses the metric system (you know what I mean). One kW is equivalent to 3412 btu/hr and 12000 btu/hr is equivalent to 1 refrigeration ton. So, 1 ton is equivalent to approximately 3.5 kW. See all capacity conversions in HVAC here.

Chilled water coil capacity is determined by calculating the building’s cooling load and analyzing the psychrometrics of the air to be processed by the coil. Chilled water coils are likely to process a mixture of return air and outdoor air. So, their capacity is not equivalent to the building’s cooling load. See how to size a chilled water coil here.

If you have seen a chilled water coil selection report, you’ll notice the capacity is separated into total cooling capacity and sensible cooling capacity. I’ll explain below.

Total Cooling Capacity

Total cooling capacity is the sum of sensible and latent cooling capacities. This is the actual capacity of the chilled water coil. At the same time, it is also the capacity of the AHU or FCU. You can also find the total cooling capacity on the first page of your AHU selection report.

Sensible Cooling Capacity

Cooling load calculation will give you sensible and latent load. In addition, outdoor air, fan motor heat gain and duct losses are all contributing to sensible load. Therefore, the sensible cooling capacity of the chilled water coil must be greater than the sum of all sensible loads to ensure the air temperature can be reduced to the desired level.

Normally, sensible cooling capacity accounts for about 80% of the total cooling capacity in a cooling coil. However, in applications where the latent load is high, for example, open offices, auditoriums and movie theatres, the sensible cooling capacity can be as low as 60% of the total cooling capacity. See more details about sensible cooling capacity here.

Chilled water coil capacity basically depends on your load calculation and psychrometric analysis. It is usually the “size” (capacity) of AHUs and FCUs that people are talking about. The next key parameter is temperature.

2. Chilled Water Coil Temperature

Perhaps the more interesting design aspect of chilled water coils is the temperature. There are two parts to it and both are equally important. In addition, there is a third part that doesn’t get mentioned as often. Let’s go into the detail now.

Water In/Out Temperature

Chilled water coils use chilled water to cool and dehumidify the air. Thus, the temperature of the chilled water is a crucial design parameter. There are two variants to it.

In a standard chilled water system, around 44°F (6.7°C) is the supply temperature (water in) and 54°F (12.2°C) is the return temperature (water out). Some systems are designed at 44.6°F (7°C) and 53.6°F (12°C) while others may see something like 42°F (5.6°C) and 56°F (13.3°C).

What I mean by standard is most cases. Before the introduction of ASHRAE Standard 90.1 which provides the minimum requirements for energy-efficiency design, the delta T or temperature difference between the chilled water supply and return is always less than 15°F, mostly 10°F.

With the introduction of the said standard, the minimum requirement for chilled water delta T is 15°F and the minimum chilled water return temperature is 57°F (13.9°C). Some people call this kind of design high delta T chilled water systems.

Unless in extreme conditions, the chilled water in/out temperature at the chilled water coil should be the same as the chilled water in/out temperature of the chiller. The reason why I say extreme conditions is because, in most systems, the chilled water pipe insulation thickness is enough to prevent any temperature drop.

With the water in/out temperature in place, the manufacturer will calculate the required chilled water flow rate and select a coil that can suit these parameters. I’ll go into the water flow rate after this.

Air In/Out Temperature

Just like DX coils, chilled water coils must also have a specified air in/out temperature – that is the air entering the coil and leaving the coil. This is also known as entering air temperature (EAT) and leaving air temperature (LAT). Alternatively, you can also call them on coil temperature and off coil temperature.

The air in/out temperature helps to determine the sensible cooling capacity of the chilled water coil. The greater the temperature difference between the air inlet and outlet, the higher the sensible cooling capacity of the coil.

During construction, cooling coils mostly follow the AHRI Standard 410. You can see the “AHRI Certified” blue color badge on the brochure of cooling coils, indicating the construction complies with the said standard.

However, most manufacturers set the entering air temperature at 27°C (80°F) for residential applications. For commercial applications, as I said earlier, the outdoor air is involved. Therefore, the mixed air temperature will be the entering air temperature.

As for the leaving air temperature, a standard value is 55°F (12.8°C) because that’s the dew point of the air at 75°F (24°C) and 50% relative humidity which is the typical indoor design conditions for human comfort.

However, due to fan motor heat gain and duct losses, the leaving air temperature may need to be reduced to ensure the cooling and dehumidifying capacity is sufficient for the application. This is some advanced stuff. If you’re interested in mastering it, I recommend you enroll in my Psychrometric Analysis Course here.

So, water and air temperatures are the two parts that make up the chilled water coil temperature. But, there is one more thing that’ll be good to know about.

Approach Temperature

Approach is a term used widely in HVAC for the temperature difference between two mediums. In the case of chilled water coils, approach temperature is the difference between the entering water temperature and the leaving air temperature. Basically, it is the efficiency of the heat exchanger coil. The better the coil at transferring heat, the lower the approach temperature. See more details about cooling oil temperature here.

Now that you understand what is chilled water coil temperature, it’s time to “go with the flow”.

3. Chilled Water Coil Flow

In a chilled water coil, we have water flow and airflow. Airflow is determined by the air in/out temperature and it is presented in the fan section of the AHU selection report. So, we are left with water flow which is what I’ll explain here.

Water Flow Rate Calculation

As I said earlier, the manufacturer of chilled water coils will calculate the required chilled water flow rate and select a coil that can suit these parameters. The calculation is simple and we can verify it on our own as follows:

Q = ρqcθ

where,
Q = cooling capacity, kW
ρ = water density, 997 kg/m³
q = chilled water flow rate, m³/s
c = specific heat of water, 4.187 kJ/kg.°C
θ = chilled water temperature difference, °C

For example, the total cooling capacity of a chilled water coil is 175.8 kW. The water in/out temperature is 7/12°C, what is the required chilled water flow rate?

Q = ρqcθ
175.8 = (997)(q)(4.187)(12-7)
175.8 = 20872.19q
q = 8.42 x 10^-3 m³/s

Convert m/s to L/s by multiplying 8.42 x 10^-3 by 1000. Therefore, the required chilled water flow rate is 8.42 L/s. See more details about chilled water flow rate calculation here (including Imperial unit).

The sum of the water flow rate of all chilled water coils is the total required chilled water flow rate for the building. However, this can also be calculated by using the total chiller capacity given that the total chilled water coil capacity is equivalent to the total chiller capacity.

Compared to capacity and temperature, flow is not a parameter that you can control. Instead, it is a result of your designed water in/out temperatures. Let’s also go through some of the other parameters that are based on the manufacturer. Some of them are important as well.

4. Chilled Water Coil Pressure Drop

Similar to the flow rate, the pressure drop across a chilled water coil also has two parts. One is water pressure drop and the other one is air pressure drop. Water pressure drop is due to the resistance of the copper tube of the coil while air pressure drop is due to the fins.

Water Pressure Drop

When the chilled water enters and leaves the chilled water coil, its pressure is slightly reduced. How much the water pressure is reduced depends on how complex is the chilled water coil. If you have more rows, the water pressure drop is likely higher. Also, if the water velocity is high, the pressure drop is also high.

The water pressure drop of a sizeable chilled water coil is typically in the range of something like 30 to 40 kPa (4.35 psi to 5.80 psi). For smaller coils or lower water velocity, the water pressure drop can be as low as 15 kPa (2.17 psi). It really depends on the overall design and is usually not a concern.

Air Pressure Drop

Now, the same goes for the air. When the air passes through the chilled water coil, its pressure is slightly reduced. However, the air pressure drop has a significant impact on the energy usage of the AHU or FCU fan.

For air pressure drop, the typical range is between 100 and 150 Pa (0.4 and 0.6 in.wg). If the total pressure of the AHU is 1000 Pa, the chilled water coil itself accounts for 10-15%. When the coil is full of dust, the increase in the air pressure drop will lead to high fan energy consumption.

The air pressure drop of the internal component of an AHU or FCU is considered internal static pressure. During the AHU or FCU selection, the manufacturer will ensure these internal static pressures are accounted for. What you need to be looking at is the external static pressure and it is mostly come from the ductwork. See the static pressure calculation process here.

5. Chilled Water Coil Face Velocity

A very important parameter but mostly done right is the face velocity of the chilled water coil. Face velocity is the velocity of the air leaving the coil.

Any coil manufacturers with a basic understanding of coil design will advise you or by default set the face velocity to below 500 ft/min (2.54 m/s) to avoid moisture carryover. Moisture carryover simply means the condensate water form on the coil “flies out” and goes into the fan and duct section, leading to all sorts of problems including mold growth and rust.

Many project specifications have stated this requirement. So, you’ll unlikely to go wrong with this.

6. Chilled Water Coil Fins

Another important parameter is fin per inch (FPI). Fin per inch is the density of the fins on the coil. Typically, coils are designed to have 12 FPI. If you increase the FPI, you can reduce the number of rows to save cost while maintaining the same capacity. But, high FPI means harder to clean and dust can easily trap inside.

The above 6 parameters; capacity, temperature, flow, pressure drop, face velocity and fins are those related to the design aspect of chilled water coils. They directly affect the performance of the coil. Next, I’ll explain the selection aspect which can impact the lifespan of the chilled water coil.

Chilled Water Coil Selection

After the manufacturer selects the coil based on your requirements, there are a few things you can change that are not related to your initial inputs. I’ll show you what are the parameters and why you want to change them.

1. Chilled Water Coil Material

Earlier, I mentioned that chilled water coils are made up of aluminium fins and copper tubes. But, there is more to it. In general, you can change 3 materials in a chilled water coil. Let’s check them out one by one.

Coil Structure Material

Chilled water coils are supported and contained within a frame known as a coil structure or just a frame. Because chilled water coils are constantly exposed to humid environments, the structure or frame is made up of stainless steel SUS304. Usually, an upgrade to the coil structure material is not necessary but if you want better rust resistance, you may opt for SUS316 which is a higher grade stainless steel. Of course, it’ll be more expensive.

Tube Material

The tube penetrating through the aluminium fins is by default made up of copper. Hence, the name copper tube. However, you can use aluminium or stainless steel. Aluminium is lighter but softer and stainless steel is more durable but more expensive.

Drain Pan Material

Drain pan is used to collect the condensate water. It is located underneath the chilled water coil. Similarly, it should be made of SUS304 but can be upgraded to SUS316 when necessary.

Pipe Header Material

Chilled water coils have a pipe header for the supply and return pipe connection. The header material is usually copper but some manufacturers use aluminium to reduce price. While aluminium pipe headers are fine, they are more prone to bending and damage, especially during pipe connections.

Here are other materials that could be upgraded/changed to.

Speaking of pipe connection brings me to the next selection parameter which is related to the site installation of the AHUs and FCUs.

2. Chilled Water Coil Connection

Like any equipment, the connection to chilled water coils depends on how you want to install the AHU or FCU and the pipe size. It’s simple.

Connection Orientation

This applies to almost all equipment in HVAC and other trades. You can choose either a left or right connection. This will depend on where you want to put the AHU or FCU in the room and where is the chilled water pipe coming from.

Connection Size

The connection size to the chilled water coil is usually the same as the chilled water pipe size. The manufacturer of the chilled water coil will propose a standard connection size. Based on my experience, if you size the chilled water pipe correctly, you should see both the pipe size and the coil connection size are the same.

Connection Type

The connection type is based on the connection size. It is either a threaded connection or a flange connection. Normally, anything equal to or below 50mm (2″) will use the threaded connection. Above that are all flange connections. However, there may be one size difference depending on the manufacturer.

Cost-saving tips: Valves above 50mm (2″) in size with threaded connection are exponentially more expensive than their flange connection counterparts. This is because the installation becomes significantly more difficult. Imagine screwing on an 80mm (3″) valve. It’s (almost) impossible!

By now, we’ve gone through 90% of the things you need to know about chilled water coil design and selection. But, there is one more thing I would like to show you.

Chilled Water Coil Piping

In a chilled water system, AHUs and FCUs are connected to chilled water pipes. What is being connected is the chilled water coil. As I explained earlier, chilled water flows in and out of the chilled water coil. So, what does the piping connection look like? Let me show you.

Piping Detail

Like many other equipment, chilled water coils will need to be serviced or replaced when the time comes. More importantly, they must be able to stop receiving chilled water in case of any problem that requires troubleshooting.

Therefore, a pair of isolation valves is needed; one at the supply line, one at the return line. In case a problem or the AHU needs to be replaced, we have a way to stop the water flow.

Other than “once in a while” functions, we also need to be able to control how much water flows through the chilled water coil because flow equals capacity. We want to be able to control the cooling capacity.

Thus, a control valve that can open and close to allow more or less water flow is needed. Nowadays, we usually use a pressure-independent balancing and control valve (PIBCV). See all types of valves used in HVAC here.

Sometimes, the AHUs are so big that they vibrate during operation. If that’s the case, we need to include a pair of flexible joints to absorb and prevent the vibration from propagating to the pipeline.

For monitoring purposes, we normally have a pressure gauge before and after the chilled water coil to check the pressure drop, and a thermometer to see the chilled water temperature.

Now, let’s put everything in one place and I’ll show you a chilled water coil piping diagram from one of my previous projects.

Piping Diagram

The above diagram is a typical installation diagram for AHUs and FCUs that run on chilled water coils. As you can see, the square box with C\C is the cooling coil. From right to left, the piping connection starts with the flexible joint and then, a y-strainer at the supply line to filter out any dust and sand.

On the return line, we have the PIBCV for flow and capacity control. Finally, both supply and return lines have an isolation valve. In this case, a butterfly valve.

If you’ve noticed, there is a BTU meter at the AHU supply line. BTU meters are used to record how much water flows through the coil and what’s the temperature difference. The BTU meter comes with two temperature sensors; one for the supply line and one for the return line. Together, they can tell how many BTUs of energy are used for this particular AHU. The data can later be used to bill the customer or calculate energy usage.

Between the supply and return lines, there is an additional pipe with an isolation valve. This is known as a bypass line. The function of the bypass line is for initial flushing purposes. After installation, the inside of the pipeline is very dirty. We don’t want the dirt to flow through the chilled water coil as it can damage the coil.

So, we put a bypass line. When we want to do flushing, we close both isolation valves on the supply and return lines and open the isolation valve on the bypass line. As such, all the dirty water will not enter the coil.

Summary

Whew! I’ve covered a lot of stuff about chilled water coils in this post. I started with an overview of chilled water coils. Then, I proceed to explain the design parameters. I included the guide on capacity, temperature, flow, pressure drop, face velocity, and fins. Subsequently, I explain parameters to which you can make changes such as coil and header material as well as connection size and orientation.

Last but not least, I show a typical chilled water piping diagram and explain what the valves needed and their functions.

There you have it. Everything you need to know about chilled water coils.

If you find this post helpful, share it with your friends and colleagues. To learn more about chilled water systems and HVAC in general, head to my online course platform. I look forward to seeing you here!

Thank you.