There are certain pieces of mechanical equipment that almost nobody thinks about until they stop doing their job.
Heat exchangers are definitely on that list.
Most people have never walked into a commercial building and thought, “Wow, the temperature feels great in here. Somebody should really congratulate that heat exchanger.”
But somewhere behind the scenes, that equipment may be quietly helping move enormous amounts of heat around the building.
I’m Jennifer Lingo with LOUMIS Air, which services locations in Louisiana and Mississippi, and heat exchangers play an interesting role in many chilled water systems. The concept is actually pretty simple once all the mechanical terminology gets out of the way.
Basically, heat needs to get from one place to another without two different fluids necessarily getting mixed together.
The heat exchanger handles the introduction.
First, What Does a Chilled Water System Actually Do?
A chilled water system doesn’t really “create cold.”
It removes heat.
A chiller cools water, and pumps circulate that chilled water through a building. The water travels to air-handling units, fan-coil units, process equipment, or other components where it absorbs heat.
Now the water is warmer.
It travels back through the system, gets cooled again, and repeats the process.
Day after day.
Hour after hour.
It’s basically the world’s least exciting water ride, but it accomplishes something extremely important.
In larger buildings, however, moving that heat around can become complicated.
That’s where heat exchangers enter the picture.
Two Fluids Walk Into a Heat Exchanger
A heat exchanger allows heat to move between two separate fluids without allowing the fluids themselves to mix.
Think about two people standing on opposite sides of a wall.
One side is warm.
The other side is cold.
Heat moves through the wall, but the people never actually meet.
A plate-and-frame heat exchanger works on a similar principle.
It contains a series of thin metal plates. One fluid moves through channels on one side of the plates while another fluid moves through neighboring channels.
Heat passes through the metal.
The fluids stay separated.
Everybody goes home happy.
Shell-and-tube heat exchangers accomplish the same general job using a different arrangement. One fluid passes through tubes while another moves around those tubes inside a larger shell.
Different design. Same basic idea.
Move the heat without mixing the fluids.
Why Keep the Water Separate?
This is where things get more interesting.
A large commercial building might have several different water loops.
- One loop might serve the central chiller plant.
- Another might serve a particular building.
- Another could serve a specific process or piece of equipment.
Those loops may operate at different pressures, temperatures, or flow rates.
A heat exchanger allows them to exchange heat while remaining hydraulically separated.
That can be particularly important in tall buildings.
Water creates pressure.
The taller the column of water, the more pressure exists toward the bottom.
At some point, simply circulating one enormous water loop from the basement to the top of a very tall building can create equipment and pressure considerations.
Dividing the system into separate loops can help manage those conditions.
Sometimes It Isn’t Even Water
Another reason to use a heat exchanger involves the fluid itself.
Some systems use glycol mixed with water.
Glycol can help prevent freezing in piping or equipment exposed to cold temperatures. That can be useful for outdoor equipment and certain process applications.
But that doesn’t necessarily mean glycol needs to circulate through every piece of equipment in the entire building.
A heat exchanger can keep the glycol loop separate from another chilled water loop while still allowing heat to move between them.
Think of it as two neighbors passing groceries over the fence without either one actually moving into the other’s house.
Heat Exchangers Can Help When Buildings Change
Buildings have a habit of refusing to stay exactly the way they were originally designed.
Someone adds another wing.
- A tenant installs new equipment.
- An office becomes a restaurant.
- A warehouse adds temperature-sensitive operations.
Suddenly, the mechanical system is being asked to do something nobody discussed 15 years earlier.
Depending on the engineering requirements and available system capacity, a heat exchanger can sometimes help connect new cooling requirements to an existing chilled water system while keeping the circuits separate.
Of course, there is more involved than simply installing two pipes and declaring victory.
- Flow rates matter.
- Temperatures matter.
- Pressure matters.
- Pumps matter.
- Controls matter.
Available cooling capacity definitely matters.
Physics continues to insist on being involved in HVAC.
Dirty Heat Exchangers Don’t Exchange Heat Very Well
Here is one of the easier concepts to understand.
Heat needs to pass through the surfaces inside the heat exchanger.
If those surfaces become covered with scale, sediment, corrosion products, or biological material, heat transfer can suffer.
Imagine putting on six winter coats and trying to cool off.
The barrier matters.
This is one reason water quality and water treatment can become important in chilled water systems.
As deposits accumulate, the equipment may not transfer heat the way it did when surfaces were clean.
Changes in temperatures, flow rates, pressure drop, or system performance can sometimes provide clues that something has changed.
Flow Matters Too
Heat exchangers are designed to operate under particular flow conditions.
If there isn’t enough flow, heat transfer may decrease.
If flow becomes excessive, other issues can develop.
Pumps, valves, sensors, and building automation controls often work together to regulate those conditions as cooling demand changes.
A commercial building at 2 p.m. on a July afternoon in Louisiana may have very different cooling requirements from the same building at 3 a.m. in February.
That’s putting it mildly.
Everything Has to Work as a System
At LOUMIS Air, with multiple locations throughout Louisiana and Mississippi, chilled water systems are a good reminder that commercial HVAC equipment rarely operates in isolation.
A heat exchanger depends on pumps moving the correct amount of water.
- Pumps depend on controls.
- Controls depend on sensors.
The chiller has to produce the appropriate water temperature.
Air handlers have to transfer heat from the building.
And somewhere in the middle, the heat exchanger may be transferring heat between two completely separate circuits.
When everything is operating correctly, nobody walking through the building notices any of this.
That’s probably the biggest compliment a mechanical system can receive.
- The building is comfortable.
- The equipment is doing its job.
And two streams of water can spend their entire working lives inches apart, exchanging heat every day, without ever actually meeting.
It’s not exactly Romeo and Juliet.
But for HVAC, it’s a pretty good relationship.