Evaporator coolers. In this article, we will focus on the evaporator of a chiller. The evaporator is located just after the expansion valve and just before the suction line to the compressor. The evaporator recovers all the unwanted heat from the building. This is where the ice water is produced. It collects heat, carries it away, and returns the water around the building to collect more heat.
Scroll down for the evaporator cooler YouTube tutorial
The chilled water leaves the evaporator and is pumped around the building, entering the AHUs
The evaporator of chillers is usually insulated. If not, it must be. In the picture above you can see the evaporator as it is covered in black. This foam is the insulation.
Insulation is very important in the evaporator because all the cold water is produced in the evaporator. This cold water is very expensive to produce, so we want to make sure that all the cooling energy produced is sent to the building to collect all the unwanted heat. We do not want to waste it recovering heat from the technical room where the chiller is located. We can, in most cases, simply use fresh ambient air to cool this room.
You want to make sure that all chilled water pipes and fittings around the building are insulated, as well as the evaporator. If your chiller and chilled water pipes are not insulated, I strongly recommend that you discuss this with your site manager.
Evaporator insulation will typically be of the vinyl nitrate polymer type. It will be about 19 millimeters thick, 3/4 of an inch, something like that. If you talk to your cooler manufacturer, they will almost certainly be able to provide you with a pre-made cover. All new refrigerators should come with this as standard.
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| inside the evaporator |
The evaporator is a shell and tube design. The tubes are inside the hull and contain the ice water. The casing contains the refrigerant. The refrigerant and water never meet or mix, they are always separated by the metal wall of the tube and only heat is transferred between them.
The tubes run the length of the evaporator and the chilled water circulates within the tubes. The refrigerant is outside the tubes and is in contact with the surface of the tubes.
The refrigerant enters from the bottom in a liquid from the expansion valve. The refrigerant has an extremely low boiling point and the heat of the incoming chilled (hot) water is around 12°C which will cause the refrigerant to boil and turn to vapor.
Learn more about refrigerants and how they work here
When the refrigerant boils, it removes unwanted heat from the incoming cold water. Imagine that when you are hot you sweat, this liquid covers the surface of your skin and when it evaporates it evacuates the heat and refreshes you. The same thing if you boil water in a saucepan, the steam that comes out is hot, it evacuates the heat.
All of the heat that is in the water is delivered to the cooler, so it simply transfers that heat through the metal tube wall to the cooler. The refrigerant collects this heat and boils. It enters the bottom of the cooler as a liquid, picks up that heat, evaporates into vapor, and goes to the suction line and compressor.
Chilled water enters the evaporator, after collecting unwanted heat from the building, at around 12°C (53.6°F), passes through the evaporator giving up its heat, and then leaves the chiller at around 6°C (42.8°F).
To keep the refrigerant inside the evaporator shell and ensure it doesn't leak out, a metal plate is welded to each end. The plate is perforated to allow the passage of the chilled water tubes. The tubes are not soldered to the plate, although a tool that increases the diameter of the tube is usually inserted anyway to create a perfect seal between the tube, which is typically a copper material, and the carbon steel end plate.
At each end of the evaporator, we install domed metal plates called water boxes. You can see the example above, there are two holes separated by a metal plate. One hole is for incoming cold water, the other hole is for outgoing cold water. The metal plate in the middle is known as a baffle, its sole purpose is to keep the supply and return flows separate. Although some heat will be transferred between the two.
Alternatively, you can have a plate like this at one end, which has no baffle in the middle. You just funnel the water so it hits this, turns 180 degrees and flows into other tubes, then out the same side it came in. This makes the cooler more efficient and more compact.
The water boxes depend on the number of water passing through the evaporator. An evaporator is often referred to by the number of passes it has. What does it mean? Well, if an evaporator is said to have had a pass, that means the cold water only goes in one side, down the tubes and into the water box at the other end, then out the other end.
Alternatively you could have a two pass evaporator. This is where the chilled water enters the first waterbox, then goes through the tubes to the final waterbox, which has no baffle, travels 180° and continues through the first waterbox. You can tell if a cooler is two-pass because the inlet and outlet will be on the same side.
The latter is a three pass evaporator. This is where the cold water enters the first water box, it is directed through the tubes to the water box at the other end which has a baffle. This funnels it through more tubes into the first water box, the baffle channels once again returning to the secondary water box where it can exit.
On the side of the evaporator you will find the manufacturer's stamp with the name and model number. You can contact the manufacturer or check their website for technical details.
The hull, water boxes and endplates are usually made of carbon steel. They will be built to a certain standard, depending on the country you are in, to contain some pressure. They typically take a giant sheet of carbon steel and roll it into a pipe, then weld it along the seam to form the casing.
Tubes, on the other hand, are usually made of a copper alloy. Sometimes they are stainless steel, it all depends on the nature of the water on the other side and the corrosion of the material. Copper is typically used for commercial office buildings.
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| evaporator tubes |
Looking inside the evaporator, you can see the refrigerant and the tubes. Please note these tubes are not smooth, they have these ridges all the way through. This is important because these fins increase the surface area of the tube. This means that more heat can be transferred from the chilled water to the refrigerant. This means that the cooler is more efficient and smaller.
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