Cooler flow measurement. In this article, we are going to look at how to measure the flow of water through a chiller. This is useful for analyzing the performance of a chiller to ensure it meets design specifications.
Scroll down for chiller flow measurement YouTube tutorial
We have already seen how to calculate the cooling capacity of a chiller, and for this you would need to know the flow of water through the chiller. If you haven't seen it, click here.
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| Chilled Water/Condenser Orifice Plate |
To measure the flow, we need to place an orifice plate in the system. An example of this is shown above, although the flange face is not visible when attached to the pipe. You can see them because they have the two thin tubes sticking out. If you have a technical drawing of the system, you should be able to locate it from the symbol which may vary slightly from the one shown above.
We also need a tool to measure the flow. So for this we need a special pressure gauge that can adapt to the pressure difference of the system. You can buy compact digital versions that are easier to transport and more accurate. However, I'm going to use an old mercury-based poddymeter to measure flow simply because that's all I had at the time.
Example: ice water
We'll look at an example for a chilled water loop, but you can also use it for the condenser side. Above is a simplified version of the actual chilled water and condenser schematic for an office building. As you can see, there are three chillers, three cooling towers, and that feeds the AHUs on both the east side and the west side of the building.
Learn to read and understand ice water diagrams chilled water
schematics here
In the example above, the ice water side has been highlighted with blue dotted lines, because that is the circuit we are measuring. Just to note that all three chillers don't need to be running. This would only happen at maximum demand.
You can see on the left hand side that the east and west wings are split from one main header and the return water is joined by another header before returning to the chillers. This separates the primary and secondary circuits. We've covered this in detail before, click here to check it out.
Also note that only one of the two pumps works on each pump unit. This is because they use a duty and standby configuration where one pump is designated as the lead pump and will run while the other pump acts as a standby pump in case the lead pump fails. These roles are reversed from time to time just to keep the hours of operation similar. He generally changes roles weekly.
First, we need to find a point in the system whose flow we want to know. In this example, we want to measure the flow of a chilled water circuit of chillers.
Therefore, an orifice plate must be placed in the chilled water drain line coming out of the chiller. We can only verify from the drawing legend that it is an orifice plate, which means we can measure here.
Next we need to find the cooler evaporator, then we follow the pipes until we find the orifice plate.
The orifice plate is likely covered with insulation. They can be a little hard to find at first. You can identify them by looking at the two thin tubes sticking out of them. The thin tubes should be colored in some way to help identify the high side and the low side. One tube will be blue, which means it is the low pressure side, and the other tube will be red, which means it is the high pressure side. There are also likely small plastic tags on each one to help identify them.
Now we open the gauge and find the high pressure side which will be colored red. Make sure the top two valves on your high and low side are fully closed and the bottom center bypass valve is fully open.
Then connect the red high pressure hose to the red high pressure tube on the orifice plate. You may need to change the connection fitting depending on the valve used. You'll also need to check that all cables are clean, as they often get covered in dust and dirt. Just hand-tighten it and make sure it won't leak. Finally, just check that you have connected the correct hose to the correct side.
You should now also check the gauge hose or tube for air pockets and flush them before proceeding, as any air pockets will cause inaccuracies in your measurements.
Then locate the blue low pressure hose and connect it to the blue low pressure side of the orifice plate.
You will now need to zero the gauge, so all you need to do is open the valve on the high pressure side of the orifice plate, as well as the red high pressure valve on the gauge.
After that you can check that the bottom of the little ball inside the gauge is level with the zero mark on the gauge.
If it is not level, you can simply move the measurement indicator up or down to align it. Now, once you are satisfied with the zero alignment, you can open the low pressure side of the orifice plate. Sometimes when you open these valves they just leak a little bit. If this happens, just tighten the connection a bit and it will stop. After that, you can open the upper blue low-pressure valve on the pressure gauge. Once it is fully open, you can begin to close the center bypass valve.
When you do this, be sure to close the bypass valve very slowly and watch the little red ball start to rise; suddenly, it can rise very quickly. If it rises too high, you must immediately reopen the center bypass valve fully to prevent mercury from escaping. Once the valve is fully open, let it sit for a moment, and once it is stable, you can take a measurement.
Note: The pellet will probably move slightly up and down, and this is only due to the pumps and turbulent flow in the pipe. Once you are satisfied that it has stabilized, you can take the pressure reading.
Above you can see that you have a reading of around 6.2 kPa, that's just the pressure difference between the high side and low side of the orifice plate. Just take note of the reading and then you can start unplugging the equipment.
This is done by simply opening the bypass valve and then closing the high and low pressure valves inside the gauge. After that, you can close the high-pressure valve on the orifice flow meter and disconnect the hose. Then do the exact same thing on the low pressure side of the orifice plate.
Now we can calculate the flow. To do this we first need to know the KVS value, this is set by the orifice plate manufacturer. The KVS value varies between manufacturers, as well as model numbers and orifice plate size as well. So make sure you use the correct value.
The manufacturer will likely provide you with a chart that you can use for quick and easy research. To do this, simply mark the pressure reading on the chart, which was 6.2 kPa. We just put it on the y axis and then we can draw a horizontal line until it hits this dark KVS line.
From there, we can draw another line vertically down to see what the flow rate is. So you can see here that it has a flow rate of about 55 liters per second. But if you want a more precise way, we can do a calculation.
For this we need to use the formula that the flow rate, Q, is equal to the KVS value multiplied by the square root of the pressure difference, divided by 36. Now we know what our KVS value is, so we can simply drop that number . . We also know what the value of the pressure difference is, so we can include that as well. We then square this value and multiply it by the KVS value. Then we divide all of that by 36 to get the answer of 54.7 liters per second. And there you go. This is the water flow rate of your chiller.
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