How to Size a Storage Tank Correctly
If you’re looking for a quick and reliable estimate of storage tank size, you’ve come to the right place.
We’ve spent the last 20 years observing the issues faced by data centres and their chilled water systems, and we’ve come up with a simple formula to help you get an estimate of size before digging into the finer details.
The day-to-day maintenance of data centre equipment rests in the hands of a chilled water system, and we know the frustrations of power outages and that tiny window of zero electricity before the generators kick in. During this time, masses of data can be lost as equipment ceases to be cooled by the chilled water system. A storage tank solves this by filling the gap.
When You Need a Storage Tank
Data centres are often the main locations of thermal storage tanks due to their reliance on the continuous operation of large electronic equipment storing data. A continuous cooling system is necessary here to ensure the large equipment doesn’t fail from overheating.
If there ever was a blackout or power shortage for some reason, the cooling system would stop operating and therefore stop cooling the equipment. This means a backup is required to prevent the loss of data – hence the introduction of a thermal storage tank.
In this case, the function of the tank is to provide thermal storage for your system. A buffer tank can be used as a decoupler or to provide extra volume for your system too, which you can read about here.
Storage tanks use stratification to fill the time gap between the power cutting out and the generators kicking in, which is usually up to 10 minutes but changes from site to site depending on the generator(s) and control system in place.
To correctly size a storage tank, we must establish the required Thermal Maintenance Period.
This is the length of time when there will be no power, but the system will require continuity of temperature at the required flow of chilled water.
This is sometimes referred to as ‘Thermal Ride Through’.
The design flow rate of the chilled water pumps multiplied by the Thermal Maintenance Period (in seconds) will determine the minimum volume of the tank.
If the Thermal Maintenance Period is unknown, go with 10 minutes to be safe.
The Formula
Design flow rate L/s x (60 seconds x Thermal Maintenance Period minutes) = Required Storage Tank Volume
Design Flow L/s x TMP seconds = Required Storage Tank Volume
Example 1:
A data centre has a heat load requiring a design flow rate of 60 litres/second. The generator supplier has indicated the Genset will take about 6-7 minutes to come up to full operation.
60L/s x (60s x 7m) = 25,200L minimum.
A safety margin should always be added. If we were to round up to 30,000L, we’d have a sufficient safety margin of 19%.
Example 2:
A data centre has a heat load requiring a design flow rate of 78L/s. The thermal maintenance period is unknown for this application.
78L/s x (60s x 10m) = 46,800L minimum.
Example 3:
A data centre has a heat load requiring a design flow rate of 41L/s. The TMP is 530 seconds.
41L/s x 530s = 21,730L minimum.
There’s a lot more involved in the storage tank design to obtain the required stratification, but with the above formula, you can quickly get an estimation of the required size. The finer details can be ironed out after this initial size has been determined with the help of your specialist mechanical consultant.
What is stratification?
Storage tanks are installed after the chiller, meaning cold water enters the tanks.
When storage tanks are installed after the chiller, cold water will be coming in to charge the tanks.
Stratification (separation of water temperature in layers) must be maintained in the tank. There needs to be a thermocline (thin but distinct layer of temperature change).
Stratification is when you have uniform cold water below the thermocline and warm water above the thermocline.
Stratification is used in situations when a power outage occurs. A tank can continue to discharge enough low-temperature water to the equipment during the several minutes it takes to implement the temporary power from the generator.
Remember that warm water rises and cold water sinks. A storage tank equipped with a sparge pipe at the inlet uses these properties to work effectively.
Learn more about sparge pipes and baffles here.
What happens if you undersize a storage tank?
If you undersize or fail to install a storage tank in a data centre, you may experience any of the following problems:
- Equipment at data centres crashing due to overheating during the time of blackout or electrical failure.
- Data lost due to this failure.
While blackouts or electrical failure may seem unlikely for your data centre, the ramifications of not preparing for these situations could result in serious data loss and equipment failure.
This is why an easy-to-use formula is essential in figuring out the correct volume for your storage tank from the beginning.
Now that you know the secret to correct storage tank calculations…
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