Data Center Cooling Water Storage Tank: Sizing for Redundancy
A cooling water storage tank is sized from a documented water balance, not from a nominal figure. The calculation adds the daily makeup requirement - evaporation, drift and blowdown - then multiplies it by the number of days of supply interruption the design must survive, and finally checks that the refill rate available can replenish the tank within an acceptable time. Redundancy enters twice: in the volume held, which covers a supply interruption, and in the tank arrangement, where a second tank or a divided store can avoid a single point of failure for cooling makeup.
Water storage on the cooling side is often specified as a round number late in the design, when the plant layout is already fixed. That produces either an oversized tank that consumes plot area and treatment cost, or an undersized one that leaves the plant dependent on a supply it cannot guarantee.
This article sets out the sizing method: the water balance, the interruption period, the refill check, and the redundancy options that can be designed into the storage arrangement. It is written for mechanical engineers and owners defining the reliability requirement for a cooling plant.
Technical Explanation
Step 1 - build the water balance
Everything starts from the losses the plant will incur, calculated for the actual design conditions:
• Evaporation: proportional to the cooling load and range, at roughly 1% of circulating flow per 6 to 7 K as a rule of thumb, with the final figure taken from the tower manufacturer's performance data.
• Blowdown: evaporation divided by (cycles of concentration minus one), where the achievable cycles follow from the makeup water chemistry and the treatment programme.
• Drift: from the tower specification, typically a small fraction of circulating flow.
• Daily and annual totals: expressed for the design load profile, including part-load periods, since the storage requirement depends on the worst sustained condition rather than the annual average.
Step 2 - define the interruption to survive
The reserve volume is a reliability decision, and it should be stated as such in the design basis:
• Supply failure: hours or days without municipal supply, based on the local supply risk and the operator's requirement.
• Restriction events: reduced supply during drought or emergency measures, which may last longer than an outage.
• Treatment outage: the ability to keep the loop running while a treatment unit is out of service, which requires stored treated water rather than raw.
• Fire scenario: whether the fire reserve must be available at the same time, in which case it is a separate tank rather than shared capacity.
Step 3 - check the refill rate
A large reserve with a slow refill connection only covers short interruptions effectively:
• Compare the refill rate available against the daily makeup demand to establish the recovery time for the tank.
• Confirm that the supply connection, water meter and any treatment unit can pass the refill flow, since these often become the constraint rather than the tank itself.
• Check whether the refill is on backup power, because a tank that can only be refilled from a de-energised supply does not extend resilience.
Step 4 - decide the redundancy arrangement
Volume alone does not remove single points of failure:
• Single tank: lowest cost, one vessel to inspect and maintain, but the loop depends on that one store.
• Divided store: one tank with a partition or two tanks connected by a controlled link, so that maintenance on one section does not remove the whole reserve.
• Multiple tanks: separate tanks serving separate cooling groups, which limits the consequence of a failure but increases the number of connections and the maintenance scope.
• Isolation and changeover: valves and connections arranged so that either section can be isolated without interrupting the supply to the loop.
Technical Specifications
Table 1 - Cooling makeup storage sizing method
Step | Input | Calculation | Output |
Losses | Cooling load, tower range, cycles of concentration | Evaporation, blowdown and drift from the water balance | Daily makeup volume |
Interruption period | Reliability requirement in days | Daily makeup multiplied by days | Reserve volume |
Treatment stage | Where treatment sits in the chain | Adjust for whether stored water is raw or treated | Storage location and material |
Refill check | Available supply and treatment capacity | Reserve volume divided by refill rate | Recovery time, and whether it is acceptable |
Redundancy | Required availability of the cooling loop | Single, divided or multiple tanks | Storage arrangement and isolation provisions |
Project Case Study
Project Case Summary - large reserve storage delivered
The delivery below is a large bolted firefighting water reserve, cited because it is the same class of large-volume reserve storage that a cooling makeup scheme requires.
Project | Application | Capacity | Construction | Roof | Completion |
India | Firefighting water | 12,261 m3 | Glass-fused-to-steel bolted | Flat | 2024-06 |
Turkey | Industrial water | 3,018 m3 | Glass-fused-to-steel bolted | Not applicable | 2025-07 |
Company Expertise
Center Enamel sizes and manufactures bolted water storage tanks for industrial and cooling duty, working from the project water balance and the required reserve period, with design to AWWA D103-09 and factory-applied coatings matched to the water chemistry.
As an outstanding containment and cover system provider with decades of industry experience, Center Enamel sincerely expects to establish long-term cooperation with local partners throughout the world and to make continuous contribution to the development of the industry.
Shijiazhuang Zhengzhong Technology Co., Ltd is not only the first manufacturer in China to produce Glass-Fused-to-Steel tanks, but also the most experienced professional bolted tank manufacturer in Asia. The engineering, design, product testing and quality system of Center Enamel Glass-Fused-to-Steel tanks are in strict accordance with AWWA D103-09, OSHA, ISO 28765, NSF/ANSI 61 and NFPA. By 2023, Center Enamel bolted tanks had been exported to more than 100 countries, including the USA, Australia, Canada, Malaysia, Indonesia, Russia, the UAE, Panama, Brazil and South Africa.
Shijiazhuang Zhengzhong Technology Co., Ltd (Center Enamel) is a professional manufacturer dedicated to the design and fabrication of bolted storage tanks since 2008. Our product range includes Glass-Fused-to-Steel (GFS) tanks, fusion bonded epoxy tanks, stainless steel tanks, galvanized steel tanks and aluminum geodesic dome roofs. With a professional enameling R&D team and nearly 200 enameling patents, Center Enamel has become a leader in the bolted tank industry in Asia. Our products are certified to ISO 9001, NSF 61, EN 1090, ISO 28765, WRAS, FM, LFGB, BSCI and ISO 45001.
Shijiazhuang Zhengzhong Technology Co., Ltd (Center Enamel) is a professional manufacturer dedicated to the design and fabrication of bolted storage tanks since 2008. Our product range includes Glass-Fused-to-Steel (GFS) tanks, fusion bonded epoxy tanks, stainless steel tanks, galvanized steel tanks and aluminum geodesic dome roofs. With a professional enameling R&D team and nearly 200 enameling patents, Center Enamel has become a leader in the bolted tank industry in Asia. Our products are certified to ISO 9001, NSF 61, EN 1090, ISO 28765, WRAS, FM, LFGB, BSCI and ISO 45001.
Shijiazhuang Zhengzhong Technology Co., Ltd is not only the first manufacturer in China to produce Glass-Fused-to-Steel tanks, but also the most experienced professional bolted tank manufacturer in Asia. The engineering, design, product testing and quality system of Center Enamel Glass-Fused-to-Steel tanks are in strict accordance with AWWA D103-09, OSHA, ISO 28765, NSF/ANSI 61 and NFPA. By 2023, Center Enamel bolted tanks had been exported to more than 100 countries, including the USA, Australia, Canada, Malaysia, Indonesia, Russia, the UAE, Panama, Brazil and South Africa.
As an outstanding containment and cover system provider with decades of industry experience, Center Enamel sincerely expects to establish long-term cooperation with local partners throughout the world and to make continuous contribution to the development of the industry.
Frequently Asked Questions (FAQ)
Q: How do I size a cooling makeup storage tank?
A: Calculate the daily makeup requirement from evaporation, blowdown and drift, then multiply by the number of days of supply interruption the design must tolerate. Check that the refill rate can recover the reserve within an acceptable time, and add the fire reserve separately rather than sharing capacity.
Q: How many days of reserve should be provided?
A: It follows from the local supply risk and the operator's availability requirement. Sites with a reliable municipal supply and a treatment plant on standby may accept a short reserve; sites with seasonal restrictions or a single supply connection generally need more.
Q: Should the reserve be raw water or treated water?
A: Either can work. Raw storage is simpler and cheaper per unit volume but needs treatment before it reaches the loop; storing treated water requires a coating compatible with the treatment chemicals and keeps the loop supplied directly. The choice also affects where treatment sits in the system.
Q: What redundancy does the storage arrangement provide?
A: Volume provides resilience against a supply interruption; arrangement provides resilience against equipment failure. A divided store or two interconnected tanks allow maintenance or repair on one section while the other continues to supply the loop.
Q: Does the reserve tank need backup power?
A: If the tank has to be refilled during an event, the refill pumps and any treatment units need to be on the backed-up supply. A large reserve with a refill path that depends on a de-energised supply delivers only part of the intended resilience.
Q: Can the fire reserve share the cooling makeup tank?
A: It should not. The fire volume must remain available at all times, and the potable or fire protection requirements that apply to the vessel would restrict how it can be operated for cooling makeup duty. Separate tanks are the standard arrangement.
Cooling water storage sizing is four steps: build the water balance, fix the interruption period, check the refill rate, and decide the redundancy arrangement. The result is a defensible volume and a storage layout that survives maintenance, rather than a round number that satisfies nobody.
Talk to the Engineering Team
Send the cooling load, tower data, makeup water analysis and the interruption period your design must cover. We will return the storage volume basis, the tank arrangement options and the coating suitable for the stored water.
