A data center cooling system does not receive all its water from the same place, and not all water in the system faces the same contamination risks.
Make-up water replaces water lost through evaporation, blowdown, leakage, or maintenance. Circulating water is already moving through the cooling system, where it can collect particles from piping, corrosion, scale, and other operating conditions.
Because these two water streams serve different purposes, they need different filtration considerations.
For data center operators, understanding the distinction helps build a water-treatment strategy that protects cooling equipment without adding unnecessary pressure drop or maintenance.
Why Water Filtration Matters in Data Center Cooling
Cooling systems depend on reliable heat transfer. When suspended solids, corrosion products, or deposits accumulate, they can contribute to fouling, restricted flow, and increased maintenance.
The water-quality challenge varies throughout the system. Make-up water may introduce contaminants from its source, while circulating water can pick up particles from system materials, installation residue, and ongoing operation.
ASHRAE’s guidance on cooling towers explains that evaporation concentrates dissolved solids and impurities in recirculating water, while airborne dust and other contaminants can enter during operation. Depending on water chemistry, these conditions can contribute to scale, corrosion, and silt deposits. ASHRAE’s cooling tower guidance discusses these water-treatment considerations.
The practical takeaway is that filtration should be selected for the water’s source, its role in the system, and the equipment it must protect.
What Is Make-Up Water?
Make-up water is added to a cooling system to replace water that has been lost.
In evaporative cooling systems, water is lost through evaporation and blowdown. Additional make-up may be needed to compensate for leaks, draining, or maintenance activities.
Depending on the source, make-up water may contain:
Suspended solids and sediment
Sand or silt
Rust particles from upstream infrastructure
Dissolved minerals that contribute to scaling
Other source-specific contaminants
Filtration can remove suspended particles before they enter downstream equipment. However, filtration alone does not remove every dissolved substance or replace a complete water-treatment program. Water chemistry, scale control, corrosion management, and biological control may require separate treatment measures.
What Is Circulating Water?
Circulating water is the water already moving through the cooling system.
In an open cooling tower system, water repeatedly passes through the tower and associated heat-transfer equipment. As it circulates, it can be exposed to airborne debris and can concentrate dissolved solids as water evaporates.
In closed loops, the water is more isolated from the surrounding environment, but it is not automatically free of particles. Corrosion products, installation debris, and material degradation can still affect water quality.
This means filtration needs to reflect the system design. An open cooling tower, a closed facility-water loop, and a technology cooling loop serving sensitive equipment do not necessarily need identical filtration arrangements.
Make-Up Water Filtration vs. Circulating Water Filtration
The difference starts with the source of contamination and the job each filtration stage performs.
Make-up water filtration: control contaminants entering the system
Make-up water filtration is intended to reduce suspended particles entering with replacement water.
The filtration technology and rating depend on the water source, the quality of incoming water, and the downstream equipment. A cartridge filter or bag filter may be suitable for particulate removal in some applications, while additional treatment may be needed to manage dissolved minerals or other water-quality parameters.
Circulating water filtration: manage contaminants already in the loop
Circulating water filtration addresses particles present within the operating cooling system.
For cooling towers, side-stream filtration can continuously treat a portion of the circulating water to help manage suspended solids. In closed loops, filtration may be used to capture corrosion products or particles introduced during installation and maintenance.
The selected system should account for flow rate, solids loading, particle characteristics, pressure drop, and maintenance requirements.
Why One Filter May Not Be Enough
A single filter cannot automatically address every contamination source throughout a data center cooling system.
A coarse filter that captures larger debris entering with make-up water may not provide the particle control needed near sensitive equipment. Conversely, placing a very fine filter in a location with a high solids load may cause rapid loading and frequent maintenance.
A staged approach can assign different filtration functions to different parts of the system.
For example:
Make-up water filtration reduces suspended particles entering with replacement water.
Cooling tower side-stream filtration helps manage debris and solids in recirculating tower water.
Facility-loop filtration can capture particles such as corrosion products or installation residue.
CDU or secondary-loop filtration is selected to meet the requirements of the technology cooling system and its components.
Brother Filtration outlines these separate filtration positions in its guide to multi-stage data center water treatment .
The exact arrangement depends on the cooling architecture and the requirements of the equipment manufacturer.
Filtration Is Not the Same as Water Treatment
It is important to distinguish particulate filtration from broader water treatment.
Filters capture particles according to their design and filtration rating. They do not necessarily control dissolved hardness, conductivity, pH, corrosion chemistry, or biological growth.
For evaporative cooling systems, water treatment may include measures for scale, corrosion, and biological control. ASHRAE notes that blowdown can help control dissolved-solids concentration in some conditions, but it does not control biological contaminants. For data center operators, filtration should therefore be coordinated with the overall water-management program rather than treated as a substitute for it.
How to Select the Right Filtration Approach
Before choosing a filter, identify where the water enters the system and what the filtration stage is expected to protect.
Consider these questions:
Is the water make-up water, cooling tower water, or water in a closed loop?
What suspended particles are expected?
What flow rate must the filter handle?
What pressure drop can the system tolerate?
How quickly is the filter expected to load?
Is the goal to protect a heat exchanger, pump, valve, or downstream filtration stage?
Are water chemistry and biological controls managed separately?
How will the filter be inspected, cleaned, or replaced?
The filtration rating should follow from these requirements. Choosing a micron rating without understanding the contamination profile can lead to unnecessary pressure drop, short service intervals, or inadequate protection.
How ASHRAE TC 9.9 Fits into Data Center Cooling
ASHRAE Technical Committee 9.9 focuses on mission-critical facilities, data centers, technology spaces, and electronic equipment. Its work includes guidance related to environmental conditions for data processing equipment.
For this article’s topic, it is useful to keep the reference in context: ASHRAE TC 9.9 is not itself a universal filter-selection specification for make-up or circulating water. Filtration requirements should be determined from the cooling-system design, water-treatment plan, equipment requirements, and applicable engineering standards.
Bringing the Filtration Strategy Together
Make-up water and circulating water are connected, but they do not have identical filtration needs.
Make-up water filtration focuses on contaminants entering with replacement water. Circulating water filtration manages particles that are present or develop within the operating system. In cooling towers, evaporation and airborne exposure create additional water-quality challenges; in closed loops, corrosion products and installation debris may be more relevant.
A well-planned strategy matches each filtration stage to its location, contamination load, flow conditions, and protected equipment.
For more information on filtration options for data center cooling systems, explore Brother Filtration’s data center make-up water filtration solutions .
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