Technology Cooling Systems (TCS) employing direct‑to‑chip liquid cooling rely on a defined range of propylene glycol concentrations to protect sensitive infrastructure. For data center operators, the risk is subtle but real: Out-of-spec glycol undermines its protective role, increases maintenance burdens, and leads to costly unplanned downtime.
The challenge for operators is there are no “fill-and-forget” coolants. Plain water is vulnerable to microbiological growth; glycol mitigates this microbial contamination risk — but only when maintained at the correct concentration and specification. In practice, this means maintaining system performance through continuous monitoring and deliberate, timely dosing to correct out‑of‑spec conditions and preserve cooling system efficiency and reliability.
EFFECTS OF SUBOPTIMAL PROPYLENE GLYCOL CONCENTRATIONS
Reduced propylene glycol (PG) concentration alone can be harmful by providing food for microbes. PG is biostatic at concentrations of 25% v/v or higher in water. However, as PG concentration decreases, PG can itself become a carbon source for microbes and can support microbial growth and proliferation within the TCS loop.
Laboratory incubation data demonstrates a definitive, concentration-dependent correlation between propylene glycol levels and the proliferation of various test organisms. Comparative analysis involving E. coli, S. pyogenes, C. albicans, and S. aureus indicates that systems lacking glycol (0% v/v) exhibit the most rapid and significant growth trajectories. Conversely, higher concentrations (30% v/v) effectively inhibit microbial growth across the identical incubation period. Intermediate concentrations ranging from 1% to 10% v/v yield varying results; while minimal concentrations provide insufficient protection and allow for detectable microbial activity, levels approaching the upper end of this range exhibit inhibitory characteristics similar to the 30% v/v threshold.
The operational implications are twofold: first, maintaining glycol at concentrations of at least 25% v/v significantly mitigates the risks associated with microbial colonization and subsequent biofilm formation. Second, suboptimal concentrations may inadvertently function as a metabolic substrate for microorganisms, thereby accelerating biodegradation and its associated consequences, including system blockages, diminished heat-transfer efficiency, and CO2 generation.
For a visualization of this data in graph form, please click here.
At lower PG concentrations and in an oxygen-rich environment, microorganisms can metabolize PG and leave behind CO2and water as waste products. In oxygen-deficient environments, such as direct-to-chip TCS loops, PG degradation can still occur, albeit at a slower rate.
The effects of biodegradation on TCS loops are two-fold: 1) glycol metabolism which allows microbes to thrive and multiply, contributing to biofilm formation, clogged pipes, and reduced heat transfer; and 2) CO2generation which increases air entrainment and decreases heat transfer further.
MONITORING REQUIREMENTS
Coolant health is paramount to the performance of any liquid-cooled system. Continuous glycol monitoring is one of the best ways to efficiently identify and address any coolant specifications issues before they can cause irreparable harm.
At a minimum, Ecolab recommends that TCS loops use:
- Propylene glycol concentrations of at least at 25% v/v to inhibit microbial growth and reduce biodegradation potential
- Routine PG concentration monitoring, ideally in real time
- Standard water analysis methods to periodically assess glycol coolant microbial load
Ecolab recommends data center operators measure several parameters continually (or periodically when real-time assessment is not possible) to develop a comprehensive picture of coolant and system health. These parameters include:
- Percentage glycol concentration to establish whether PG levels are in the recommended range
- Microbial counts to determine microorganism contamination levels pH to assess whether the system is at risk for corrosion
- Conductivity to indirectly track coolant salt concentrations
- Reserve alkalinity level to measure the coolant’s buffering capacity, which contributes to corrosion prevention
- Pressure differential to determine if any system blockages are present and if pumps are working correctly
- Copper and iron levels to detect corrosion within the system
- Total hardness to assess ingress of hard ions (Ca or Mg) from contaminated, hard water
Continuous and in-depth monitoring of coolant health provides a more complete picture of both the coolant and the overall system health. Proper data analysis and timely corrective actions not only ensure peak system performance but also protect valuable assets, maximize system longevity, and mitigate breakdown risk.
