Calculating HTI Allows You to Properly Protect and Operate Open Cooling Tower Systems

Each open cooling tower system seems to be unique, often making it a challenge to get consistent results in protecting the systems. Make-up water quality, heat load, unit operations in the system, the environment, and cooling tower design are all factors. Many people consider these factors when determining operating parameters (like conductivity settings) and chemical treatment programs, but many water treaters overlook the Holding Time Index (HTI) of the systems.

What is the HTI? It is defined as the half-life of water and treatment chemical, in the system. It is calculated by dividing the system’s water volume by the blowdown rate and multiplying this value by the natural log of 2 [ln(2) = 0.693]. The HTI number gives you an approximate age of the water and the treatment chemicals in the system. Treatment chemicals are good, but only for a given amount of time. The longer treatment chemicals are in a system, the more they degrade. If left in a system too long, they will start failing to do the job they were intended to do.

Does this really make a difference? I had two towers at the same facility on the same make-up water and same treatment program. Preventing scale in one system was always a challenge, while the other was not. The only difference was the problem tower had a much larger sump than the other. (The engineer thought “If some is good, then more must be better!”) This system had a much higher HTI than the smaller system at the same recirculation and heat load rates. That very large sump caused the treatment chemicals to be in the system longer than desired (three times longer!), subjecting the chemicals to higher stresses (especially oxidizers and heat). This led to the treatment program being less effective than the smaller sump system.

The takeaway — please consider the HTI of a system when determining treatment programs and operating parameters, like conductivity, for controlling cycles of concentration.

In an upcoming post, we’ll talk about how HTI MUST be considered when adding non-oxidizing biocides to an open cooling tower system.