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Water Treatment · Calculate & Specify

Cooling Water Cycles and Blowdown Calculator

Enter what you measure — circulation rate and temperature drop, or evaporation directly, plus drift and your target or measured cycles. The tool returns the rest of the water balance and the resulting makeup demand. It states its equations openly and it does not decide how hard your tower can safely run.

This calculator does not determine the safe or appropriate operating cycles for a cooling-water system.

It converts operating data you supply into water-balance quantities. Whether a cycles value is achievable or appropriate on your system depends on factors this arithmetic never sees:

  • Makeup-water chemistry, and how much it varies through the year
  • Calcium hardness and alkalinity, and the carbonate saturation they produce at skin temperature
  • Silica, where the makeup water carries it
  • Sulfate, chloride and total dissolved solids relative to the metallurgy in the system
  • Bulk and heat-transfer surface temperature, and heat flux
  • Corrosion risk for every material in the loop, including any galvanised, copper-alloy or stainless components
  • The treatment programme itself — inhibitor chemistry, halogen regime and their stability limits
  • Equipment and material constraints, and the manufacturer's operating requirements
  • The site water management programme and the engineering and public-health requirements that apply to it

Operating data

Every figure below comes from your system. Nothing is filled in on your behalf.

Evaporation

Water balance

Formula, units and assumptions are shown with every figure.

Withheld — inputs missing

  • Evaporation is not known. Enter a measured evaporation rate, or enter circulation flow, range and an evaporation factor to use the simplified estimate. It is not assumed.
  • Drift is not known. Enter drift as a percentage of circulating flow together with the circulation rate — the manufacturer's drift-eliminator rating is the usual source. Drift is not assumed to be zero.

Evaporation

Entered directly

—m³/h

Drift

D = circulation × drift% ÷ 100

—m³/h

Other losses

Entered directly; treated the same way as blowdown in the balance

0m³/h

Blowdown required to hold the entered cycles

B = E ÷ (cycles − 1) − D − other losses

—m³/h

Makeup

M = E + B + D + other losses

—m³/h

  • Other losses (side-stream filter backwash, leaks, process bleed, deliberate draw-off) are taken as none because none were entered. If your system has them, enter them — they concentrate the water balance in the same way blowdown does.
  • Annual volumes are not shown because annual operating hours were not entered.

What the result means. These are flow quantities implied by a steady-state mass balance on the numbers you entered. They are not a treatment programme, a dose, or a statement that the entered cycles can be held safely on your water.

Worked example

Every conversion is shown; none of it is hidden in code.

A tower circulates 500 m³/h with a 6 °C range. Evaporation has not been metered, so the simplified estimate is used with the documented rule-of-thumb factor of 0.18% of circulation per °C. The drift-eliminator rating on the nameplate is 0.005%. The plant is operating at 5 cycles and runs 6,000 hours a year.

  1. 1.Evaporation (estimate) = 500 m³/h × 6 °C × 0.18% = 5.40 m³/h.
  2. 2.Drift = 500 m³/h × 0.005% = 0.025 m³/h.
  3. 3.Blowdown = evaporation ÷ (cycles − 1) − drift = 5.40 ÷ 4 − 0.025 = 1.325 m³/h.
  4. 4.Makeup = evaporation + blowdown + drift = 5.40 + 1.325 + 0.025 = 6.75 m³/h.
  5. 5.Annual makeup = 6.75 m³/h × 6,000 h = 40,500 m³.

Every figure here follows from the four numbers entered. Whether 5 cycles is appropriate for this water, this metallurgy and this treatment programme is an engineering question that this arithmetic does not answer.

Use this when

  • You need blowdown and makeup rates for a cycles value you are already operating at or evaluating.
  • You want the water-balance consequence of moving cycles before you discuss it with your treatment engineer.
  • You need an annual makeup or blowdown volume for a budget, a permit conversation or an enquiry.

How the output is produced

  • Evaporation may be entered directly, or estimated as E = circulation rate x range x evaporation factor — a simplified planning estimate that assumes essentially all heat rejection is evaporative.
  • Drift is entered as a percentage of circulation rate; it is never assumed to be zero silently.
  • Water balance: makeup = evaporation + blowdown + drift, and cycles = makeup / (blowdown + drift). Solving for blowdown: blowdown = evaporation / (cycles - 1) - drift.
  • Cycles may instead be estimated from a conservative tracer ratio — recirculating concentration divided by makeup concentration — where the species is neither added nor removed by the programme.

What this page does not determine

  • IT DOES NOT DECIDE THE SAFE OPERATING CYCLES FOR YOUR SYSTEM. Maximum cycles depend on site water chemistry, materials, heat flux and the treatment programme, and require engineering validation.
  • It does not calculate a chemical treatment dose, an inhibitor requirement or a biocide regime.
  • It does not predict scaling, corrosion, fouling or microbiological risk, and it does not evidence any public-health or discharge compliance obligation.
  • The evaporation estimate is a simplified planning figure. Where accuracy matters, use measured makeup and blowdown.

Relevant Allzone products

These are catalogue identities relevant to this decision. A listing here is not a statement that the material is suitable for your system, and it is not a certification, performance or drinking-water claim.

Ask for a quotation or a specification

Where this comes from, and what it cannot tell you

  • Cooling-water engineering references on evaporative system water balance (ASHRAE Handbook — HVAC Systems and Equipment, cooling tower chapter; standard cooling-water treatment texts)

    Supports: The steady-state mass balance for an evaporative cooling system: makeup = evaporation + blowdown + drift, and cycles of concentration = makeup / (blowdown + drift), with the simplified form cycles = evaporation / blowdown + 1 when drift and other losses are neglected.

    Limitation: The balance is a steady-state model. It ignores transients, side-stream filtration losses, leaks and any chemical consumed or precipitated. It says nothing about whether a chosen cycles value is safe for a given water chemistry.

    Engineering literature — general practice · Established engineering.

  • Cooling-water treatment literature on determining cycles from a conservative tracer (chloride, conductivity or an inert tracer ratio)

    Supports: That cycles of concentration can be estimated as the ratio of a conservative species in recirculating water to the same species in makeup water, provided the species is neither added nor removed by the treatment programme.

    Limitation: Conductivity is not strictly conservative and chloride can be added by chlorination chemistry. A tracer ratio is an estimate, not a measurement of chemical stress.

    Engineering literature · Established practice.

  • Public-health and standards guidance on building water systems and cooling towers (e.g. ASHRAE Standard 188; provincial public-health guidance for cooling towers in Canada)

    Supports: That cooling-tower operation carries a recognised microbiological risk requiring a site-specific water management programme, and that operating parameters must be set within such a programme rather than by generic calculation.

    Limitation: Requirements are jurisdiction-specific and property-specific. Nothing on this site sets a compliance obligation or discharges one.

    Canada — provincial and municipal requirements vary · Verify the requirements currently in force for your site.

  • Scaling-index literature (Langelier Saturation Index, Ryznar Stability Index and their documented limitations)

    Supports: That calcium-carbonate scaling tendency is commonly screened using saturation indices calculated from pH, temperature, calcium, alkalinity and total dissolved solids.

    Limitation: Saturation indices are directional screening tools for calcium carbonate only. They do not predict sulfate, silica, phosphate or iron deposits, they do not quantify deposition rate, and they are widely documented as unreliable when used as a control setpoint.

    Engineering literature · Long-established, with well-documented limitations.

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