Compare cooling tower makeup and blowdown budgets
Recirculating water is not the same as newly purchased water.
A tower reuses its circulating water, while makeup replaces evaporation, drift and blowdown losses.
This calculator compares two cycles-of-concentration plans using the same evaporation and drift rates, then estimates incremental water and sewer budgets from your confirmed billing inputs.
It helps small-factory owners, building managers and facilities staff check a treatment proposal without overlooking discharge costs.
It does not recommend the cheaper plan, authorize a higher concentration, certify water quality or approve safe operation.
The engineering balance is internationally applicable, but local rates and billing eligibility must come from your own utility or contract.
Understand the four water streams
Evaporation E and drift D
Evaporation is water lost as vapor.
Drift is liquid droplets carried away by the air stream; this model assumes those droplets contain dissolved solids at the circulating-water concentration.
Enter both rates directly in m³/h using measurements or a separately justified estimate.
Blowdown B and makeup M
Blowdown is intentional discharge used to control concentration.
Makeup is the external water required to replace evaporation, drift and blowdown.
Do not count a loss already included in evaporation or drift again as blowdown.
The model does not automatically estimate evaporation from circulation flow or temperature difference.
Significant leaks, overflow, cleaning discharge or changes in stored water can invalidate the simplified balance.
Ask your treatment professional to define the water-balance boundary rather than hiding those flows inside a drift estimate.
Cycles of concentration and the equations
M = E + B + D
C = M / (B + D)
B = E / (C − 1) − D
Cycles of concentration C describe dissolved-solids concentration in the circulating water relative to makeup under the model assumptions.
The balance assumes steady state, constant makeup concentration, no dissolved-solids loss through evaporation, and circulating-water concentration in both drift and blowdown.
Solute additions and losses from precipitation, leaching and chemical dosing must be negligible; ask a professional whether this assumption fits your site.
With drift included, the denominator is B + D, not blowdown alone.
C must be greater than 1.
At a fixed evaporation rate, a higher C reduces calculated blowdown until the model reaches its feasibility boundary.
Actual allowable cycles depend on source-water chemistry, treatment, corrosion, scale, microbiological control and equipment condition.
An algebraically feasible result is not an operating approval.
Prepare the inputs and confirmations
- Common E and D: use rates representing the same load and operating conditions, and record the measurement period and basis.
- Operating hours and days: use actual daily run time and operating days for the period, not calendar days by default.
- Plan A and B cycles: enter each proposed value and record whether a qualified professional has reviewed it.
- Incremental water and sewer rates: use verified rates for the change in volume; an empty rate is unknown, while zero means confirmed no charge.
- Sewer billing basis: confirm whether billing uses all makeup or separately recognized blowdown, and retain the utility or contract basis.
Input limits are software scope limits, not statutory thresholds or equipment safety ratings.
Retain source records with units and matching time periods so differences can be traced later.
Loading the fictional example fills demonstration values only; it does not establish normal conditions at your site.
From hourly rates to period budgets
- Calculate total run time T = operating hours per day × operating days.
- Calculate B for each plan, then calculate M = E + B + D.
- Multiply each hourly rate by T to obtain evaporation, drift, blowdown and makeup volumes.
- Calculate water budget = makeup volume × confirmed incremental water rate.
- Calculate sewer budget = confirmed billable volume × sewer rate, and add the two budgets.
Fixed charges, block tariffs, taxes, chemicals, electricity, maintenance, meter installation and treatment-system capital costs are outside this calculation.
Do not add a cost twice if your chosen incremental rate already includes it.
Zero operating hours produce zero modeled variable volumes and budgets, not a promise of a zero utility bill.
For seasonal operation, calculate separate monthly conditions rather than assuming one peak month represents the entire year.
Fictional example: 3 versus 5 cycles
Assume E = 10 m³/h, D = 0.1 m³/h and 8 hours per day for 25 days, giving T = 200 hours.
For illustration only, assume confirmed incremental water and sewer rates of KRW 1,200/m³ and KRW 800/m³, with sewer billed on blowdown only.
These are fictional rates, not a local tariff, and neither concentration is a recommendation.
Fictional 200-hour plan comparison with sewer billed on blowdown| Metric | Plan A: C = 3 | Plan B: C = 5 |
|---|
| Blowdown rate | 4.9 m³/h | 2.4 m³/h |
| Makeup volume | 3,000 m³ | 2,500 m³ |
| Blowdown volume | 980 m³ | 480 m³ |
| Water budget | KRW 3,600,000 | KRW 3,000,000 |
| Sewer budget | KRW 784,000 | KRW 384,000 |
| Total incremental budget | KRW 4,384,000 | KRW 3,384,000 |
Plan B uses 500 m³ less makeup and discharges 500 m³ less blowdown, for a fictional budget reduction of KRW 1,000,000.
Each plan still evaporates 2,000 m³ and loses 20 m³ through drift.
If confirmed sewer billing instead uses all makeup, the totals become KRW 6,000,000 and KRW 5,000,000.
The billing basis changes the totals even when the engineering volumes stay unchanged.
Holds, boundary cases and signed differences
A negative calculated B places that plan on hold.
The assumed drift already removes more dissolved solids than the target balance allows, so the tool must not clamp blowdown to zero and claim that the target has been achieved.
For example, E = 10, D = 3 m³/h and C = 5 produces this hold.
At E = 10, D = 2.5 m³/h and C = 5, B is exactly zero.
This mathematical boundary is not approval for zero-blowdown operation.
Missing professional review or unconfirmed rates and sewer basis also require attention to the displayed hold reasons.
Valid engineering volumes may remain available while the budget is withheld until its confirmation requirements are satisfied.
Differences are Plan A minus Plan B.
A positive difference means B has a lower volume or budget, while a negative difference means B is higher.
Reversing the plans reverses the sign, and identical cycles produce zero differences.
None of these signs determines safety or investment return.
Conductivity ratio is an optional reference
The optional reference divides circulating-water conductivity by makeup-water conductivity.
Use the same units and comparable temperature-compensation conditions for both measurements.
If either value is missing, no reference ratio is calculated.
The ratio does not automatically set a plan C or alter the water balance.
Chemical addition, water composition, meter error and differences in sampling location or timing can affect interpretation.
A matching ratio does not certify complete water quality or safe operation.
Share conductivity records alongside makeup and blowdown meter records with your treatment professional, and investigate unit, calibration and operating-condition mismatches rather than adjusting inputs to obtain a preferred result.
Three practical uses
Compare proposals
Use one common water balance so a change in tower load is not confused with the effect of different cycles.
Obtain treatment-cost quotations separately.
Plan seasonal budgets
Separate peak-season and low-season operating schedules.
Check modeled volumes against monthly meter records before projecting a year.
Prepare utility questions
Evaporation does not automatically qualify for a sewer deduction.
Ask the utility about recognized metering, billable volume and applicable rates.
The downloadable text report retains inputs, confirmation notes, fictional-example status and hold reasons.
A report with holds is a request for missing information, not an operating approval request.
The tool does not automatically send the report externally; review notes for sensitive site information before sharing it yourself.
Frequently asked questions
Are higher cycles always better?
No.
This compares modeled water volumes and incremental budgets only.
Treatment costs, water-quality risks and potential equipment damage require separate review.
Can I enter circulation flow as makeup?
No.
Circulation repeatedly moves the same water, while makeup is new external supply.
Enter supported evaporation and drift rates directly.
Does this apply a sewer deduction automatically?
No.
Confirm whether the utility recognizes makeup-based or blowdown-based billing.
A discharge meter alone does not establish deduction eligibility.
Does an empty rate mean free water?
No.
An empty rate remains unknown and does not establish a budget.
Enter zero only when no charge has actually been confirmed.
Can negative blowdown be replaced with zero?
No.
It signals an incompatible target and drift condition in this model.
Keep the plan on hold and review the inputs and balance assumptions.
Does matching conductivity prove safe operation?
No.
The reference ratio does not approve chemistry, microbial control, corrosion or scaling conditions.
It supports professional review only.
Why does the result differ from my utility bill?
This is an incremental budget rather than a complete bill.
Fixed charges, tariff blocks, taxes, other water use and meter-period differences require separate reconciliation.
What about zero hours or large cleaning discharges?
Zero hours give zero modeled period volumes.
Cleaning discharge and changing storage can violate steady state, so obtain a separate water balance rather than hiding those flows in this result.
Official sources and verification date
Official references were checked on 2026-10-23.
This is a non-regulatory engineering budget model, not a determination of statutory rates, discharge limits or deduction eligibility.
Reconfirm allowable cycles, site assumptions and utility billing conditions when using it.
The approximate M/B ratio applies only when other losses such as drift are negligible; it is not the exact concentration definition used here.
The calculator derives results from the entered balance rather than importing a general savings percentage from guidance.
Revisit saved reports after source updates or changes in site water quality and operating conditions.
Bring the evidence before changing operation
Collect meter records, evaporation and drift calculations, professional cycle reviews and confirmed water and sewer billing terms.
Compare water-volume changes separately from budget changes, then share the report to make your questions specific.
Implement an operating or budget change only after the treatment professional and utility have confirmed the relevant conditions.