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    HVAC / Plant

    Cooling tower and chiller plant

    The plant behind the coil: tower range and approach, evaporation, drift, blowdown and makeup water, plus the chiller power, COP and running cost for the same duty.

    Duty1000 kWCycles4.0kW/ton0.35
    Range / approach
    6.0 / 5.0 K
    Water drop / to wet-bulb
    Evaporation
    0.41 L/s
    makeup 0.59 L/s
    Chiller input
    100 kW
    COP 10.0
    Annual energy cost
    $2.400e+4
    2.000e+5 kWh
    Tower and plant
    Thermal performance, water balance and chiller power.
    Tower and plant
    QuantityValueUnit
    Range6.0K
    Approach5.0K
    Duty implied by flow and range1256kW
    Evaporation0.408L/s
    Drift0.050L/s
    Blowdown0.136L/s
    Makeup0.594L/s
    Chiller tons284.3ton
    Chiller COP10.00—
    Chiller input100kW

    Range is the water temperature drop; approach is the leaving water minus the wet-bulb.

    Inspect the model
    Choose a quick result view, a scaled comparison, or the complete diagnostic set.
    Makeup water against approach
    A tighter approach means a bigger tower but the same evaporation.
    Makeup water against approach0.0000.1330.2660.3990.5320.6652 K2 K3 K3 K4 K4 K5 K5 K6 K6 K8 K8 K10 K10 KL/s
    Canonical equations
    The exact relationships implemented by this engine.

    Q = ṁ·cp·ΔT · cp 4.187 kJ/kg·K, 1 L/s ≈ 1 kg/s.

    E = Q / h_fg · h_fg ≈ 2,450 kJ/kg.

    B = E / (COC − 1) · Keeps dissolved solids in balance.

    engineering methodology

    A clear path from inputs to results.

    The plant behind the coil: how much water a cooling tower evaporates, how much makeup it needs, and what the chiller costs to run at the same duty.

    Step-by-step methodWorked exampleAssumptions explained

    How to use

    1. Enter the heat rejection and the water temperatures.
    2. Set the design wet-bulb and the cycles of concentration.
    3. Read the water balance and the chiller power and cost.

    The formula

    E=Qhfg,B=ECOC−1E = \frac{Q}{h_{fg}}, \qquad B = \frac{E}{COC - 1}

    Reference: Latent heat of vaporisation at about 2,450 kJ/kg; blowdown from the cycles of concentration.

    Worked example

    1,000 kW tower, 35/29 °C, 24 °C wet-bulb

    Inputs
    • 4 cycles, 0.1% drift.
    Calculation path
    1. 1

      Range 6 K, approach 5 K. Evaporation = 1000/2450 = 0.41 L/s. Blowdown = 0.14 L/s. Makeup 0.59 L/s.

    Result

    0.59 L/s of makeup; chiller 200 kW at COP 5.

    Interpreting the result

    Evaporation is set by the duty, not the tower size. A tighter approach buys a smaller tower but the same water loss.

    Assumptions & limitations

    • Steady-state water balance.
    • Latent heat taken at 2,450 kJ/kg.
    • No windage beyond the drift allowance.
    • Chiller part-load follows a straight line.

    Frequently asked questions

    Why does blowdown depend on cycles?

    Because dissolved solids concentrate by the evaporation; blowdown is what keeps them at the cycle limit.

    Is kW/ton the same as COP?

    They are reciprocals in different units: COP = 3.517 / (kW/ton).

    What is a good approach?

    3–5 K is typical; below 3 K the tower grows disproportionately.

    Does the chiller load equal the tower duty?

    No — the tower rejects the chiller load plus the compressor power.

    Methodology last reviewed 2026-10-22. Results are informational and not financial or engineering advice — see our disclaimer.