EnergyPlus Cooling Tower Sewage Calculator

System Input Parameters

1. Heat & Flow Rates

Heat Rejection Load (kW)

Cooling Range ΔT (°C)

Recirculation Water Flow (L/s)

2. System Operational Metrics

Cycles of Concentration (CoC)

Drift Loss (% of Recirculation)

Annual Operating Hours (hrs)

3. Utility Cost Parameters

Water Utility Rate ($/m³)

Sewer Discharge Rate ($/m³)

Formulas and Physics Principles Used

The EnergyPlus cooling tower sewage module uses core thermodynamic heat balance and chemical conservation principles to model water loss, drift, and sewage volume:

Evaporation Rate ($E$): Derived from the latent heat of vaporization ($h_{fg} \approx 2430\text{ kJ/kg}$) and heat rejection load ($Q_{rej}$):
$$E = \frac{Q_{rej}}{h_{fg}}$$
Drift Loss Rate ($D$): Computed as a fractional percentage of total recirculation water flow ($V_{circ}$):
$$D = V_{circ} \times \%_{\text{drift}}$$
Blowdown (Sewage Discharge) Rate ($B$): Formulated via mass balance relative to Cycles of Concentration ($\text{CoC}$):
$$B = \frac{E - D(\text{CoC} - 1)}{\text{CoC} - 1}$$
Total Makeup Water Rate ($M$): Sum of all system fluid losses:
$$M = E + B + D$$

How to Use This Calculator

Enter Thermal Load Parameters: Input the heat rejection capacity ($kW$), designated temperature range ($\Delta T$), and total circulation rate ($L/s$).
Configure System Water Quality: Enter your targeted Cycles of Concentration ($\text{CoC}$) and manufacturer drift loss percentage.
Provide Utility Pricing & Operational Time: Input local municipal water tariffs, sewer discharge rates, and total planned operational hours.
Process Simulation: Click the Calculate EnergyPlus Metrics button. Results will instantly appear above the input form for quick review.

Optimizing Cooling Tower Sewage and Water Usage in EnergyPlus Simulations

Evaporative cooling towers are essential components in commercial HVAC design and industrial refrigeration systems. Because cooling towers exchange heat directly through water evaporation, they account for significant municipal water usage and sewage effluent generation in modern facilities. Understanding and quantifying these fluid dynamics via EnergyPlus engineering principles is crucial for energy auditors, MEP engineers, and building sustainability managers.

Understanding Tower Water Balance Dynamics

A cooling tower water loop operates on continuous mass balance principles. The total makeup water added to the system must balance all outgoing mass flows: evaporation, windage drift, and intentional blowdown. Evaporation removes pure $H_2O$, leaving dissolved mineral solids behind in the remaining circulating water. As these solids build up, they cause scaling and corrosion unless controlled. Blowdown is the periodic or continuous release of mineral-rich water into the municipal sewer line to maintain manageable concentration thresholds.

Role of Cycles of Concentration in Sewage Reduction

The Cycles of Concentration ($\text{CoC}$) define the ratio of dissolved solids in circulating water relative to incoming fresh makeup water. Operating at a higher $\text{CoC}$ dramatically lowers blowdown volumes, directly reducing municipal sewage costs and total water footprint. However, pushing $\text{CoC}$ beyond maximum thresholds risks severe heat exchanger fouling and scale formation. Finding an optimal equilibrium allows facility operators to maximize thermal efficiency while maintaining water conservation goals.

Frequently Asked Questions

Blowdown contains high levels of dissolved solids, minerals, and chemical treatment agents drained directly into the sewer system. Because this water cannot be evaporated into air, municipal authorities bill it under sewer discharge fees.

Increasing $\text{CoC}$ lowers the required blowdown rate. This significantly decreases the total volume of wastewater sent to municipal sewers, generating substantial financial savings on utility bills.

Modern high-efficiency cooling towers equipped with modern drift eliminators achieve drift losses between $0.001\%$ and $0.005\%$ of total recirculating water flow.