1. The Energy Balance Equation

A chiller is essentially a heat pump. It moves heat from a low-temperature source (Evaporator) to a high-temperature sink (Condenser). The First Law of Thermodynamics dictates that energy must be conserved:

This means the condenser must reject not only the heat removed from the building ($Q_{evap}$) but also the heat generated by the compressor motor ($W_{comp}$).
Commissioning Tip: If your calculated $Q_{cond}$ differs significantly (>10%) from $(Q_{evap} + W_{comp})$, you likely have a sensor error (flow meter calibration or temp sensor drift) or fouling issues.

2. Accurate Capacity Calculation ($Q$)

The standard formula is $Q = \dot{m} \cdot c_p \cdot \Delta T$.
In Imperial units, engineers often use the shortcut: $Tons = \frac{GPM \times \Delta T}{24}$.
WARNING: This shortcut assumes pure water at standard conditions ($c_p=1.0$, $\rho=8.33$). If you are using Glycol, the specific heat drops significantly (e.g., 30% PG has $c_p \approx 0.90$). Using the "24" factor on a glycol system will overestimate capacity by ~10-15%. This calculator uses the actual fluid properties at the mean temperature to give a precise result.

3. Decoding Efficiency Metrics

Chiller efficiency is expressed in multiple ways depending on region and size:

• kW/Ton: The standard for large centrifugal chillers in the US. Lower is better.
  Excellent: < 0.55. Good: 0.60. Old/Poor: > 0.80.
• COP (Coefficient of Performance): Unitless ratio of Output/Input ($W/W$). Higher is better. $COP = \frac{3.517}{kW/Ton}$.
  Typical Water-Cooled: 5.5 - 7.0. Air-Cooled: 2.8 - 3.2.
• EER (Energy Efficiency Ratio): $Btu/hr$ output per $Watt$ input. $EER \approx 3.412 \times COP$. Common for smaller packaged units.
• IPLV (Integrated Part Load Value): A weighted average efficiency simulating a typical year of operation (100%, 75%, 50%, 25% load). Chillers often run at part load where they can be extremely efficient due to larger heat exchanger surface area relative to load.

4. The Hidden Cost: Pumping Energy

Chillers don't work alone. Moving water through the evaporator and condenser consumes significant energy.
Low Delta-T Syndrome: If your building coils are dirty or valves are bypassing, the return water comes back cold (Low $\Delta T$). To satisfy the load ($Q$), you must pump much more water ($GPM \propto 1/\Delta T$).
Since Pump Power $\propto Flow^3$, doubling the flow requires 8x the pump power. This calculator estimates pump energy to highlight this often-overlooked cost.

5. Compressor Technologies

• Centrifugal: Best for large loads (>300 Tons). Very efficient at full load. Subject to "Surge" at low loads/high lift. Uses magnetic or oil bearings.
• Screw (Helical Rotary): Robust, good for 70-500 Tons. Excellent part-load performance. Ideally suited for VFDs.
• Scroll: Used in smaller modular chillers (< 60 Tons). Simple, reliable, often manifolded together.
• Maglev (Magnetic Bearing): Frictionless centrifugal compressors. Extremely efficient at part load, require no oil, very quiet.

6. Glycol Application Guide

Ethylene Glycol (EG): Better heat transfer, lower pumping viscosity. Toxic. Standard for industrial.
Propylene Glycol (PG): Non-toxic (food safe). Higher viscosity (more pumping energy), lower heat transfer efficiency.
Derating: Adding glycol derates chiller capacity. A chiller rated for 100 Tons of water might only do 85 Tons with 40% PG due to poorer heat transfer in the evaporator tubes.