Condenser Water Free Cooling Plate Exchanger Calculator

Water-Side Economizer Plate Heat Exchanger (PHE) Principles

A water-side economizer ("free cooling") utilizes winter or shoulder-season ambient wet-bulb temperatures to chill the building's chilled water loop directly via the cooling tower, bypassing energy-hungry chiller compressors completely.

1. Log Mean Temperature Difference (LMTD)

Plate and frame heat exchangers operate in true counter-flow. The effective thermal driving force across the plates is governed by the LMTD:

$$\Delta T_1 = T_{\text{chw,ret}} - T_{\text{cw,out}}, \quad \Delta T_2 = T_{\text{chw,sup}} - T_{\text{cw,in}}$$

$$\text{LMTD} = \frac{\Delta T_1 - \Delta T_2}{\ln(\Delta T_1 / \Delta T_2)}$$

2. Heat Exchange Surface Area Sizing

From total cooling duty (Q = \text{Tons} \times 12,000\text{ BTU/hr}) and the overall heat transfer coefficient (U) (typically 900 to 1,100 BTU/hr·ft²·°F for corrugated stainless steel plates):

$$A = \frac{Q}{U \cdot \text{LMTD}}$$

3. The Ultra-Low Approach Requirement

To maximize annual free cooling hours, the heat exchanger approach ((T_{\text{chw,sup}} - T_{\text{cw,in}})) must be engineered as tight as 2°F to 4°F. A 2°F approach enables free cooling whenever outdoor wet-bulb drops below 40°F, unlocking hundreds of additional compressor-free operating hours each winter.