1. Calculation Scope & Governing Equations
This rating sheet provides sizing and thermal verification for air-cooled heat exchangers (fin-fan coolers) in accordance with API Standard 661 / ISO 13706 and ASME Section VIII Division 1.
$$Q = \dot{m}_{tube} C_{p,tube} (T_{tube,in} - T_{tube,out}) = \dot{m}_{air} C_{p,air} (T_{air,out} - T_{air,in})$$
$$\text{LMTD}_{counter} = \frac{\Delta T_1 - \Delta T_2}{\ln(\Delta T_1 / \Delta T_2)}, \quad \Delta T_1 = T_{tube,in} - T_{air,out}, \quad \Delta T_2 = T_{tube,out} - T_{air,in}$$
$$A_{\text{req,bare}} = \frac{Q}{U_{bare} \cdot F_t \cdot \text{LMTD}}, \quad A_{\text{avail,ext}} = A_{\text{avail,bare}} \times \left(\frac{A_{ext}}{A_{bare}}\right)$$
$$\frac{1}{U_{bare}} = \frac{1}{\eta_o \cdot h_{air,bare}} + R_{f,air} + \frac{d_o \ln(d_o / d_i)}{2 k_{tube}} + \frac{d_o}{d_i}\left(\frac{1}{h_{inside}} + R_{f,tube}\right)$$
$$P_{motor} \ge 1.10 \times \left(\frac{Q_{fan} \cdot \Delta P_{static}}{1000 \cdot \eta_{fan} \cdot \eta_{drive}}\right) \quad \text{(API 661 Clause 7.2.8 Driver Margin)}$$
2. Standard Parameters & API 661 Design Criteria
| Service / Fluid Type | Typical Overall $U_{bare}$ | Recommended Design Limits |
|---|---|---|
| Hydrocarbon Gas (Compressor Discharge) | $250 - 450\text{ W/m}^2\cdot\text{K}$ | $v_{tube} = 8 - 20\text{ m/s}$; allowable $\Delta P \le 0.5\text{ bar}$. |
| Naphtha Overhead Vapor (Condenser) | $600 - 950\text{ W/m}^2\cdot\text{K}$ | Single pass or split header; allowable $\Delta P \le 0.15\text{ bar}$ to prevent column backpressure. |
| Glycol / Water Closed Loop | $500 - 800\text{ W/m}^2\cdot\text{K}$ | $v_{tube} = 1.2 - 2.5\text{ m/s}$; ensures turbulent flow without erosion. |
| Crude Oil / Heavy Hydrocarbons | $150 - 300\text{ W/m}^2\cdot\text{K}$ | $v_{tube} \ge 1.0\text{ m/s}$; winterization louvers to prevent wax deposition. |
| Air Face Velocity ($v_{face}$) | $2.5 - 3.5\text{ m/s}$ ($500 - 700\text{ ft/min}$) | API 661 optimal window balancing heat transfer coefficient with fan power. |
| Fan Tip Speed Noise Limit | $v_{tip} \le 61.0\text{ m/s}$ ($12,000\text{ ft/min}$) | API 661 Clause 7.2.4 acoustic threshold; $\le 45\text{ m/s}$ for low noise. |
3. Engineering Assumptions & Boundary Limits
- Fouling Resistances: Default calculations include standard API 661 fouling allowances ($0.00035\text{ m}^2\cdot\text{K/W}$ for hydrocarbon gas, $0.00018\text{ m}^2\cdot\text{K/W}$ for ambient air side).
- Surface Overdesign Margin: Minimum $+10\%$ to $+25\%$ excess bare area required over clean/dirty duty to accommodate summer ambient peaks and dirt deposition.
- Fan Projected Coverage: Total fan swept area must cover $\ge 40\%$ of bundle face area for forced draft, and $\ge 45\%$ for induced draft per API 661.
- Site Elevation Correction: Barometric pressure and ambient air density are automatically adjusted via the barometric atmospheric formula for exact altitude.