Quick conversions
Quick field numbers for checks on the job (page idea from Jay Ganz). For an exact head factor at your glycol mix and temperature, use the specific gravity calculator.
PSI to feet of head
Ft of head = PSI × factor. PSI = ft ÷ factor. Glycol factors: ethylene/propylene glycol, % by mass, at 50°F (ASHRAE). For other temps use the Specific Gravity calculator.
Evaporator (chilled water)
GPM = tons × 24 ÷ ΔT · Tons = GPM × ΔT ÷ 24. Enter tons or GPM; the other is figured. Rule of thumb 2.4 GPM/ton (10 °F ΔT).
Condenser water
GPM = tons × 30 ÷ ΔT · Tons = GPM × ΔT ÷ 30. Rule of thumb 3.0 GPM/ton (10 °F ΔT).
Reference
| Item | Formula / value |
|---|---|
| Water head | ft = PSI × 2.31 |
| 30% ethylene glycol | ft = PSI × 2.20 |
| 35% ethylene glycol | ft = PSI × 2.18 |
| 40% ethylene glycol | ft = PSI × 2.16 |
| 45% ethylene glycol | ft = PSI × 2.15 |
| 50% ethylene glycol | ft = PSI × 2.13 |
| 30% propylene glycol | ft = PSI × 2.25 |
| 35% propylene glycol | ft = PSI × 2.24 |
| 40% propylene glycol | ft = PSI × 2.23 |
| 45% propylene glycol | ft = PSI × 2.22 |
| 50% propylene glycol | ft = PSI × 2.21 |
| Evaporator rule of thumb | 2.4 GPM per ton |
| Condenser rule of thumb | 3.0 GPM per ton |
| Evaporator GPM | tons × 24 ÷ (EWT − LWT) |
| Evaporator tons | GPM × ΔT ÷ 24 |
| Condenser GPM | tons × 30 ÷ (LWT − EWT) |
| Condenser tons | GPM × ΔT ÷ 30 |
Glycol factors: ethylene/propylene glycol, % by mass, at 50°F (ASHRAE). For other temps use the Specific Gravity calculator. See also the formulas page.