Engineering library
Technical notes behind our calculators: the equations, symbols and units, assumptions and limits, and a worked example for each. Free to read and print. All symbols are listed on one page: symbols and units.
HVAC 15 notes
Psychrometrics: moist-air properties
Saturation and actual vapour pressure, humidity ratio and enthalpy of moist air from dry-bulb temperature, relative humidity and air pressure.
HVAC · HVAC-03Mixing of two air streams
Dry-air flow, humidity ratio and enthalpy after two air streams mix, for example outdoor air and return air ahead of a coil.
HVAC · HVAC-04U-value and heat transfer through a layered wall or roof
Layer resistances, total thermal resistance, U-value and steady heat flow through a wall, roof or floor made of several layers.
HVAC · HVAC-05Dehumidification: water removed from air
Water removed from an air stream as it is cooled and dehumidified, from the dry-air flow and the entering and leaving humidity ratios.
HVAC · HVAC-06Humidification: moisture added to air
Moisture a humidifier must add to an air stream to reach a target humidity ratio.
HVAC · HVAC-07Evaporative cooling: leaving air temperature
Air temperature leaving a direct evaporative cooler, and its approach to the wet bulb, from the cooler effectiveness.
HVAC · HVAC-08Electric air heater duty
Heat added to an air stream and the electrical input of the heater, from airflow and temperature rise.
HVAC · HVAC-09Cooling tower range and heat rejection
Cooling-tower range and heat rejection from the condenser-water flow and the hot and cold water temperatures.
HVAC · HVAC-10Supply airflow from sensible load or air changes
Supply airflow in m³/s, m³/h, L/s and cfm, from a room sensible load and supply-to-room temperature difference, or from room volume and air changes per hour.
HVAC · HVAC-11Fan air power, shaft power and motor input
Air power, fan shaft power and motor input power from airflow, fan total pressure and the fan, drive and motor efficiencies.
HVAC · HVAC-13Pump total dynamic head
Pump total dynamic head as the sum of static, pipe friction, fitting, equipment, terminal and other heads along the critical path.
HVAC · HVAC-15Pipe internal diameter from flow and velocity
Internal pipe diameter needed to carry a flow at a chosen velocity.
HVAC · HVAC-17Chilled-water flow from cooling load
Chilled-water mass and volume flow from the cooling load and the chilled-water temperature difference.
HVAC · HVAC-18Pipe weight with its water content
Mass per metre and total mass of a pipe and the fluid inside it, from outside diameter, wall thickness and densities. Used for support and hanger loads.
HVAC · HVAC-21Pipe water volume
Internal cross-section and water volume of one pipe, or the total water content of several pipe segments. Used for expansion tanks and chemical dosing.
Electrical 6 notes
Electrical load: connected, demand and diversified
Connected, demand and diversified load totals from a load schedule, one row per load group.
Electrical · ELEC-03Cable voltage drop
Voltage drop in volts and percent, and the receiving-end voltage, for single-phase or balanced three-phase circuits from conductor resistance and reactance.
Electrical · ELEC-05Transformer kVA from load
Transformer kVA needed for an active load at a given power factor, with an optional design margin.
Electrical · ELEC-08Motor full-load current, input power and torque
Motor full-load current, electrical input, apparent power and shaft torque from the rated output power, voltage, efficiency and power factor.
Electrical · ELEC-10Power factor correction: capacitor kvar
Capacitor kvar needed to raise the power factor of a load, with the reactive and apparent power before and after correction.
Electrical · ELEC-12Lighting design by the lumen method
Number of luminaires for a target average illuminance in a room, and the illuminance they give, by the lumen method.
Plumbing 7 notes
Rectangular water tank: capacity and plan size
Water capacity of a rectangular tank from its dimensions, or the plan length and width for a required volume, water depth and length-to-width ratio.
Plumbing · PHE-07Pump hydraulic, shaft and motor power
Hydraulic power, pump shaft power and motor input power from flow, total head, fluid density and the pump and motor efficiencies.
Plumbing · PHE-09Hydro-pneumatic tank volume
Nominal volume of a hydro-pneumatic pressure tank from the required drawdown and the tank acceptance factor.
Plumbing · PHE-10Hot water demand: daily and peak hour
Daily and peak-hour hot-water demand totals from a schedule of usage rows (apartments, rooms, beds, fixtures and so on).
Plumbing · PHE-11Water heater duty and input
Water heating duty and heater input from water flow and temperature rise.
Plumbing · PHE-12Hot water storage volume
Net and design hot-water storage volume from the peak demand, the heater recovery during the peak and a storage margin.
Plumbing · PHE-13Hot water recirculation flow
Recirculation flow that holds the hot-water return temperature drop to the allowed value, from the loop heat loss.
Cleanroom 1 note
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Every equation is checked against its calculator
Each of the 29 notes describes one of our verified calculators. Before the pages are published, all 82 equations are recalculated and compared with the calculators on 450 sets of input values, including the verification tests, so the equation on the page is the one the calculator uses. The worked examples are those verification tests.
The working, not just the answer
Each note shows the equations in order, with every symbol and unit, so a result can be checked by hand.
What each method leaves out
Assumptions and limits are listed with every note: what is an input you must supply, and what is not covered.
Preliminary checks
The notes support engineer review. The project calculation to the applicable codes, authority requirements and specification governs.
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