ICE CREAM CABINET - Commercial refrigerator which operates at approximately -18°C; used for storage of ice cream.

IGNITION QUALITY - The ability of a fuel to ignite when it is injected into the compressed-air charge in a diesel cylinder. It is measured by an index called the cetane number.

IMMISCIBLE - Not capable of mixing (as oil and water).

IMPEDANCE (Z) - The quantity in an AC circuit that is equivalent to resistance in a DC circuit, inasmuch as it relates current and voltage. It is composed of resistance plus a purely AC concept called reactance and is expressed, like resistance, in ohms.

IMPELLER - A rotating set of vanes designed to impart rotation to a mass of fluid.

IMPINGEMENT - High-velocity flow of water or gas over a metal surface, causing premature failure by abrasion.

IMPULSE PRINCIPLE -

INDUCTANCE - The process when a second conductor is placed next to a conductor carrying AC current (but not touching it), the ever-changing magnetic field will induce a current in the second conductor.

INDUCTION - The capture of part of the ambient air by the jet action of the primary air stream discharging from a controlled device.

INDUCTION HEATING - Heating by combined electrical resistance and hysteresis losses induced by subjecting a metal to varying magnetic field surrounding a coil carrying alternating current.

INDUCTIVE LOADS - Loads whose voltage and current are out-of-phase. True power consumption for inductive loads is calculated by multiplying its voltage, current, and the power factor of the load.

INDUCTOR - A fundamental element of electrical systems constructed of numerous turns of wire around a ferromagnetic or air core.

INERT GAS - A gas that does not readily enter into or cause chemical reactions.

INFILTRATION - Air flowing inward as through a wall, crack, etc.

INFLUENT - The solution which enters an ion-exchange unit.

INHIBITOR - An additive used to retard undesirable chemical action in a product. It is added in small quantities to gasoline’s to prevent oxidation and gum formation, to lubricating oils to stop color change, and to corrosive environments to decrease corrosive action.

INORGANIC MATERIAL - Are substances not derived from living things.

INRUSH CURRENT - The current that flows the instant after the switch controlling current flow to a load is closed. Also called "locked rotor current".

INSTANTANEOUS RATE - Method for determining when load shedding should occur. Actual energy usage is measured and compared to a present kilowatt level. If the actual kilowatt level exceeds a designated set point, loads will be shed until the actual rate drops below the set point.

INSULATION, THERMAL - Material which is a poor conductor of heat; used to retard or slow down flow of heat through wall or partition.

INSULATOR - A material of such low electrical conductivity that a flow of current through it can usually be neglected. Similarly, a material of low thermal conductivity, such as that used to insulate structures.

INTERCOOLED CYCLE - Refers to a gas turbine employing two compressors. The compressed air from the first compressor is cooled before being discharged to second compressor.

INTER-COOLING - Removal of heat from compressed gas between the compression stages.

INTER-GRANULAR CRACKING - Cracking or fracturing that occurs between the grains or crystal in a polycrystalline aggregate. Also called intercrystalline cracking. Contrast with Tran granular cracking.

INTERMITTENT BLOW DOWN - The blow down is taken from the mud drum, waterwall headers or the lowest point of circulation.

INTER-STAGE DIFFERENTIAL - In a multistage HVAC system, the change in temperature at the thermostat needed to turn additional heating or cooling equipment on.

ION - An atom or radical in solution carrying an integral electric charge, either positive (cat ion) or negative (anion).

ION EXCHANGE - A reversible process by which ions are interchanged between solids and a liquid.

ION EXCHANGE RESIN - Cross linked polymers that form salts within ions from aqueous solutions.

IONIC STRENGTH - A measure of strength of a solution based on both the concentrations and valences of the ions present.

IONIZATION - The process of separation of a molecule into its electrically charged atoms or parts.

IRON - A metallic element found as an impurity in water in very small amounts. Also a metal which is widely used in the construction of HVAC and plumbing equipment; the major component of steel.

IRON BACTERIA - Are filamentous organisms encountered in iron-bearing water.

ISENTROPIC PROCESS - A process carried out reversibly without energy interchange as heat. Also a processes carried out with no entropy change.

ISOBARIC PROCESS -  An isobaric process is a thermodynamic process in which the pressure remains constant. This is usually obtained by allowed the volume to expand or contract in such a way to neutralize any pressure changes that would be caused by heat transfer.

ISOCHORIC PROCESS OR ISOMETRIC- A process during which the specific volume remains constant.

ISOMETRIC PROCESS - A process carried out at constant volume.

ISOTHERMAL - Changes of volume or pressure under conditions of constant temperature.

ISOTHERMAL EXPANSION AND CONTRACTION - Action which takes place without a temperature change.

ISOTHERMAL PROCESS - For a constant temperature process involving an ideal gas,