HVAC Fundamentals

91 flashcards by MentisLibrary

The refrigeration cycle, superheat and subcooling, airflow and static pressure, load calculation, refrigerants, and the troubleshooting logic that separates a low charge from a restriction.

What is conduction, and where does it show up in HVAC?
Heat transfer through direct contact between molecules of a solid or between touching materials.
How does convection move heat, and what is the difference between natural and forced convection?
Heat moves with a fluid in motion. Natural convection relies on warm fluid rising on its own; forced convection uses a blower or pump to move the fluid.
What is radiant heat transfer?
Heat transferred as infrared energy through space, with no fluid or solid carrier needed.
What is the difference between sensible heat and latent heat?
Sensible heat changes the temperature of a substance and shows on a thermometer. Latent heat changes its state with no temperature change.
What is a BTU?
A British Thermal Unit, the heat needed to raise one pound of water one degree F.
A ton of refrigeration equals how many BTU per hour, and where does that number come from?
12,000 BTU/h. It is the rate of cooling produced by melting one ton of ice over 24 hours.
What are the three air side heat formulas a tech uses in the field?
Sensible: $Q_s = 1.08 \times CFM \times \Delta T$. Latent: $Q_l = 0.68 \times CFM \times \Delta grains$. Total: $Q_t = 4.5 \times CFM \times \Delta h$.
What is wet bulb temperature and how is it read?
The temperature of air after adiabatic saturation, read with a wetted wick on a thermometer moving through the airstream, as with a sling psychrometer or a digital psychrometer.
What is dew point temperature?
The temperature at which air becomes fully saturated and moisture begins to condense out of it.
What does relative humidity actually express?
The percentage of moisture the air holds compared to the maximum it could hold at that same dry bulb temperature.
What is a psychrometric chart used for?
It plots every property of moist air, so knowing any two properties lets you read all the rest.
What is enthalpy in air conditioning work?
The total heat content of the air, sensible plus latent, expressed in BTU per pound of dry air.
How does an air conditioner dehumidify?
The evaporator coil surface runs below the dew point of the return air, so moisture condenses on the fins and drains away.
Name the four essential components of the vapor compression refrigeration cycle and what each does.
Compressor raises pressure and temperature of the vapor, condenser rejects heat and condenses it to liquid, metering device drops the pressure, evaporator boils the refrigerant and absorbs heat.
What is the compressor doing in the cycle besides moving refrigerant?
It is the pump of the system. It lowers pressure on the evaporator so refrigerant boils cold, and raises pressure on the condenser so refrigerant condenses hot enough to reject heat outdoors.
Where is the dividing line between the high side and the low side of a system?
The compressor discharge and the metering device. From compressor outlet through the condenser to the metering device inlet is high side; from the metering device outlet through the evaporator back to the compressor suction is low side.
What is saturation temperature?
The temperature at which a refrigerant boils or condenses at a given pressure. At saturation, liquid and vapor exist together.
What is the pressure temperature relationship, and how is a PT chart used?
For any pure refrigerant, pressure and saturation temperature are locked together. Reading gauge pressure and converting it on a PT chart gives the saturation temperature inside the coil.
What is superheat and how do you measure it?
Superheat is the number of degrees a vapor is above its saturation temperature. Measure suction pressure, convert to saturation temperature, then subtract that from the measured suction line temperature at the same point.
What is subcooling and how do you measure it?
Subcooling is the number of degrees a liquid is below its saturation temperature. Measure liquid line pressure, convert to condensing temperature, then subtract the measured liquid line temperature from it.
How does a reciprocating compressor work, and what are its traits?
Pistons driven by a crankshaft draw vapor in through suction valves and compress it out through discharge valves.
What makes a scroll compressor different from a reciprocating compressor?
Two interleaved spiral scrolls, one fixed and one orbiting, trap and progressively squeeze pockets of vapor toward the center.
Where are rotary and screw compressors typically used?
Rotary compressors, using a rolling piston or vanes, are used in small capacity equipment such as window units, mini splits, and small heat pumps. Screw compressors, using meshing helical rotors, serve large commercial and industrial chillers.
How does a thermostatic expansion valve control the evaporator?
A sensing bulb on the suction line drives a diaphragm against evaporator pressure and a superheat adjustment spring, opening or closing the valve to maintain a constant superheat regardless of load.
What is a fixed orifice metering device, and how does it behave differently from a TXV?
A piston or capillary tube with a fixed bore. Flow varies only with the pressure difference across it, so superheat floats with load and outdoor temperature.
What advantage does an electronic expansion valve have over a TXV?
A controller reads pressure and temperature sensors and drives a stepper motor, so it can hold tighter superheat, respond faster, operate over a much wider capacity range, and close tight on shutdown.
What does a suction line accumulator do, and where is it required?
It sits in the suction line and traps liquid refrigerant, metering it back slowly with oil through a small bleed hole so the compressor gets vapor.
What is a liquid receiver for?
It stores liquid refrigerant on the high side after the condenser so the system has a reserve and the metering device always sees solid liquid.
What does a filter drier do, and when must it be replaced?
It filters particulate and adsorbs moisture and acid from the refrigerant using a desiccant core.
What can and cannot be concluded from bubbles in a sight glass?
Bubbles mean flash gas in the liquid line, which is usually low charge or a restriction, but they can also come from low subcooling or a partly closed valve.