Evaporator Physics
This fundamental process in the evaporator absorbs heat by changing refrigerant from a low-pressure saturated liquid into a low-pressure saturated vapor.
What is vaporization (boiling or phase change)?
This common air-cooling construction stacks thin metal plates onto copper or aluminum tubing to maximize surface area.
What is a Tube-and-Fin Evaporator?
The metering device separates the high-pressure side of the system from this side.
What is the low-pressure side (evaporator side)?
Thermostatic Expansion Valves (TEVs/TXVs) are designed to maintain a constant value of this parameter at the evaporator outlet.
What is Superheat?
Turning a TEV stem in a clockwise direction increases spring pressure, which changes superheat in this direction.
What is an increase in superheat (starves the evaporator)?
This type of cooling load involves removing moisture from the air (dehumidification) as water condenses on the coil surface.
What is Latent Cooling (or Latent Load)?
Low-temperature freezer coils use wide fin spacing (1–4 FPI) primarily to prevent this problem from blocking airflow.
What is ice/frost buildup (frost bridging)?
Liquid refrigerant enters the metering device as subcooled liquid and leaves as a saturated mixture containing approximately this percentage of vapor.
What is 25% vapor (and 75% liquid)?
In the TEV force-balance equation, bulb pressure acts as an opening force, while spring pressure and this pressure act as closing forces.
What is Evaporator Pressure ($P_{evap}$)?
On suction lines 7/8" OD or larger, the sensing bulb must be mounted at the 4 or 8 o'clock positions to avoid reading this liquid resting at 6 o'clock
What is oil (or oil/liquid puddles)?
As entering air wet-bulb temperature increases, more system capacity is diverted to latent cooling, causing this value across the coil to drop.
What is the air temperature drop (dry-bulb temperature drop)?
Modern Microchannel coils are manufactured entirely out of this material to eliminate formicary ("ant-nest") corrosion leaks.
What is Aluminum?
This term describes the pressure drop across a restriction occurring without external heat transfer, maintaining constant enthalpy.
What is Adiabatic Expansion?
This type of expansion valve maintains a constant outlet pressure inside the evaporator, making it ideal for compressed air driers.
What is an Automatic Expansion Valve (AEV / AXV)?
Low suction pressure, high superheat, and high subcooling indicate this type of system restriction.
What is a Restricted Metering Device (or Liquid Line Restriction)?
This ratio represents the percentage of air passing through an evaporator without making contact with the fins or tubes.
What is the Bypass Factor (BF)?
In a flooded evaporator, these zigzag baffle plates force vapor to make abrupt turns so heavy liquid droplets fall back down before entering the suction line.
What are Eliminators (or Eliminator Baffles)?
Capillary tubes generate pressure drop through both their small internal diameter and this physical dimension.
What is their length?
An externally equalized TEV is required whenever a coil has a high pressure drop or uses this component to split fluid into parallel paths.
What is a Liquid Distributor?
Rapid, wild fluctuations in suction pressure and superheat caused by an oversized TEV or poor bulb contact is called this.
What is Hunting?
Exceeding an air velocity of this speed across a wet evaporator coil will cause condensate blowoff, pulling water droplets off the fins into the ductwork.
What is 550 Feet Per Minute (FPM)?
When using face control to unload a multi-circuit evaporator coil, this section must always be the first circuit turned on and the last circuit turned off.
What is the bottom circuit/section?
On a fixed-orifice system operating in cool outdoor weather, decreased head pressure pushes less refrigerant through the orifice, causing superheat to do this.
What is increase (climb/starve the coil)?
Gas-charged sensing bulbs provide an MOP feature to protect the compressor motor from overloading. What does the acronym MOP stand for?
What is Maximum Operating Pressure?
Before torch-brazing suction lines near a TEV, technicians must protect or unstrap the sensing bulb for this critical reason.
What is excessive heat creates high pressure in the bulb that will rupture the internal valve diaphragm?