Unit 3 — Refrigeration System Fundamentals & Maintenance
Section 1 — Fundamental Concepts
1.4 Energy, Work and Power
Understanding energy, work, power, pressure, and thermodynamic properties
is essential for analyzing how refrigeration and HVAC/R systems operate,
measure performance, and transfer heat.
The capacity to do work or produce heat. In refrigeration, we are especially
concerned with thermal energy (heat) and
mechanical energy.
SI: joules (J), kilojoules (kJ), kilowatt-hours (kWh) Imperial: British Thermal Units (Btu), ton-hours
Refrigeration systems move heat from a low-temperature region (evaporator) to a
higher-temperature region (condenser) using mechanical work supplied to the compressor.
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Work
Done when a force moves an object over a distance. In a compressor,
mechanical work is performed to compress refrigerant vapour and increase its pressure
and temperature.
SI: joules (J), where 1 J = 1 N·m Imperial: foot-pounds force (ft·lbf)
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Power
The rate at which work is done or energy is transferred.
SI: watts (W), where 1 W = 1 J/s; kilowatts (kW) Imperial: horsepower (HP)
System efficiency is evaluated by comparing delivered cooling/heating capacity
(kW of heat moved) to electrical input power (kW).
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Example — Two Fan Motors
Two fan motors are installed on similar air-handling units. One motor is rated at
1 HP and the other at 2 HP. At first, the 2 HP motor
appears to be doing more work simply because it is larger.
However, both fans are moving the same airflow against approximately the same static
pressure, and both motors are drawing nearly the same electrical current at the same
voltage. This shows that both motors are under a similar load and are delivering about
the same power to the fan.
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Key Idea
Motor size tells you the maximum power the motor can handle, but the
actual work being done depends on the airflow and pressure conditions. A bigger motor
does not automatically mean more work is being done.
1.4.2 — Pressure and Vacuum
Pressure is the amount of force pushing on a certain area. In HVAC work, pressure is
usually measured in psig, psia, or inches of water column. Standard atmospheric pressure
at sea level is about 14.7 psia.
A vacuum is any pressure lower than atmospheric pressure. In HVAC/R, vacuum is usually
measured in microns or inches of mercury (in. Hg) below
atmospheric pressure. A deep vacuum is needed when evacuating a system before charging it
with refrigerant, to remove air and moisture.
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Absolute Pressure (psia)
Measured from a perfect vacuum. Includes atmospheric pressure in
the reading. Used in thermodynamic calculations and pressure-temperature charts.
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Gauge Pressure (psig)
Measured from atmospheric pressure. This is what most gauges read
— it shows how much pressure is above (or below) the surrounding air.
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Converting Between psig and psia
Technicians often need to convert between psig and psia when using
pressure-temperature charts and other system information.
psia = psig + 14.7
psig = psia − 14.7
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Example — Tire Gauge
A tire gauge does not measure all the pressure in the tire; it only measures how much
the tire pressure is above the outside air. That is psig. If you add the
outside air pressure, you get psia.
If a tire gauge reads 32 psig, that means the air in the tire is
32 psi above atmospheric pressure. At sea level, atmospheric pressure is about
14.7 psi, so the same tire has an absolute pressure of 46.7 psia.
Flat Tire
Gauge pressure: 0 psig
Absolute pressure: 14.7 psia
Properly Inflated Tire
Gauge pressure: 32 psig
Absolute pressure: 46.7 psia
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Choosing the Right Unit
Both psig and psia are used for relatively higher
pressure measurements, such as refrigerant pressures in an air conditioning system.
Inches of water column (“w.c.) is used for lower pressure
measurements, such as the static pressure of air in ducts.
1.4.3 — Tons of Refrigeration
A ton of refrigeration is a way to describe cooling capacity —
how much heat an air-conditioning or refrigeration system can remove.
The term comes from the early days of refrigeration, when ice was used to keep food and
buildings cool. Before mechanical refrigeration systems were common, ice was cut from lakes
and rivers in winter, stored in ice houses, and then used for cooling during warmer weather.
Engineers later used ice as a standard for measuring cooling. They found that if one short
ton of ice (2,000 lb) melts over a 24-hour period, it
absorbs 288,000 Btu of heat. Dividing by 24 hours gives 12,000 Btu/h.
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Tons of Refrigeration — Key Values
1 ton of refrigeration = 12,000 Btu/h
2 tons = 24,000 Btu/h
3 tons = 36,000 Btu/h
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In Practice
In HVAC work, air-conditioning equipment is often rated in tons. For example,
a 2-ton system can remove 24,000 Btu of heat per hour, which is
the same as removing 288,000 Btu over 24 hours.
1.4.4 — Enthalpy and Entropy
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Enthalpy
The total heat energy in a refrigerant — includes the heat
stored in the refrigerant and the energy related to pressure.
Units: Btu/lb (kJ/kg)
Shown on the horizontal axis of a pressure-enthalpy (P-h) diagram.
If the refrigerant gains heat, its enthalpy increases. If the refrigerant loses heat,
its enthalpy decreases.
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Entropy
A property that helps describe how energy moves and how efficient
a process is. Can be thought of as a measure of how spread-out energy is.
Used in refrigeration to explain:
How close a process is to ideal operation.
How efficient compression is.
How heat transfer affects system performance.
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Using Enthalpy in HVAC/R Practice
Technicians use enthalpy differences to find:
The heat absorbed in the evaporator.
The heat rejected in the condenser.
The work added by the compressor.
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Refrigeration Effect
The refrigeration effect is the difference in enthalpy between the
refrigerant entering and leaving the evaporator.
