Service technicians often confuse motor horsepower (hp) with tons of refrigeration. One common misconception is that 1 ton of refrigeration equals 1 hp. This statement is only true in some of the higher temperature applications like air conditioning. In fact, in the medium- and low-temperature applications of refrigeration, 1 hp is hardly ever equal to 1 ton of refrigeration.
A ton of refrigeration is a rate of heat transfer, not an amount of heat. One ton is equal to the heat absorbed in melting 2,000 pounds (1 ton) of ice at 32°F in 24 hours (1 day). This equates to 12,000 Btu/hr or 12,000 Btuh. Btu/hr and Btuh are often used interchangeably and mean the same: 12,000 Btuh is also equivalent to 200 Btu/min since there are 60 minutes in 1 hour. The mathematical formula below proves how 1 ton of cooling equates to 12,000 Btuh.
(2,000 lb ice) x (144 Btu/lb) ÷ (24 hours) = 12,000 Btuh
so, 1 ton of cooling = 12,000 Btuh or 200 Btu/min
Note: The latent heat of fusion for ice is 144 Btu/lb, which means that 144 Btu are absorbed for each pound of ice that is melted to water.
When refrigeration or air conditioning equipment is rated for 1 ton of cooling, it means that the equipment should remove heat at a rate of 12,000 Btuh. However, equipment that is removing 12,000 Btu in 30 minutes would have a capacity of 2 tons. If the cooling equipment is removing 12,000 Btu in 4 hours, its capacity is only ¼-ton. Notice, in all three scenarios, 12,000 Btu of heat energy is removed; however, the time rate or how fast these systems removed the 12,000 Btu determined the capacity of the system in tons.
There are variables in each type of system that will determine the capacity of that system.
Evaporator pressure: A higher pressure in the evaporator would mean that the cylinder volume of the compressor is experiencing a higher pressure also. This means that the cylinders experience a denser vapor each down stroke. This higher density vapor inside the compressor cylinders increases the mass flow rate of refrigerant vapor through the compressor and thus increases capacity.
Any time you fill a fixed volume (compressor’s cylinder) with a higher pressure, more refrigerant gas molecules will be present, causing a higher refrigerant density. The mass flow rate of refrigerant through the compressor is a product of the piston displacement and the density of the refrigerant filling the cylinder. The units for mass flow rate are in pounds/minute:
Mass flow rate = (Piston displacement) x (Refrigerant density)
pounds/minute = cubic feet/minute pounds/cubic feet
Volumetric efficiency: As evaporator pressure is increased, compression ratio is decreased, which in turn, will increase the volumetric efficiency of the compressor’s cylinders. Both the high and low side system pressures can be expressed as a ratio called a compression ratio. Compression ratio is defined as the absolute discharge pressure divided by the absolute suction pressure:
Compression ratio = Absolute discharge pressure ÷ Absolute suction pressure
Most service technicians realize that their service gauges read zero when not connected to a system, even though there is a pressure of approximately 15 psi on the gauges exerted from atmospheric pressure. That’s because these gauges are calibrated to read zero at atmospheric pressure. Therefore, in order to use the true or absolute discharge and suction pressure at zero gauge pressure or above, a technician must add 14.696 psi, or approximately 15 psi, to the gauge reading.
When referring to absolute pressure, psia is used to label the pressure's magnitude, and psig labels the pressure's magnitude when referring to gauge pressure. A compression ratio of 6 to 1 is expressed as 6:1 and simply means that the discharge pressure is six times the magnitude of the suction pressure.
A high volumetric efficiency means that more of the piston's cylinder volume is being filled with new refrigerant from the suction line and not re-expanded clearance volume gases. The higher the volumetric efficiency, the greater the amount of new refrigerant that will be introduced into the cylinder each down stroke of the piston, and thus more refrigerant will be circulated each revolution of the crankshaft. The system will now have better capacity and a higher efficiency. Also, the higher the suction pressure, the less re-expansion of discharge gases, because of the discharge gases experiencing less re-expansion to the higher suction pressure and the suction valve will open sooner.
Discharge pressure: The lower the discharge pressure, the less re-expansion of discharge gases in the clearance volume of the cylinder in order to get to the suction pressure. How much of the piston displacement is filled by new refrigerant vapors depends on system pressures and valve design. A service technician can control, to a certain extent, how high or low the discharge and suction pressure will go. If the discharge (condensing) pressures can be kept low and the suction (evaporating) pressure can be kept as high as possible without affecting the refrigerated product temperature, the compression ratio will be low and the volumetric efficiency will be high. This will cause a higher mass flow rate of refrigerant to flow through the compressor and increase the capacity of the system.
Compressor superheat: By taking the temperature of the suction line entering the compressor and the suction pressure at that point and convert it to a saturation temperature, the difference between the two is the compressor superheat. The higher the compressor superheat, the hotter the refrigerant gases will be coming into the compressor. This will cause a lower refrigerant density and a lower mass flow rate of refrigerant through the compressor. The service technician can make sure that the compressor does not have too much compressor superheat.
There is an approximate 1% change in capacity for every 10°F of total superheat change. As total superheat increases, capacity decreases. This rule of thumb should be used for service purposes only and not design purposes. Other factors that affect how much compressor superheat the compressor sees are length and insulation of the suction lines, ambient or surrounding temperature the suction line is exposed to, and liquid/suction line heat exchangers present. Always consult with the compressor manufacturer to find out what is the ideal compressor return gas temperature or what is the maximum return gas temperature allowed for your compressor application.
Subcooling: The more subcooled the refrigerant is before it enters the evaporator, the more cooling capacity the system will have. This phenomenon happens because the cooler liquid entering the evaporator does not have to flash as much to cool itself down to the evaporating temperature associated to the evaporator pressure. The less flashing, the more of a net refrigeration effect (NRE) there will be in the evaporator. There is approximately a half percent change in capacity for every 1°F of liquid subcooling change. As subcooling increases, capacity increases. This rule of thumb should be used for service purposes only and not design purposes.
In conclusion, keeping the refrigerant return gas to the compressor as dense as possible will increase system capacity. Also, keeping suction pressures as high as possible without sacrificing product temperature and keeping the head pressure as low as possible will also increase system capacity. Also, the more liquid subcooling there is entering the evaporator, the higher will be the system capacity.