Horsepower (metric) (PS) to Tons of Refrigeration (RT) conversion

What is Horsepower (metric)?

This section will provide a comprehensive overview of metric horsepower, including its definition, origins, calculation, and real-world applications.

Definition and Origin

Metric horsepower (PS, PferdeStärke in German, or cheval-vapeur in French) is a unit of power defined as the power required to raise a mass of 75 kilograms against Earth's gravitational force over a distance of one meter in one second. It is slightly less than the imperial horsepower.

Calculation

The value of one metric horsepower is:

$$1 \text{ PS} = \frac{75 \text{ kg} \cdot 9.80665 \text{ m/s}^2 \cdot 1 \text{ m}}{1 \text{ s}} \approx 735.499 \text{ watts}$$

Therefore, approximately:

$1 \text{ PS} \approx 0.98632 \text{ hp}$ (Imperial Horsepower)

$1 \text{ hp} \approx 1.01387 \text{ PS}$

Historical Context

The term "horsepower" was originally coined by James Watt to compare the output of steam engines to the power of draft horses. While Watt's original definition is related to the imperial horsepower, the metric horsepower evolved separately on the European continent, primarily for similar comparisons involving machinery and animal power.

Real-World Examples

• Automobiles: Engine power is commonly specified in metric horsepower in many parts of the world. For example, a typical family car might have an engine rated at 150 PS.
• Motorcycles: Motorcycle engine power is also frequently stated in metric horsepower.
• Agricultural Machinery: Tractors and other farming equipment often have their power output measured in PS.
• Industrial Pumps and Motors: The power of pumps, fans, and electric motors used in industrial applications can be rated in metric horsepower. For example a pump may be rated at 5 PS.

Fun fact

While not commonly named after any specific person or law, it's interesting to know how metric horsepower is used across Europe. It gives the user a good understanding of the "power" of a machine.

What is the Ton of Refrigeration?

The ton of refrigeration (RT) is a unit of power that measures the rate of heat removal in cooling and air-conditioning systems. It is widely used in the HVAC industry to rate chillers, air conditioners, and industrial refrigeration equipment.

Definition

One ton of refrigeration is the rate of heat transfer needed to freeze 2,000 pounds (one short ton) of water at 0 °C into ice at 0 °C over 24 hours. Using the latent heat of fusion of water, this equals exactly 12,000 Btu/h, which is:

$$1\ \text{RT} = 3516.85\ \text{W}$$

Equivalently, $1\ \text{RT} = 12{,}000\ \text{Btu/h} = 3.51685\ \text{kW}$. It is a power (heat-flow) unit, not a unit of mass or energy; the NIST Guide to the SI (SP 811) lists the ton of refrigeration (12 000 Btu(IT)/h) as 3.516 853 × 10³ W.

Origin and History

The unit dates to the 19th century, when mechanical refrigeration competed directly with the natural-ice trade. Cooling capacity was rated by how much ice a machine could effectively replace, so one "ton" of refrigeration described equipment capable of melting one ton of ice per day. The measure stuck even as ice harvesting disappeared.

Law and Notable Facts

The ton of refrigeration is not an SI unit; the SI unit of power is the watt. It remains an industry standard in North America, where residential air conditioners are commonly sized in tons (for example, a "3-ton" central AC unit). Note that the "ton" here refers to heat-removal rate, not the weight of the equipment. A closely related metric variant, the frigorie-based commercial ton, is defined slightly differently in some regions, but the U.S. value of exactly 12,000 Btu/h is the most common.

Real-World Examples and Conversions

• A typical residential central air conditioner rated at 3 RT removes about 36,000 Btu/h, or roughly 10,551 W of heat.
• A single ton of refrigeration (3516.85 W) is comparable to the cooling output of a large window AC unit sized for a big room.
• Large commercial chillers are often rated in the hundreds or thousands of tons; a 500 RT chiller handles about 1.758 MW of heat load.
• To convert tons to kilowatts of cooling, multiply by 3.51685: a 2 RT mini-split delivers about 7.03 kW of cooling.