In this pair of articles I will be dissecting the fundamentals of cooling and refrigeration from an IT engineering perspective, then going over the process of sizing a cooling system. As infrastructure engineers, we likely deal with the topic of cooling on occasion, and those occasions tend to be during a facility buildout/upgrade or outage (get the portable fans!). Since there is typically little an infrastructure engineer can do during an environmental control outage, I will focus on what you need to know to be an asset during the design/build of a new facility or the upgrade of an existing one.
The purpose of this article is to equip you with the concepts and general understanding required to effectively communicate with mechanical engineers in regards to datacenter cooling and environmental control.
For handy references and info, download my Power & Cooling Cheat Sheet to the right.
Approx Reading Time: 10-15 Minutes
Generally speaking, heating is simple. Typically, heating systems (in the HVAC world) just convert energy from one form to another; this can be by burning a fuel like natural gas (chemical potential energy converted to heat) or by running electricity through a heating element (electrical energy to heat). Relative to a refrigeration system, heating is simple because it is just simple energy conversion to create heat from something else (this is excluding heat pumps, which I will explain later). Refrigeration or cooling, on the other hand is a bit more complicated.
Since “cold” is really just a lack of thermal energy (heat), refrigeration involves moving thermal energy from one place to another; extracting heat from the place you want to cool. The method used to move this heat is based on the thermodynamic property of a liquid, which has a high vapor pressure at the desired (cold) temperature, to absorb heat and evaporate into a gas. This absorption of heat is what allows the refrigerant to move that heat from one place to another.
Frosted Propane Tanks
If you have ever used a Bar-B-Que which is powered by propane, then you have likely seen the above stated refrigeration/cooling effect as a cold and sometimes frosted propane tank after it has been significantly drained.
This happens because the propane tank contains liquid and gaseous propane under high pressure. Propane is a substance which, at regular atmospheric pressures and temperatures, has a very high vapor pressure and a boiling point of about -40°C. When you release the pressure in the tank by letting out gaseous propane, the liquid boils (quickly evaporates) and produces more gas (which continues to flow out of the tank and into your BBQ). The action of evaporation (boiling) is an endothermic process where a liquid absorbs heat from its surroundings and converts into a gas. This is true any evaporation process including boiling a pot of water, where the pot of water absorbs heat from the heat source (stove or otherwise), reaches its boiling point temperature (100C), and stays at that temperature: releasing the excess heat as water vapor into the air.
The Refrigeration/Cooling Cycle
The example of the propane tank shows us the evaporation portion of the refrigeration/cooling cycle. This is the portion that happens inside of an evaporator which is inside of an air handler and cools the air for distribution inside the facility. Two other main parts in the system are the compressor and the condenser. These act to reuse the refrigerant and cycle it through the system to move the heat once it has been absorbed into the refrigerant, then expel it outside through the condensation process.
To start with something similar to the propane tank example, we will begin the refrigeration cycle with the expansion valve
- The high pressure side the of the expansion valve contains high pressure liquid refrigerant which is at room temperature or slightly above. As that liquid passes through the expansion valve, it moves into the evaporator.
- Inside the evaporator there a much lower pressure than the area behind the expansion valve. The reduction in pressure causes the refrigerant to flash boil and evaporate into a gaseous form.
- This occurs because the refrigerant, at low pressure and regular room temperature, has a very low boiling point (comparable to propane). In order for the liquid to evaporate, it must absorb thermal energy from its surroundings which causes the evaporator to lose a large amount of heat and become cold.
- The evaporator, which looks very similar to the radiator in a car and is designed to transfer heat into the air, is placed inside the ventilation system and in the path of the moving warm air. As the warm air moves by, it transfers heat to the evaporator; this cools the air (which is our primary desired effect) and also keeps the evaporator warm enough to continue evaporating the refrigerant.
- After passing through the evaporator the cool, low pressure gaseous refrigerant, is piped to the compressor which compresses it into a hot high pressure gas. The gas comes out of the compressor hot due to the same effect (just in reverse) as what happened at the expansion valve.
- It is important the gaseous refrigerant comes out of the compressor compressed enough to be hotter than the temperature outside of the building. If it is not, the refrigerant cannot release its heat into the outside air.
- The hot gaseous refrigerant, after coming out of the compressor, moves into the condenser. Here the refrigerant loses a large amount of heat as it moves through the condenser and ends up condensing back into a warm liquid. The now liquid refrigerant is piped back to the expansion valve and evaporator to start the cycle over again.
- The condenser is, like the evaporator, shaped similar to a giant radiator and is usually placed outside the building. Typically condensers have a large fan which moves air through the condenser fins to aid in the releasing of heat from the refrigerant.
- The compressor usually sits right next to or inside of the condenser unit. Since the condenser usually is accompanied by a large fan on top to move air through the coils, the compressor is typically placed at the bottom of the condenser and uses the same power supply as the fan. If you look into a common household condenser unit from the top, you will see the compressor sitting at the bottom.
And there you have it. The single-stage vapor compression refrigeration cycle. There are other methods of cooling/refrigeration available such as the vapor absorption cycle (which actually uses a heat source for power, and is typically limited to use in mobile vehicles where electricity is not available in large quantities), but they are not typically used for environmental control. The most commonly used system is the vapor compression cycle.
**NOTE: A “heat pump” is a refrigeration unit which runs in reverse. The system is able to change the direction of heat transfer; pulling heat from the outside and depositing it inside. The benefit over conventional heating systems (like the ones described above) is that a heat pump is simply moving heat rather than creating it from different energy sources; making it more efficient.
Refrigeration and cooling capacity is always measured in units of power, which also translate to units of heat. There are a plethora of units for heat and power, (Watts, Joules per second, horsepower, calories, BTUs, ergs, etc.) but refrigeration systems are usually measured in British Thermal Units (BTUs) or tons.
One BTU is equal to the amount of work required to raise the temperature of one pound of water by one degree Fahrenheit, and is approximately equivalent to the amount of heat produced by a stick match once completely consumed by fire.
One ton of refrigeration/cooling is equal to the amount of work required to freeze one short ton (2000lb) of water in 24 hours (after is has already reached 0°C). Note that once water has reached 0°C, extra heat must be extracted from it to force it through the phase transition to ice. It is this extra heat extraction for phase transition which is measured in tons. The unit traces back to the early days of the ice making industry where ice manufacturers needed a way to measure how much ice they could produce using a particular refrigeration system.
The most efficient way to measure cooling (or heat production) in the IT world is using the Watt. This is because all electrical power consumed by IT equipment in a datacenter is converted to heat, and we can often easily measure the total wattage being consumed by our equipment by looking at statistics on the power backup and distribution systems. Once we know the power consumption, we also know the heat production. Since watts, BTUs, and tons are all measurements of power, here is a conversion chart.
|Unit||Unit 1||Unit 2||Equivalent To|
|1 British Thermal Unit (BTU)||0.0000833… Tons (of Refrigeration)||0.29307107 Watts||Burning of a stick match or enough heat to raise the temperature of 2 cups of water by 1°F|
|1 Ton (of Refrigeration)||12,000 BTU per hour||3,516.85 Watts||The amount of cooling needed to lower the temperature of the water in a large spa by 1°F|
|1 Watt||3.412 BTU per hour||0.000284 Tons (of Refrigeration)||Enough heat to raise the temperature of 1/2 gal of water by almost 1°F|
To Be Continued…
Check out Back to Basics: Cooling – Part 2 for the second and final part of this topic!