Refrigeration – The Start
If a building engineer were tasked to sit down and list all of the systems he operates and maintains, he’d be there for seconds if not minutes. Nah, he could be there for days. When we get down to it though, the nitt-ay gritt-ay of building engineering, there would only be a few vital, critical systems that we all should know the fundamentals of.
And that would be refrigeration.
From building engineers, to landlords, to homeowners, it would probably help you out a little by knowing at least the basics of refrigeration. Things like the definition of refrigeration, sensible and latent heats which always confused the beejeezus out of me.
By us at least having a handle on the fundamentals of refrigeration, cooling, heating, you’ll know in your car if your air conditioning is working properly. You’ll know at home if the repairman is telling you big fat lies when he tries to bill you for your window unit.
Do I have to point out that when you hear the term HVAC, the ac on the end is for air conditioning? And refrigeration. Which is why we are discussing it here.
What is refrigeration?
Refrigeration is the process of removing heat from materials, objects, or spaces and maintaining them at a temperature below that of the surrounding atmosphere.
Heat will always, always, always flow in its natural direction – warmer to colder. Therefore refrigeration generally refers to an artificial way of lowering the temperature. Mechanical refrigeration is a system, device, or piece of equipment that transfers this heat between substances.
To produce a refrigeration effect, the object or material you want cooled simply needs to be exposed to a colder environment.
We’ve talked about chillers and refrigeration machines on the site before. We know they remove heat from spaces just like the ac in your home and car. Quickly, let’s recall roughly what a chiller, a chilled water system, & condenser water system do to cool down a building.
- The refrigerant goes through a pressure drop.
- In a heat exchanger (remember?) it passes by some chilled water.
- The chilled water gives up the heat it picked up from a space to the refrigerant.
- The refrigerant then gives up the heat it picked up from the chilled water to the condenser water system.
- The condenser water goes outside and gives up its heat to the atmosphere.
It’s still all about heat exchange and heat transfer!
What is heat?
The object of refrigeration is to keep those objects, devices, or systems at lowered temperatures. When was the last time you added ‘coolness’ to anything? Answer? Never because we can only remove heat from an object. Refrigeration cools through removing heat.
It is not this:
Difference between heat & temperature
Heat is a form of energy (like we talked about in the fundamentals of engineering post when we shoved that hapless victim off the cliff – heat, work, light, sound, etc.) Temperature is a measurement of the intensity of the heat.
Temperature is measured in degrees Farenheit, Celsius, Kelvin. Heat is measured in btus, or British thermal units. So the amount of heat energy contained in a substance is measured in btus. Temperature indicates the intensity of the heat of the substance. It DOES NOT indicate the number of btus in the substance though.
1 btu is the amount of heat required to raise the temperature of 1 lb of water 1F.
Example for heat and temperature:
We have 1 cup o’ coffee and we have 1 bucket full o’ coffee. Which temperature is probably hotter? Which one has more heat? The heat in the cup is more intense so its temperature would be higher. The bucket of coffee has more heat energy (btus,) but its heat is less intense.
What is sensible heat?
Here is where learning refrigeration in the military lost and confused me. When we got into sensible and latent heats, I was how you say, a little confused. We are going to go over them as quickly as possible lol. They are important terms though for us to have a grasp on because they are all over refrigeration systems. And if you get it down real good like, come back and teach me!
Sensible heat is the heat absorbed or emitted by a substance that is not in the process of a physical change of state. Adding or removing heat always causes a change in the temperature of the substance. Sensible heat can be measured with a thermometer.
What is latent heat?
Latent heat is the heat absorbed or given off by a substance while in a physical state change. The heat absorbed or given off does not cause a temperature change in the substance – hence latent.
So – sensible heat is heat that impacts the temperature of things, latent heat is the heat that affects the physical state of things,
What is specific heat?
All substances can absorb or reject heat in varying amounts depending on the substance. The spcific heat of water (the standard by which all others are measured) is 1.0. All other substances are then measured in comparison to the holy water.
The ability of a substance to lose or absorb heat is its specific heat.
To get qualified in whatever on a submarine, you have to go through like a year or 2 of questions, quizzes, and interviews. People above you sometimes try to trip you up, or make finding an answer difficult. One favorite question of the old, salty sea dogs was to define a refrigeration ton.
A refrigeration ton is based on the cooling effect of 1 ton of ice at 32F melting in a 24 hour period.
The capacity of a refrigeration unit is usually expressed in refrigeration tons. It basically tells us the heat removal capacity of a refrigeration machine. Keep in mind though, it does not show us the amount of ice a machine can produce or make.
Pressure, volume, and temperature
All related and interconnected like in bred cousins. When one changes, the others can change. Up, down. It is important that we start to grasp how these values relate to engineering, building engineering, and just basics in general.
Say the device that controls the temperature of your hot water heater at home goes haywire. Instead of keeping the water at a nice 105F, it allows the water temp to skyrocket to 180F! What happens to pressure inside the tank? It goes up and up until something gives.
The boiling point of any liquid varies according to the pressure on the liquid (think cooking at altitude.) The higher the pressure, the higher the boiling point.
Water boils at 212F at atmospheric pressure (about 14.6 psi.) At 600 psi, water doesn’t boil until almost 500F! At a 29″ vacuum, water boils at a cool 80F! Crazy huh?
The refrigerant used in most plants or devices usually have way, way lower boiling points than water at any pressure. Like R-12, an old common refrigerant. At atmospheric pressure, R-12 is boiling at -21F. This shows us that R-12 cannot exist in liquid form at ordinary temperatures unless it is confined in order to maintain a pressure.
If we keep raising the temperature of a liquid to the boiling point at that pressure and continue to apply heat, the liquid will boil and eventually vaporize. The vapor stays at the same temperature as the boiling liquid, as long as the two are in contact. A vapor cannot be superheated while it remains in contact with the liquid from which it was generated.
There are a bunch of pressure=temperature relationships. We are right now concerned with 3 of them and these are again expressed by frickin’ laws:
- Boyle’s Law
- Charles’ Law
- General Gas Equation
states that the volume of any dry gas varies inversely with its absolute pressure, if the temperature remains constant.
- V1P1 = V2P2
states that the volume of a gas is directly proportional to its absolute temperature if pressure remains constant.
- V1T1 = V2T2
General Gas Equation:
combines Boyle’s law and Charles’ law. It connects volume, temperature, and pressure into an equation.
- P1V1/T1 = P2V2/T2
All 3 of these laws/equations for pressure, temperature, and volume are interconnected and show the interrelationships between them.
Important concepts for us to take from these laws:
- When volume is held constant, increasing the temperature of the gas causes a proportional increase in pressure. Decreasing the temperature causes a decrease in pressure (proportional.)
- When pressure is held constant, increasing the temperature of the gas causes a proportional increase in volume. Again – same with a decrease.
- When temperature is held constant, increasing the pressure on a gas causes a proportional DECREASE in volume. A decrease in pressure results in an INCREASE in volume.
Yo – concepts like these apply across the board. Refrigerant like R-12 or R-134A or steam. These principles or laws must be observed by everything! So when we look at those 3 things just above, make sure inside your head you connect that a chiller behaves concept-wise like a boiler. Even though one heats water to the point of boiling and harnessing that steam energy and the other device tries to get water as cold as possible, the concepts are the same.
Heat exchange, pressure changes, adding or removing heat – awesome right?
Wrapping up the basics of refrigeration
We went over this because concepts like this, and laws that certain objects or things have to observe applies very heavily in refrigeration. Laws of pressure, gases, volume and temperature are all key to understanding ideas like how a basic refrigeration cycle works, why some choose certain refrigerants, etc.
The temperature and volume of a gas are different after a change in pressure. A temperature change usually happens in a gas during the pressure change. By compressing a gas – we raise the temp, by allowing the gas to expand, we can drop its temp. Cool right?
Leave a comment, stay tuned, share. To keep going and read about the basic 4 step refrigeration cycle, click here.