Heat Pumps for dummies (beginners guide)

[Daox]

This site has accumulated a pretty huge wealth of heat pump information. However, there are many who are completely baffled by how these magical machines work at over 100% efficiency. Now throw in all the technical terms used to explain the different bits of the system and you can really get confused. This thread will attempt to go over the basics of how a heat pump works, and demystify things for beginners.

Table of Contents
- What is a heat pump?
- How do heat pumps work?

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What is a heat pump?

OK... to start off, a Heat Pump is really a pump. But it pumps heat instead of, for example, water.

Like a water pump, a heat pump moves stuff from one place to another place... only, since it is a heat pump, it moves heat from one place to another place.

A common kind of heat pump, that is used often, gathers heat that is in air, and moves that heat inside a house to keep us warm.

Here is a photo of a very common type of heat pump:

It will have a large fan in the top that will draw a very large amount of air through the unit, and extract heat from the air which will heat a liquid.

It then sends the heated liquid into the house through a small pipe (that is barely visible on the lower left side of the heat pump).


Here's another photo of a heat pump... this one is extracting heat from cold winter air, and sending it into the house to keep everyone inside warm and cozy...

So as you can see, heat pumps can even extract heat form cold air to help us warm our house.

Look at this diagram, it shows how a heat pump can work:

Here is another picture of a house that is heated with a heat pump...

You may be asking yourself, "Where is the heat pump?"

You can't really see the heat pump because it is in the basement, an another important part is under the ground...

Before this house was built, deep trenches were dug and long coils of strong plastic pipe were placed in the trenches and then they were buried. Water is pumped through the many feet of plastic pipe, and this time, the heat pump is able to extract heat from the many, many tons of earth... and then the heat is sent into the house to keep everybody comfortable and happy during the cold winter season. So, the pipes are in the ground, and the heat pump is in the basement, working reliably and quietly doing its job.

If you had X-ray vision, you would be able to see something like this:

The simplest, most common heat pump is a refrigerator or deep freezer. It moves heat from inside the box to outside the box. A thermostat inside the box maintains a set temperature, so when the box gets warm, it starts pumping heat out of the box. When the box is cooled enough, the pumping stops until it warms again.

Many people would think that this is all terribly complex. Why would anyone go to all this bother with compressors, and refrigerants, and loops and other complex things?

In the winter, when we want to stay warm, many people burn natural gas or oil or wood to stay warm, it seems to work OK?

The problem is that all of these fuels are becoming more expensive and harder to obtain just about every year.

* * *

The physicists tell us that even when it is winter, and the cold winds are blowing, and lakes and rivers are covered with thick sheets of ice, and the earth is frozen and the ground become very hard under our feet, that there is still some heat there. It is low 'grade heat' and we can't use it directly to heat our houses. In fact if we tried, our homes would just get colder.

But a wonderful thing about heat pumps and their complex parts, is that they are able to gather up this 'low grade heat' and make it usable for us to heat our homes. The wonderful thing about low grade heat is that there is a huge amount of it, and heat pumps can gather it up for less money than if we were to heat with, for instance, oil.

So the question you may be thinking is, "How can a heat pump gather any heat from air that is colder than ice, or from the ground in winter, or from the water in a frozen lake? I mean, if you fell through the ice and into the cold water, you might die!

So, exactly how can a heat pump gather heat from air, or earth, or water that is so cold?!?!?

Well, you are now going to read just how this is done...

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Things we should cover:
- basics of operation
- diagram of a heat pump and its parts
- different refrigerant types, and their pros & cons
- tools for working on heat pumps
- abbreviations (what the heck is a txv?)
- ???


I personally do not know all that much so I am relying on you guys to compile some good links, images, and descriptions of parts to help out other beginners. As information is compiled I'll keep updating the first post to make a nice beginners guide to heat pumps. I'm going to sticky this so that we can get more people understanding, contributing, and using these wonderful pieces of equipment.

[AC_Hacker]

How do heat pumps work?

To explain how heat pumps work, we need to understand a process that is basic in all of nature, but isn't fully understood by most people. That process is called "Change of State".

Change of state refers to the event of a substance changing from gas to liquid, or from liquid to solid. Water is an example that everyone is familiar with. Water can exist in different 'states' as a vapor, or as a liquid, or as a solid. When water is in each of these states, it is still water, it is still H2O.

When water is in the vapor state, it behaves in every way as a gas would behave. It is highly compressible, it can expand to fill the container it may find itself in. But it is still water, still H2O. "Steam" is another term for water vapor, but the word 'steam' also carries the connotation of heat... which gives us a clue as to what is going on here.

When water is in the liquid state, it behaves in every way as a liquid would behave. It is almost completely incompressible, but it can take the shape of the container it may find itself in. It is still water, still H2O. This is the most familiar form of water we know. When we hear the word "water", we think of liquid water. This is because in the temperature and pressure levels that humans are comfortable in, water is most frequently seen as a liquid.

When water is in the solid state, it behaves in every way as a solid would behave. It is almost completely incompressible, but it will not take the shape of the container it may find itself in. It is still water, still H2O.

What causes the differences between these different states of water? Why is water a vapor in some instances, and a liquid in other instances, and a solid in still other instances?

Our ordinary experience give us a clue that heat has something to do with it. So you could take a pan, put some ice in it, put the pan on a stove and turn on the heat.

After a while, you would see the ice start to melt, turn into liquid water, and then a while after that the heat from a stove would cause the water to boil and then turn to steam, or water vapor, until the pot was dry.

OK, great... Is it reversible?



So, where does the heat come from that is used to heat the house?

Well, if you have ever pumped up a bicycle tyre you will probably have noticed that the pump body gets warm or hot. How come?

When you compress a gas it heats up - pumping the pump with mechanical energy (your arm or leg) compresses the gas to a higher pressure (as the exit from the pump is a small hole so the pressure in the pump body increases) and the gas heats up.

This is what the compressor does - compresses the refrigerant which is in the form of a cold gas with mechanical energy (a motor) and in doing so as the gas compresses and it heats up.

The hot gas is then pumped to the place where the heat is removed (some form of heat exchanger) so you then have a cold liquid. This cold liquid is vaporised into an even colder gas through a metering device. The very cold gas then easily absorbs heat from the atmosphere or ground (in another heat exchanger) to become a cold gas and this then goes back into the compressor to be compressed back into a hot gas.




Most residential heat pump (and air conditioning) systems use what is called a "vapor compression cycle" or "phase change cycle". They use a volatile chemical (like freon, puron, propane, etc) for a heat transfer fluid. The chemical "refrigerant" is in a sealed plumbing loop, and a compressor is used to move the chemical by pressure differential. The process is very stable and very energy efficient.

This process takes advantage of the energies of evaporation and condensation or "boiling and distilling". As with water on the stove, it takes a lot less heat (sensible heat) to bring the fluid to boiling temperature than it does to actually boil all the water(latent heat). The heat pump takes advantage of this large latent heat transfer, illustrated below in the phase changes that occur when water is heated from ice to steam:

As you can see, it takes many more joules of heat energy to melt the ice and boil the water at constant temperatures than it does to raise it from -50 to 0, 0 to 100, and 100 to 150 degrees celsius.

In the sealed refrigerant loop, internal pressures determine the boiling point of the chemical rather than just raw temperature. So by changing the pressure inside the plumbing, the chemical can be forced to change from a liquid to a gas and vice versa at whatever temperatures we want or need them to. When the chemical changes from a liquid to a gas (evaporation), it must absorb heat to do so. When it changes from a gas to a liquid (condensation), it has to release the same amount of heat.

The device that separates the pressures is called a metering device. It is basically a highly engineered blockage in the plumbing. When the compressor runs, it builds up pressure on the discharge side due to this blockage. As the discharge pressure builds up, the chemical gets hot inside and releases heat through the piping wall. This release of heat at high pressure causes the chemical to condense into liquid form. The higher the pressure, the more the liquid builds up in front of the blockage. The longer the wait, the more the liquid is cooled in the plumbing. If the liquid refrigerant is cooled below its boiling temperature, we say it is "subcooled".

At some point, the built up liquid forces its way through the blockage. Making its way through, it encounters a massive pressure drop, which forces the liquid to boil violently and absorb heat in the process. The liquid has to seek a lower temperature in order to absorb heat, since at this lower pressure, the boiling temperature is many degrees lower than it was on the other side of the metering device. The longer the plumbing is between the blockage and the compressor intake, the more heat is absorbed along the way. When (and if) the liquid all boils off and begins to absorb sensible heat from the plumbing, we say the chemical has become "superheated".

In well-designed heat pumps, the plumbing in between the compressor and metering device is made so the heat flows easily as the chemical changes states of matter (phase change).This area of plumbing surface is known as a heat exchanger. Heat exchangers are designed so that the refrigerant can absorb or release all the latent heat it needs to make the change and then some. The heat extracted or released can be upwards of 5 times as much as the energy used by the compressor to actually move the refrigerant.

Below is a basic diagram of an air-to-air heat pumping system:

In this picture, red represents high pressure gas, orange represents high pressure liquid, blue represents low pressure liquid, and green represents low pressure gas.

[jeff5may]

The simplest, most common heat pump is a refrigerator or deep freezer. It moves heat from inside the box to outside the box. A thermostat inside the box maintains a set temperature, so when the box gets warm, it starts pumping heat out of the box. When the box is cooled enough, the pumping stops until it warms again.

Large, industrial heat pump systems use either steam or ammonia water as a heat transfer fluid. These systems are not very energy efficient and fall out of the scope of this thread. On a massive sized installation, they usually capitalize on waste heat generated from another process (power generation, machine cooling, etc.), so the heat reclaimed is a "freebie". They function just as an automobile heater does: they use some of the engine heat that would otherwise be cooled in the radiator to heat the passenger compartment.

Most residential heat pump (and air conditioning) systems use what is called a "vapor compression cycle" or "phase change cycle". They use a volatile chemical (like freon, puron, propane, etc) for a heat transfer fluid. The chemical "refrigerant" is in a sealed plumbing loop, and a compressor is used to move the chemical by pressure differential. The process is very stable and very energy efficient.

This process takes advantage of the energies of evaporation and condensation or "boiling and distilling". As with water on the stove, it takes a lot less heat (sensible heat) to bring the fluid to boiling temperature than it does to actually boil all the water(latent heat). The heat pump takes advantage of this large latent heat transfer, illustrated below in the phase changes that occur when water is heated from ice to steam:
As you can see, it takes many more joules of heat energy to melt the ice and boil the water at constant temperatures than it does to raise it from -50 to 0, 0 to 100, and 100 to 150 degrees celsius.

In the sealed refrigerant loop, internal pressures determine the boiling point of the chemical rather than just raw temperature. So by changing the pressure inside the plumbing, the chemical can be forced to change from a liquid to a gas and vice versa at whatever temperatures we want or need them to. When the chemical changes from a liquid to a gas (evaporation), it must absorb heat to do so. When it changes from a gas to a liquid (condensation), it has to release the same amount of heat.

The device that separates the pressures is called a metering device. It is basically a highly engineered blockage in the plumbing. When the compressor runs, it builds up pressure on the discharge side due to this blockage. As the discharge pressure builds up, the chemical gets hot inside and releases heat through the piping wall. This release of heat at high pressure causes the chemical to condense into liquid form. The higher the pressure, the more the liquid builds up in front of the blockage. The longer the wait, the more the liquid is cooled in the plumbing. If the liquid refrigerant is cooled below its boiling temperature, we say it is "subcooled".

At some point, the built up liquid forces its way through the blockage. Making its way through, it encounters a massive pressure drop, which forces the liquid to boil violently and absorb heat in the process. The liquid has to seek a lower temperature in order to absorb heat, since at this lower pressure, the boiling temperature is many degrees lower than it was on the other side of the metering device. The longer the plumbing is between the blockage and the compressor intake, the more heat is absorbed along the way. When (and if) the liquid all boils off and begins to absorb sensible heat from the plumbing, we say the chemical has become "superheated".

In well-designed heat pumps, the plumbing in between the compressor and metering device is made so the heat flows easily as the chemical changes states of matter (phase change).This area of plumbing surface is known as a heat exchanger. Heat exchangers are designed so that the refrigerant can absorb or release all the latent heat it needs to make the change and then some. The heat extracted or released can be upwards of 5 times as much as the energy used by the compressor to actually move the refrigerant.

Below is a basic diagram of an air-to-air heat pumping system:




And a reversible heating / cooling system diagram:

[Acuario]

Good explanations so far but where does the heat come from that is used to heat the house?

Well, if you have ever pumped up a bicycle tyre you will probably have noticed that the pump body gets warm or hot. How come?

When you compress a gas it heats up - pumping the pump with mechanical energy (your arm or leg) compresses the gas to a higher pressure (as the exit from the pump is a small hole so the pressure in the pump body increases) and the gas heats up.

This is what the compressor does - compresses the refrigerant which is in the form of a cold gas with mechanical energy (a motor) into a liquid and in doing so as the gas compresses and as it turns to liquid it heats up. The liquid refrigerant then boils into a gas absorbing even more energy and becoming even hotter (super heat).

The super heated gas is then pumped to the place where the heat is removed (some form of heat exchanger), the gas cools and condenses into a liquid releasing its heat so you then have a cold liquid. This cold liquid is vaporised into an even colder gas through a metering device. The very cold gas then easily absorbs heat from the atmosphere or ground (in another heat exchanger) to become a cold gas and this then goes back into the compressor to be compressed back into a hot liquid then super heated gas.

[AC_Hacker]

Many people would think that this is all terribly complex. Why would anyone go to all this bother with compressors, and refrigerants, and loops and other complex things?

In the winter, when we want to stay warm, many people burn natural gas or oil or wood to stay warm, it seems to work OK?

The problem is that all of these fuels are becoming more expensive and harder to obtain just about every year.

* * *

The physicists tell us that even when it is winter, and the cold winds are blowing, and lakes and rivers are covered with thick sheets of ice, and the earth is frozen and the ground become very hard under our feet, that there is still some heat there. It is low 'grade heat' and we can't use it directly to heat our houses. In fact if we tried, our homes would just get colder.

But a wonderful thing about heat pumps and their complex parts, is that they are able to gather up this 'low grade heat' and make it usable for us to heat our homes. The wonderful thing about low grade heat is that there is a huge amount of it, and heat pumps can gather it up for less money than if we were to heat with, for instance, oil.

So the question you may be thinking is, "How can a heat pump gather any heat from air that is colder than ice, or from the ground in winter, or from the water in a frozen lake? I mean, if you fell through the ice and into the cold water, you might die!

So, exactly how can a heat pump gather heat from air, or earth, or water that is so cold?!?!?

Well, you are now going to read just how this is done...

-AC

[AC_Hacker]

To explain how heat pumps work, we need to understand a process that is basic in all of nature, but isn't fully understood by most people. That process is called "Change of State".

Change of state refers to the event of a substance changing from gas to liquid, or from liquid to solid. Water is an example that everyone is familiar with. Water can exist in different 'states' as a vapor, or as a liquid, or as a solid. When water is in each of these states, it is still water, it is still H2O.

When water is in the vapor state, it behaves in every way as a gas would behave. It is highly compressible, it can expand to fill the container it may find itself in. But it is still water, still H2O. "Steam" is another term for water vapor, but the word 'steam' also carries the connotation of heat... which gives us a clue as to what is going on here.

When water is in the liquid state, it behaves in every way as a liquid would behave. It is almost completely incompressible, but it can take the shape of the container it may find itself in. It is still water, still H2O. This is the most familiar form of water we know. When we hear the word "water", we think of liquid water. This is because in the temperature and pressure levels that humans are comfortable in, water is most frequently seen as a liquid.

When water is in the solid state, it behaves in every way as a solid would behave. It is almost completely incompressible, but it will not take the shape of the container it may find itself in. It is still water, still H2O.

What causes the differences between these different states of water? Why is water a vapor in some instances, and a liquid in other instances, and a solid in still other instances?

Our ordinary experience give us a clue that heat has something to do with it. So you could take a pan, put some ice in it, put the pan on a stove and turn on the heat.

After a while, you would see the ice start to melt, turn into liquid water, and then a while after that the heat from a stove would cause the water to boil and then turn to steam, or water vapor, until the pot was dry.

OK, great... Is it reversible?

(* To Be Continued... *)

-AC

[Daox]

Thanks guys, its starting to come together!

Sorry, but I am going to have to commandeer some of you guys posts near the beginning. We quickly hit the max character length for a post, so I am going to edit your posts to keep everything together.

I'm also trying to knit all of your descriptions together, so lets try to play off of others work. If I have anything wrong, or something doesn't make good sense or you have any suggestions, just let me know.

I really like the use of images, I think that helps people out (like me) vs reading a ton of text.

[AC_Hacker]

Daox,

You have your work cut out for you, slicing and dicing this information into a coherent narrative. If you don't understand how vapor compression works, hopefully you will by the time you get through.

-AC

[Daox]

Haha, I also did think of that. This is a great way to learn for me!

[mejunkhound]

Let's see. Hmm, try to explain this really simple so even a democrat (or republican, take your pick) can understand.

It takes heat to boil water right? OK.
Freon, propane, etc. boil at much lower temperatures, the engineers (pick a fluid that boils below zero, of which there are many, like freon, propane, CO2, even air itself.

So, we send liquid freon out into the cold air outside, even cold it boils the fluid as the cold air is hotter than the boiling point of the freon, so now it is a gas. In a heat pump, that happens in the coil outside.

Remember how the lid on a pot that has boiling water in it gets water drops on the inside of the lid? Similarily, we send the gas that we boiled from the liquid in the outside coil (called the evaporator since it evaporates the liquid) to the inside coil by way of compressor.

Recall that when you pump a bicycle tire the bottom of the pump gets warm -that is what the compressor does, it compresses the cold gas, which then gets hot.
Now we get to the part like the lid of the pot, which is like the coil inside the house. We blow the room air in the house thru that coil (which gets hot inside due to the hot gas) and the room air gets warmer and we blow that out the ducts to heat the house.

Again, just like the water drops inside the lid of the pot with boiling water, the hot gas condenses to a cooler liquid. We then send that back to the evaporator and it all starts over again.

Simplistically, we boil a fluid outside, compress that cold gas to make it hot, cool it down using air from the house thereby heating the house. Since it got cool it liquifies, we send that liquid back outside to boil and start the process all over again.

Process called the vapor refrigeration cycle.
What it does is 'pump' heat from outside to the inside by cooling outside air and heating inside air. It takes less energy to compress the gas than the energy the process takes from the outside air to boil the freon and then gives up to the inside air, hence we get more heat than the energy put into the compressor.