# Heat pumps: Questions about compressor pump refrigerant gas and evaporator

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• GreenWombat
GreenWombat
TL;DR Summary
A heat pump is a sealed unit that contains the refrigerant within a constant volume. How does the compressor increase the pressure within the high-pressure compressor and condensation spaces? Does it do this by pumping refrigerant gas from the evaporator and into the high-pressure space? Is this part of why the refrigerant in the evaporator gets cold?
A heat pump is a sealed unit that contains the refrigerant within a constant volume. How does the compressor increase the pressure within the high-pressure compressor and condensation spaces? Does it do this by pumping refrigerant gas from the evaporator and into the high-pressure space? Is this part of why the refrigerant in the evaporator gets cold?

I am retired from work and exploring hot-water heat pumps using propane as a refrigerant. I have many questions. Is it best to pose these questions individually in this thermodynamics forum?

GreenWombat said:
I am retired from work and exploring hot-water heat pumps using propane as a refrigerant. I have many questions. Is it best to pose these questions individually in this thermodynamics forum?
It's probably best to start with this question and see how the thread goes. These questions can go either here in the Thermo forum or in the Mechanical Engineering forum. PM a Mentor if you want a thread moved.

It would also help if you could post more about where you are in understanding heat pumps, and give links to the reading you've been doing about them. Thanks.

I completed 4 years of science, including a thermodynamics unit, but that was way back. I am refamiliarising myself with things like latent heat and specific heat capacity.

I’ve found Wikipedia helpful and have questions about the Wikipedia pressure-volume diagram. The temperature-entropy diagram is beyond me just now.
https://en.wikipedia.org/wiki/Vapor-compression_refrigeration

berkeman
A compressor compresses the gaseous refrigerant, causing it to heat up. Then it goes to a condenser where it is cooled and condenses at constant pressure. Then it goes through the throttling valve where the pressure drops and it cools. The diagram on that wiki page should make it clear what the four devices are that are associated with the processes in the p-v diagram:

Lnewqban
Thanks for including those diagrams.

I get the four stages of heat pump operation. I've been wrestling with the concept of how a heat pump compressor can compress a gas that it holds in a fixed volume.

At first, I thought of the compression stage as like a bicycle pump, heating the air in the tyre by pumping air from the exterior into the tyre. Then I saw that heat pumps are sealed units containing a fixed mass of refrigerant within a fixed volume, so there is no external propane to pump into the compression space.

I now assume that the compression does not occur due to the system pumping extra gas from the evaporator into the compressor.

I may have worked it out overnight.

Does a heat pump with a piston compressor:
. close the valve between the compressor and the evaporator trapping refrigerant gas in the compression space,
. then draw, say, 10% of the gas in the compression space into a piston chamber,
. then close the valve between the compression space and the piston chamber,
. then use electric energy to push the piston into the chamber, compressing the gas and heating it,
. then open the valve to the piston chamber,
. then expel the compressed gas back into the heat pump compression space with the rest of the gas,
. then the heated gas from the chamber expands and cools and warms the rest of the gas in the heat pump compressor area,
. then, after repeated movements of the piston, the work done by the electric motor driving the piston heats and pressurises all the gas in the heat pump compression space.

Am I on the right track now?

GreenWombat said:
I get the four stages of heat pump operation. I've been wrestling with the concept of how a heat pump compressor can compress a gas that it holds in a fixed volume.

At first, I thought of the compression stage as like a bicycle pump, heating the air in the tyre by pumping air from the exterior into the tyre. Then I saw that heat pumps are sealed units containing a fixed mass of refrigerant within a fixed volume, so there is no external propane to pump into the compression space.
They are indeed usually reciprocating compressors, like a bicycle pump. The volume of the compressor cylinder is obviously not fixed. The fact that the rest of the system has a fixed volume doesn't have any impact. Any closed system that moves a working fluid will have different pressures in different parts of the system. It should be intuitively obvious.
Does a heat pump with a piston compressor:
. close the valve between the compressor and the evaporator trapping refrigerant gas in the compression space,
. then draw, say, 10% of the gas in the compression space into a piston chamber,
. then close the valve between the compression space and the piston chamber,
. then use electric energy to push the piston into the chamber, compressing the gas and heating it,
. then open the valve to the piston chamber,
. then expel the compressed gas back into the heat pump compression space with the rest of the gas,
. then the heated gas from the chamber expands and cools and warms the rest of the gas in the heat pump compressor area,
. then, after repeated movements of the piston, the work done by the electric motor driving the piston heats and pressurises all the gas in the heat pump compression space.

Am I on the right track now?
It works just like a bicycle pump; a piston with two check valves. But I don't know what you mean by "compression space". The gas moves into the cylinder from the evaporator through one check valve, then gets compressed and pushed out toward the condenser through the other:

https://en.m.wikipedia.org/wiki/Reciprocating_compressor

Lnewqban
Thanks for confirming that. I’ve focused on heat pumps for about a month now and going nutty. I’ve asked three clever and experienced science/engineering friends and was surprised when they said they couldn’t tackle my questions.

*** How can heat pump evaporation occur at constant pressure?

The pressure-volume diagram (see above) shows evaporation happening at constant pressure while the volume increases.

I do not understand this because:
. The manufacturer fills the heat pump with refrigerant and seals it, leaving a fixed volume for the refrigerant.
. During evaporation, the refrigerant gas can expand into every space of the heat pump, and the volume of this space is constant.
. as the air heats the liquid refrigerant, more gas will boil off, and the pressure will build.
. It’s like cooking pasta with the lid on; the pressure builds until the lid rattles and steam escapes.

Perhaps this is how it works: You boil pasta for a long time, giving ample time for the pressure to build. Perhaps the vaporisation period for each heat pump cycle is brief, say five seconds, and over that short period, the pressure doesn’t build, so you can think of it as expansion at constant pressure.

** Can we assume that the expansion occurs at constant pressure because the heat pump expansion stage is always brief?

** What is a typical time period for one complete evaporation, compression, condensation and expansion cycle?

** Condensation at constant pressure

The diagram also shows the condensation occurring at constant pressure. Is this also because the condensation stage occurs over a brief time? Otherwise, I would expect pressure to fall with condensation in a fixed volume. Again, it could be like pasta; when you turn the heating off and leave the lid on the uneaten pasta, you must pull hard to oppose the vacuum and get the lid off.

** Example temperatures and pressures for a propane heat pump

I’m trying to understand the operation of a hot water heat pump using propane as the refrigerant, and I'm trying to imagine what the whole system could look like over one short period.

Again, looking at the above pressure-volume diagram, the diagram shows:
. a blue, cold temperature (Tc)
. a red, hot temperature (Th)
. a pressure of condensation (Pc), and
. A pressure of evaporation. (Pe)

These variables will change during the propane heat pump operation, but are the following values plausible for an instant in time:

. Pressure of evaporation (Pe) = 2 atmospheres (atm)
. Blue, cold temperature (Tc) = at or below minus 25.6°C, say minus 27°C.
(The boiling point of propane at 2 atm = minus 25.6°C)

. Pressure of condensation (Pc) = 30 atm
. Red, hot temperature (Th) = 90°C

The boiling point of propane at 30 tm = 78.7°C

This would have the throttling /expansion stage:
. starting with a liquid at 90°C and pressure 30 atm, and
. ending with a liquid/gas at minus 27°C and 2 atm.
This seems wild, an enormous temperature jump!!

All these values are below the
. Critical Temperature 96°C, and
. Critical Pressure = 618 psi = 43 atmospheres

Other key values:
. the target heat for the hot water = 60°C
. the water temperature in the hot water tank = 50°C
. the air temperature = 15°C.

There is a lot here. Thanks for reading this far.

russ_watters said:
The gas moves into the cylinder from the evaporator through one check valve, then gets compressed and pushed out toward the condenser through the other:

Hello Russ,

After re-reading what you wrote, I am back to my original uncertainty. Apologies; sometimes, it's complicated to sort out my misunderstandings.

******* Possibility A:

You wrote, “The gas moves into the cylinder [piston chamber] from the evaporator through one check valve, then gets compressed and pushed out toward the condenser through the other.”

Your writing seems to support this possibility A, which means that the answer to my very first question (Monday at 11:12 PM) was YES. The compressor DOES pump refrigerant gas from the evaporator and into the high-pressure space.

In possibility A:
. As in your diagram, there is (1) an evaporator, (2) NO Valve, (3) the compressor, (4) the condenser, & (5) an expansion valve.
. Initially, the inlet valve opens, allowing gas to move between the evaporator and the piston chamber.
. Also, the outlet valve closes, preventing gas from moving from the piston chamber into the condenser.
. During the piston’s intake stroke, the piston moves away from the piston chamber, increasing the volume of the chamber and sucking cold gas from the evaporator into the piston chamber via the intake valve.
. The intake valve then closes, and the outlet valve opens.
. The piston moves towards the compression chamber, decreasing the volume of the chamber, compressing the gas, heating it, and forcing it out of the piston chamber via the outlet valve and into the high-pressure space.
.
. The compressor space is only the piston chamber, and the high-pressure area is the condenser space.
. The piston can only work on each gas molecule once per cycle.
.
. ** As the piston sucks gas from the evaporator, does it reduces the gas pressure over the pool of refrigerant and so cool the refrigerant in the evaporator?

***** Alternative possibility B.

If possibility B is correct, then the answer to my original question was NO. The compressor DOES NOT pump refrigerant gas from the evaporator and into the high-pressure space.

In possibility B:
. there is (1) an evaporator, (2) A VALVE, (3) the compressor, (4) the condenser, & (5) an expansion valve
. There are two spaces:
. (1) The low-pressure evaporator space and
. (2) The high-pressure condenser space, which entirely contains the compressor.
. This compressor takes in gas from the high-pressure space, compresses it and then releases it back into the same high-pressure space.
.
. In the evaporator, there is a pool of liquid and gas above the liquid.
. A valve closes, separating (1) the pool of liquid and a little of the gas in the evaporator space from (2) the upper gas in the evaporator, which, after the valve closes, becomes gas in the soon-to-be high-pressure space.
. This compressor takes in gas from the high-pressure space, then the piston compresses it and pushes it back into the same high-pressure space.
. The expelled gas is hot at high pressure and heats the other gas in the high-pressure space.
. After repeated movements of the piston, the work done by the electric motor that drives the piston heats and pressurises all the gas in the high-pressure space.
. The compressor can work on the same molecule of refrigerant several times in one cycle to get the pressure up to a desired level before the compressor stops and allows condensation to start.

*** Is possibility A correct, and possibility B a misunderstanding?

Is there any website on this that you would recommend?

Thanks Russ.

GreenWombat said:
No, you're fine.
*** How can heat pump evaporation occur at constant pressure?

The pressure-volume diagram (see above) shows evaporation happening at constant pressure while the volume increases.

I do not understand this because:
. The manufacturer fills the heat pump with refrigerant and seals it, leaving a fixed volume for the refrigerant.
. During evaporation, the refrigerant gas can expand into every space of the heat pump, and the volume of this space is constant.
. as the air heats the liquid refrigerant, more gas will boil off, and the pressure will build.
No, the refrigerant gas does not expand into every space, it only expands into the half of the cycle from the expansion valve to the pump. Since gas is simultaneously being expanded into the low pressure side by the throttling valve and pulled out by the compressor on the other side, the mass of refrigerant in that part of the cycle is fixed. The states at every location in the cycle are constant and the mass flow rates are constant (except inside the compressor cylinder). The states change only as the refrigerant moves from one location to another (such as through the evaporator). This is called a Steady State, continuous process(cycle).
. It’s like cooking pasta with the lid on; the pressure builds until the lid rattles and steam escapes.

Perhaps this is how it works: You boil pasta for a long time, giving ample time for the pressure to build.
No, that's not how cooking pasta works. The pressure in a boiling pot is constant, even with the lid on (you would not use a sealed pressure cooker to cook pasta).
Perhaps the vaporisation period for each heat pump cycle is brief, say five seconds...

** Can we assume that the expansion occurs at constant pressure because the heat pump expansion stage is always brief?
No, there is no "vaporization period". Vaporization is always happening. Again: steady state, continuous process.

GreenWombat said:
Your writing seems to support this possibility A, which means that the answer to my very first question (Monday at 11:12 PM) was YES. The compressor DOES pump refrigerant gas from the evaporator and into the high-pressure space.
I might call that the high pressure side of the cycle. But yes, that sounds correct.
** As the piston sucks gas from the evaporator, does it reduces the gas pressure over the pool of refrigerant and so cool the refrigerant in the evaporator?
No, the pressure is constant on the low pressure side of the cycle, even when drawing the refrigerant in. While the compressor is doing that more refrigerant is flowing into the high pressure side of the cycle through the throttling valve, so the amount of refrigerant in the low pressure side is constant (or nearly so, as the volume of the compressor cylinder is much smaller than the volume of the rest of the system).
***** Alternative possibility B.
...
. This compressor takes in gas from the high-pressure space, then the piston compresses it and pushes it back into the same high-pressure space.
*** Is possibility A correct, and possibility B a misunderstanding?

Is there any website on this that you would recommend?
Possibility B is wrong. There's nothing hidden here: it's not in the picture I posted or the P-V diagram (refrigerant flowing backwards), so it's not part of the cycle. The wikipedia article explains this fully, and there's no need to go anywhere else.

I think you just need to get your arms around the idea of a continuous, steady state process and understand that we're not dealing with an intermittent batch process.

Lnewqban
@GreenWombat you are getting hung up on words. You are 'overthinking it'. Don't worry about how the compressor works until you've wrapped your head around the refrigeration cycle.
-
Closed system, open system, it makes no difference. We compress a gas and it heats up. We then cool that gas by blowing ambient air over the container the gas is in. The compressed gas is then ideally at ambient temperature. Then we reduce the pressure of the gas by allowing it to expand into a larger chamber. The gas then cools below ambient absorbing heat from the environment. The gas is then drawn into the compressor to repeat the cycle. We could do this with air on an open system but the properties of using air as a refrigerant are not well suited. There are much better choices for refrigerants and in order to utilize them we need a closed system. In those cases there are parts of the cycle where the refrigerant is a liquid and other parts a gas.
-
I don't understand why you are hung up on how a compressor works. Are you having trouble with the claim of a fixed volume system and yet a piston type compressor changes the volume within the compressor?
-
Edit: What @russ_watters says is true. All these things are happening at once within the system. The refrigerant itself 'sees' these different parts of the cycle as it travels the loop.

Lnewqban, berkeman and russ_watters
GreenWombat said:
Is there any website on this that you would recommend?
https://www.torr-engineering.com/the-refrigeration-cycle/

https://okmarts.com/news/how-does-a-refrigeration-compressor-work-with-animation-for-each-type.html

Thermal energy naturally flows from a hot/warm source to a cold/cool one (a sustained difference of temperatures induces that flow).

If external energy is not added, that flow eventually stops after the cold/cool source is sufficiently heated/warmed at the expense of the thermal energy in the hot/warm source (natural equalization of temperatures).

Likewise, in a closed loop of tubes, all pressures and associate temperatures naturally tend to a unique value, if left alone.

If we need to keep heat unnaturally moving from a cold source to a hot one (think refrigerator, air conditioning, freezer), we need to constantly introduce external energy into that dynamic movement (think electricity, motor, gas compressor).

That constantly introduced external energy induces a point in the loop at which there is a huge sharp difference of pressures between upstream and downstream that point (think of a roller coaster section that increases the height of a car, from ground level to its highest point).

The molecules of the gas contained within that closed loop will then naturally flow form the points of high pressure (downstream or discharge of the compressor) to the points of not-so-high pressure (upstream or suction of the compressor).

In a rapid cyclic way, the compressor steals gas molecules form the low-pressure side and transfer those into the high-pressure side, in order to keep the desired flow of refrigerant inside the closed loop.

How it does the magic?
It takes advantage of the equation Pv=nRT=constant value.

After stealing a fixed number of gas molecules from the low-pressure side, in each stroke (if reciprocating type compressor) it reduces the volume of that portion of gas that it has previously stolen, forcing it into the high-pressure side.

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russ_watters
Wow. You fellows are fantastic. I'm experiencing a little revolution as I digest the concept of a " Steady state continuous process cycle." I'll check my conclusions with you in a bit. I need to do some shopping now. Thanks.

russ_watters and Lnewqban
Thanks for your help. After a radical re-think, this is how I see it. Does it make sense now?

The heat pump process is a steady-state, continuous cycle. It is not a sequence of stop-and-start processes. The compressor does not stop and start, and the expansion valve does not alternate between fully open and closed. These are continuous operations.

The refrigerant is in a different steady state after each process of the heat pump cycle:
. after the evaporator,
. after the compressor,
. after the condenser, and
. after the expansion valve.

After the heat pump starts up and comes into a steady state, at each point in the cycle, we see constant:
. pressure
. temperature, and
. mass flow rates.

The constant pressure and temperature at each point depends on:
. the external air temperature
. the goal hot-water temperature,
. the current hot water temperature, and
, the refrigerant used.

The mass of refrigerant in the high-pressure space of the heat pump stays close to constant, as does the mass of refrigerant in the low-pressure space. This is because the system equalises the rates of flow through the:
. compressor, which pushes refrigerant into the high-pressure space, and
. expansion valve, which passes refrigerant out of the high-pressure space.

(The compressor piston has an intake and expulsion stroke, so the compression is a pulsing operation. However, the volume of the compressor cylinder is much smaller than the volume of the entire system, so the refrigerant movement out of the evaporator and into the condenser is a rapid, small pulse. It’s close to a continuous flow.)

Thanks to your help, I now have an answer to my question.

I thought the heated refrigerant evaporating into the fixed volume between the expansion valve and the compressor would increase pressure. However, there is no pressure increase because the compressor keeps moving this gas into the high-pressure condenser. Also, the system monitors the temperature and pressure of the refrigerant before the compressor and adjusts the expansion valve to maintain a constant state.

The heat pump process is a steady-state, continuous cycle. It is not a sequence of stop-and-start processes.

I hope I’ve got this right now. I’m working on some further questions about the temperatures and pressures in a propane heat pump. I can now significantly improve the figures I provided earlier.

Thanks

Lnewqban and russ_watters
Awesome, you have pretty much everything right! Two minor things though:
GreenWombat said:
. the current hot water temperature
Your email address says you are in Australia. In the US almost all heat pumps are air to air or water to air...though air to water and water to water are starting to become more common. This doesn't make a big difference in the cycle but had some implications for the control:
[/quote]
Also, the system monitors the temperature and pressure of the refrigerant before the compressor and adjusts the expansion valve to maintain a constant state.
[/QUOTE]
Typically the expansion valve is just a fixed orifice with no control. Evaporator side pressure is regulated by regulating the evaporator fan speed for an air source heat pump, but small system heat pumps may have little or no active control of the cycle. If it's set up correctly and operates in a known range of temperatures it doesn't need any.

GreenWombat said:
... and the expansion valve does not alternate between fully open and closed.
As @russ_watters has pointed above, the device in charge of reducing the pressure is not always a valve.
It can be a calibrated orifice or a length of capillary tube, in systems not facing big variations of thermal loads (also reason for which the compressor can be of fixed displacement and rpm's).

Big commercial and industrial cooling systems require greater range of adaptability or flexibility.

https://en.wikipedia.org/wiki/Thermal_expansion_valve

Additional information regarding the continues cycle we have been discussing:
* There are compressors that are not reciprocating, supplying a more continuous flow of refrigerant.

* All compressors get damaged if refrigerant in liquid form reaches its suction port. Overheating the suction gas is a necessary evil to protect the machine.

* There is also undesired heat loss or gain in air handlers and condenser units, and in the tubes connecting both, both of which affect our ideal cycle. Insulation is the normal way to reduce those deviations.

* All works as it should as long as the working fluid can freely change phases (hot gas-hot liquid-cool gas-cool liquid).
In cases when the system is accidentally contaminated with water or air, problems related to internal ice formation (in refrigeration systems) and impaired condensation (air remains in gaseous form through the condenser, stealing surface of heat exchange).

I am trying to show how a heat pump works by roughly identifying one set of temperatures and pressures for each stage of the heat pump cycle. I want to understand how propane can be useful as a refrigerant when it boils at minus 43°C at one-atmosphere pressure.

I am considering:
. an air-source, hot-water, heat pump,
. with a propane refrigerant,
. the heat source is the air outside the house,
. the hot water tank is also outside the house,
. the target hot water tank temperature = 60°C
. the current hot water tank temperature = 50°C, and
. the air temperature outside the house used as the heat source = 17°C.

The pressure-volume diagram suggests there are only a few temperatures and pressures to work out. In the cold, low-pressure space of the heat pump, there is:
. one low pressure (Pc), and
. one low temperature (Tc), which is the boiling point of propane at this pressure.

In the hot, high-pressure space, there is:
. one high-pressure (Ph)
. one high-temperature (Th), which is the boiling point of propane at this pressure, and
. a second superheated temperature (Ts) which is higher than Th.

****** Propane Boiling points

Here are propane boiling points from the internet calculator. Below I use the rounded values in brackets to keep it simpler.

Pressure Propane boiling point
1 Atm Minus 43°C
5.2 Atm (say 5 Atm) 14.9°C, (say 15°C)
16 Atm 74.47°C (say 74°C}

https://www.omnicalculator.com/chemistry/boiling-point#boiling-point-definition

***** The low-pressure space

To estimate the temperature in the heat pump's cold, low-pressure space, consider the temperature after evaporation and before compression. The pressure-volume diagram shows that the evaporated propane is a saturated gas. Since the gas is at its boiling point, the pressure will be the vapour pressure of the propane at this temperature.

This refrigerant has passed through a heat exchanger and is warmed by air at 17°C. I guess that the refrigerant will be cooler than the air, So I assume that Tc = 15°C.
** Is this reasonable, or should it be 17°C?

From my propane boiling point table, the propane vapour pressure at 15°C is 5 atm, meaning that Pc = 5 atm.
** Is this reasonable?

A controller keeps this refrigerant slightly superheated to prevent liquid from entering the compressor and damaging the heat pump. I will ignore this.
** Would it be easy to include this in my figures?

The propane after the expansion valve and before the evaporator will be at the same Tc = 15°C and Pc = 5 atm, still at boiling point but with a mixture of propane liquid and vapour.

****** The high-pressure space

To estimate the temperature in the heat pump's hot, high-pressure space, I consider the temperature after condensation and before expansion. Here, the pressure-volume diagram shows that the condensed propane is a saturated liquid. This liquid is at its boiling point, so the pressure will be the vapour pressure of the propane at this temperature.

This refrigerant needs to heat the hot water to 60°C, so I have assumed that the propane needs to get a good bit hotter than that, so Th = 74°C.
** Is this reasonable?

From my propane boiling point table, the propane vapour pressure at 74°C is 16 atm, meaning that Pc = 16 atm.
** Is this reasonable?

The pressure-volume diagram shows that, after compression, the propane is at the same pressure as after condensation, but it is now a superheated vapour. Its temperature will be higher than that of the saturated liquid after compression, Th = 74°C. Assume Ts = 80°C.
** Is this reasonable?

*******Is this real or an ideal

** Is this pressure-volume representation of a heat pump an ideal?
** Does a real propane heat pump come close to this?

Now I wonder how a real propane heat pump could behave like this ideal. (1) In vaporisation, the heat pump transfers exactly the latent heat of vaporisation into the propane, and (2) the compression does superheat the propane, so condensation transfers to the hot water its superheat and then exactly the latent heat of vaporisation.
** Any references to help here?

That's pretty much all fine. Only a couple of minor comments:

-Your approach temperatures(minimum difference between the working fluid and the source/sink) are a little close - but fine for a sample problem.
https://enggcyclopedia.com/2019/05/heat-exchanger-approach-temperature/

-There's less control on small systems than you seem to think. For most small systems there is zero active control other than start/stop. There might be fan on/off but even that is more of a safety. The hot side and cold side conditions will vary somewhat to achieve an equilibrium, as the source and sink temperatures change. To avoid liquid in the compressor there's usually some superheat. The cycle isn't and doesn't need to be precise otherwise.

Lnewqban
Once again, you've got me thinking. I'm working out my next post.

Many hot water heat pumps have (1) a fan and evaporation at the top and (2) a condenser/heat exchanger embracing the water tank nearly to the base. I wondered how the refrigerant gets from the bottom of the condenser near the bottom of the water tank to the evaporator and fan at the top. Here is how I imagine this happens.

In the condenser, the high-pressure vapour presses down on a pool of liquid refrigerant at the bottom of the condenser.

The expansion valve is at the bottom of the pool of refrigerant in the condenser. The high pressure forces the liquid downwards through the expansion valve and into the low-pressure evaporator.

The evaporator: The refrigerant liquid sprays out into the evaporator with some vaporising, taking in the latent heat and cooling the vapour and liquid mixture. The liquid refrigerant pools at the bottom of the evaporator while the vapour moves to the top. The vapour is then ready for compression.

** There is a problem here! My liquid refrigerant remains at the bottom of the evaporator, near the bottom of the water tank and far from the fan and the warming air. How does it warm and vaporise?

*** Does this make sense, even if some details are fanciful?

** Does an individual heat pump have a quasi-steady state with the steady state slowly changing as (1) the hot water temperature approaches the target temperature and (2) the weather changes the air temperature?

** Do these quasi-steady states vary over a wide range?

** Alternatively, does the heat pump have one steady state regardless of the external factors?

GreenWombat said:
In the condenser, the high-pressure vapour presses down on a pool of liquid refrigerant at the bottom of the condenser.

The expansion valve is at the bottom of the pool of refrigerant in the condenser. The high pressure forces the liquid downwards through the expansion valve and into the low-pressure evaporator.

The evaporator: The refrigerant liquid sprays out into the evaporator with some vaporising, taking in the latent heat and cooling the vapour and liquid mixture. The liquid refrigerant pools at the bottom of the evaporator while the vapour moves to the top. The vapour is then ready for compression.

** There is a problem here! My liquid refrigerant remains at the bottom of the evaporator, near the bottom of the water tank and far from the fan and the warming air. How does it warm and vaporise?
There are no pools or spray nozzles, only tubes. The evaporator and condenser are nothing but coils of tubing (with fins) the refrigerant is forced to flow through by the pump.
** Does an individual heat pump have a quasi-steady state with the steady state slowly changing as (1) the hot water temperature approaches the target temperature and (2) the weather changes the air temperature?
Yes.

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Thanks again Russ,

You say, “There are no pools, only tubes. … the refrigerant is forced to flow through by the pump.”

No pools, I’m not understanding something here.

In my propane heat pump scenario, the propane is at 16 atm and between 74C and superheated 78C. After it has passed through the condenser, to the bottom of the condenser, before the expansion valve, I think it will be liquid propane at 74C and under pressure.
Where does it gather before moving through the expansion valve?
It sounds like it does not pool in the tubes before the expansion valve.
What is the input to the expansion valve?
Is it 100% condensed saturated liquid or a mixture of vapour and liquid droplets?
Does propane vapour pass through the expansion valve too?

I asked how the liquid propane gets from the bottom of the evaporator to the top where the fan is. You say the pump forces it. For the flow through the evaporator heat exchanger pipes, there must be a pressure drop and this will push the flow. So, there is a pressure drop across the evaporator! Now it seems to conflict with the pressure-volume diagram. I thought that the diagram showed the experience of one gram of propane as it moved around the heat pump, with the propane’s trip through the evaporator occurring at a constant pressure.

The same difficulty will arise with a pressure drop over the condenser too.

I hope all these questions are not too demanding. I am afraid that I have still more.

GreenWombat said:
Where does it gather before moving through the expansion valve?
It sounds like it does not pool in the tubes before the expansion valve.
"Gather" and "pool" are not words I would use to describe liquid in a tube. It's just liquid in the tube. That's it, there's nothing complicated about that.
What is the input to the expansion valve?
Is it 100% condensed saturated liquid or a mixture of vapour and liquid droplets?
Does propane vapour pass through the expansion valve too?
What does the P-V diagram say?
I asked how the liquid propane gets from the bottom of the evaporator to the top where the fan is. You say the pump forces it. For the flow through the evaporator heat exchanger pipes, there must be a pressure drop and this will push the flow. So, there is a pressure drop across the evaporator! Now it seems to conflict with the pressure-volume diagram. I thought that the diagram showed the experience of one gram of propane as it moved around the heat pump, with the propane’s trip through the evaporator occurring at a constant pressure.
It's too small to be visible on the P-V diagram and can be ignored.
I hope all these questions are not too demanding. I am afraid that I have still more.
It's fine, but you should spend more time looking at the provided sources and less speculating on your own. Or at least after you guess, check the sources before asking us. Most of these questions are answered in what has already been provided.

berkeman
GreenWombat said:
(The compressor piston has an intake and expulsion stroke, so the compression is a pulsing operation. However, the volume of the compressor cylinder is much smaller than the volume of the entire system, so the refrigerant movement out of the evaporator and into the condenser is a rapid, small pulse. It’s close to a continuous flow.)
At least in a home refrigerator, the compressor is usually a Scroll Compressor. It's one of those magic designs that does not use a piston with its pulsing output. It does continuous compression while running.

The Low pressure inlet (gas) is at the outer ends of the spirals.
The High pressure outlet is at the center, into the page.

https://enggcyclopedia.com/2012/03/scroll-compressors/

GreenWombat said:
No pools, I’m not understanding something here.

russ_watters said:
"Gather" and "pool" are not words I would use to describe liquid in a tube. It's just liquid in the tube. That's it, there's nothing complicated about that.
Think of a garden hose. You wouldn't normally consider the water in a garden hose to be a "pool" when watering the lawn.

Cheers,
Tom

Nik_2213 and russ_watters
Your 'constant' situation changes as soon as the compressor starts up.
The dynamics are very different...

Another factor is what will your worst-case source / sink temperatures be ? IIRC, some heat-pumps struggle in the cold --per 'Kitchen' vs 'Garage' freezers-- and others struggle to dump heat during 'exceptional' heat waves.

Seems silly, but a friend's big freezer's content suffered when a cold-snap took region to a dozen degrees of frost. Though set in an enclosed porch / conservatory, the freezer's over-chilled heat-pump just shut down, could not maintain a dozen degrees cooler. Like-wise, a neighbour found their holiday villa's A/C simply could not cope with an 'exceptional' heat-wave, the third or fourth that year...

My kitchen's 'eco' fridge-freezer easily holds freezer at -17º~~ -20 ºC, but struggles to keep the 'larder' side more than a dozen degrees below ambient. Worse, the F/F radiator array is not an exposed grille on the back, but in upper-left side-wall. So, wary off-set positioning to allow convection is essential. FWIW, in 'High Summer', I put a hand-span desk-fan on adjacent work-top to boost warm side-wall's 'natural' convection. Brings 'larder' temperature --Nominally 5ºC-- an extra 3Cº~5Cº down from 'Ambient Minus A Dozen', to stay in 'single figures' by day.
'Plan_B' is to cycle several ice-packs from freezer to larder...

Tom.G said:
At least in a home refrigerator, the compressor is usually a Scroll Compressor.
The last time I took one apart it had a piston. When did things change?

Low temperature can be a real problem for dehumidifiers. I bought one and it iced up in a cold room. Took hours to sort it out. Later, I found out that I could have bought one that would work in sub zero conditions. Kitchen freezers can ice up if the thermocouple's not in the right place to make sure the defrost period lasts long enough.

Nik_2213
sophiecentaur said:
The last time I took one apart it had a piston. When did things change?
Wiki is a wonderful source. The time I was looking at (playing with) fridge compressors was actually before the scroll compressor was commonly used. (Forty years is but a blink of the eye to me.)
They're very Archimedes.

Tom.G
I’ve been thinking about worst case situations, like an uncommonly cold night in Australia with the air at minus 1C.

Russ wrote:
[My heat pump scenario] is pretty much all fine. Only a couple of minor comments. Your approach temperatures (minimum difference between the working fluid and the source/sink) are a little close - but fine for a sample problem.

The article Russ suggested has a table of “acceptable approach temperatures”. I’ve taken this table to give acceptable differences between:
. my propane and the air that heats it: 14C, and
. my propane and the water that it heats: 8C
** Have I got that right?

Now, I’m trying to get more realistic values for this scenario where:
. the heat pump heats water,
. uses propane as a refrigerant.
. Air temperature = 17C
. water temperature = 50C
. target water temperature = 60C

***** My condenser temperature
With target water temperature = 60C, I guessed a condenser temperature of 74C.
So, as the water nears its target temperature, the approach-temperature nears 74 – 60 = 14C
This is greater than the acceptable minimum of 8C
** Would the scenario be more realistic with a larger approach temperature?
** Could you suggest a more realistic condenser temperature? 84C?
** Is it sensible to select a condenser temperature for this scenario by considering the highest water temperature you want the heat pump to generate?

***** Superheating in the condenser
I guessed that the compressor would superheat the propane by 4C making the superheated propane in the condenser 74 + 4 = 78C.
** Is the superheating of 4C realistic?
** Could you suggest a more realistic superheating? 14C?

******* Propane in the evaporator:
• The acceptable approach-temperature for air is 14C.
• For the heat pump to be efficient down to minus 5C, the temperature in the evaporator would have to be 14C less, i.e., minus 19C.
• This implies a pressure of 2.17 atm (From the internet calculator)
• With an air temperature of 17C, this gives an approach temperature of 17 + 19 = 31C.
For the evaporator, I was suggesting T = 15C and P = 5 atm.
Now it’s T = minus 19C and P = 2.17 atm.
** Is this 31C a realistic approach temperature?
** Is it sensible to select the evaporator temperature for this scenario by considering the lowest air temperature you want the heat pump to handle?

Are you trying to design and build one or just understand how they work? Either way, this focus on minutiae won't lead anywhere useful if you still harbor major misunderstandings about the basics. Since you've not responded to my last post, I don't have a high confidence you've resolved these misunderstandings and I don't really want to put in the effort to wade deeper if this isn't going anywhere useful. I'm on vacation right now and have somewhat limited time...

Last edited:
Hi Russ. In your contribution (Saturday, 24 August), you wrote that the pressure drop across the evaporator was “too small to be visible on the P-V diagram and can be ignored.” For me, that was brilliant because it resolved the contradiction of (1) the pressure-volume diagram showing constant volume and (2) refrigerant moving across the evaporator without any pressure drop.

On 21 Aug, you said there were “no pools”. To me, “no pools” seemed like “no liquid”, and that seemed very strange, “no liquid”!! On 24 Aug, you said that the refrigerant was “just liquid in the tube” on the condenser side of the expansion valve. Tom said it was like a [trickling] garden hose. Resolved.

My last post followed up on your comment (14 August) that my “approach temperatures are a little close, but fine for a sample”.

I’m trying to understand how heat pumps work by constructing a realistic example of one heat pump.

I looked at your Russ’s scope website and remembered my amazement as I leaned against my washing line with my binoculars to see Jupiter and some of its moons, a few centuries behind Galileo. Your spiral galaxy M51 tearing apart the smaller galaxy caught my attention. The cosmos is alive.

Thanks for helping me work my way through major misunderstandings. And please enjoy your vacation.

russ_watters
Tom.G said:
At least in a home refrigerator, the compressor is usually a Scroll Compressor. It's one of those magic designs that does not use a piston with its pulsing output. It does continuous compression while running.
View attachment 350343

The Low pressure inlet (gas) is at the outer ends of the spirals.
The High pressure outlet is at the center, into the page.

https://enggcyclopedia.com/2012/03/scroll-compressors/

Think of a garden hose. You wouldn't normally consider the water in a garden hose to be a "pool" when watering the lawn.

Cheers,
Tom
Tom, That scroll compressor moving diagram is fantastic. Your hose was helpful. Thanks.

Tom.G
GreenWombat said:
Hi Russ. In your contribution (Saturday, 24 August), you wrote that the pressure drop across the evaporator was “too small to be visible on the P-V diagram and can be ignored.” For me, that was brilliant because it resolved the contradiction of (1) the pressure-volume diagram showing constant volume and (2) refrigerant moving across the evaporator without any pressure drop.
My error: I meant "the pressure-volume diagram showing constant pressure"

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