Evap sizing tools

[MikeHolm]

Hi there,
I am building a 2 stage ASHP to run with solar for floor heating and DHW and must be good down to -20C. In Canada 99% of all the ASHPs are designed to do air heating and to me that sucks. I am a wet head and hate ductwork. Also, we cannot get inverter/variable speed compressors here for some reason Danfoss and others don't sell them (unless my research is shyte)

I am looking for a good sizing tool for the outdoor coil so I can design a few different models. Does anyone know of a generic sizing tool that would work here.

Mike

[desA]

http://www.coolit.co.za/

Some very useful software on this site.

[MikeHolm]

Thanks for your info on the site. I'm not sure it has the tools I need such as fin spacing and tube diameter and spacing etc. but maybe I need to look deeper.

BTW advice on making heat pumps that will have a COP of at least 2.5 at -20C would be appreciated. The system will always be air to water and for heating and DHW only. My customers are seldom interested in cooling.

[mad fridgie]

Thanks for your info on the site. I'm not sure it has the tools I need such as fin spacing and tube diameter and spacing etc. but maybe I need to look deeper.

BTW advice on making heat pumps that will have a COP of at least 2.5 at -20C would be appreciated. The system will always be air to water and for heating and DHW only. My customers are seldom interested in cooling.

What water temp are you after?
You will need to be a real smart cookie to sort this one, not the sort of thing you will find in a text book

[desA]

It has ALL the tools you need to design a decent evaporator, or condenser.

[MikeHolm]

There is a real push for low heat loss custom houses and retrofits where i live. The houses are often too close together to make GSHPs financially viable (must do expensive vertical drilling) and we rely on nucs and hydraulic for most of our electricity so from a CO2 point of view, getting off gas is good, even though we have lots of it.
Matching floor heating (30-40C) temps with two stage ASHPs seems a good way to go. The only problem is we have occasional -20C winters and 35C summers with high RH so sizing is a challenge and I am wanting to the most out of this at -20 as i can. As yet, no larger (Daikin, Mits etc) manufacturer makes an air to water HP for sale in North america although that is going to change.
So, I am combining it with some active solar heat, a solar wall as an input to preheat the evap coil but designing the right coil size is very important.
On the condenser side, I am looking at the temps above for most use + 50C for DHW and I will be using BPHEs with long thermal length to transfer to a water tank. Bit of a task.....and I do need some help.

[mad fridgie]

There is a real push for low heat loss custom houses and retrofits where i live. The houses are often too close together to make GSHPs financially viable (must do expensive vertical drilling) and we rely on nucs and hydraulic for most of our electricity so from a CO2 point of view, getting off gas is good, even though we have lots of it.
Matching floor heating (30-40C) temps with two stage ASHPs seems a good way to go. The only problem is we have occasional -20C winters and 35C summers with high RH so sizing is a challenge and I am wanting to the most out of this at -20 as i can. As yet, no larger (Daikin, Mits etc) manufacturer makes an air to water HP for sale in North america although that is going to change.
So, I am combining it with some active solar heat, a solar wall as an input to preheat the evap coil but designing the right coil size is very important.
On the condenser side, I am looking at the temps above for most use + 50C for DHW and I will be using BPHEs with long thermal length to transfer to a water tank. Bit of a task.....and I do need some help.

You do need help?
-20C ambient, 120W/M2, 200M2 house 24hours a day
576Kwhr/day, solar again in your neck of the woods is around 2Kwhrs/day/M2 (in winter) lets say 25% by solar, you would need 72M2 of solar surface area, this does exclude any losses that may occur.
So what is wrong here, is it -20C all day long? If not then you would not need 120w/M2. You will always have losses 24/day, but does the heating run 24Hrs/day. More than likely not, so your actual watt/M2 includes recoving heat when the heating is Off, and is oversized to allow for a reasonable reaction time, which also requires hotter water to enable quick response of heat transfer through the floor.
You need to do a load profile, of losses (ambient related), work out your averages for performance. Rate your heat pump around these averages.
Also Have alook at vapour injection on R404a, very big working enverlope

[MikeHolm]

Hi Fridgie,
the 120w is a design load, not an average. I come from the boiler world and we have a lot easier time designing the system. Typically, find the worst load, add a bit for pick up, know your supply temp and pop in the boiler. At least for me it is that easy.

HPs are different in many parts of Canada. From a greenhouse gas point of view (BTW, the current Canadian govt does not believe in global warming...I'm not kidding ) , it is better for me to use electricity which is currently made up of 25-30% hydraulic, 40% nuc (and that won't change) and most of the balance is NG. Coal is being phased out and we have the best FITs for solar PV at $.80/kwh. I want to get off gas.

Anyway. Electrical prices in Ontario are about $.09 for peak hours and $.05 for off peak 9pm to 7am. Gas right now is the equiv of about $.03/kwh

From a pricing point of view, the HP makes sense. I already build basements with 200mm of mass in the floor and store heat during the night for radiation during the day but I don't always have that opportunity.

Anyway, for my house, the peak at -20 is probably about 60w/m2 at this point. The average in January here would be a -15 night temp and a -7 or so daytime temp. The last time i recall -20, 24 hrs a day was in 1994 when we had it for 30 days straight. We were out fixing a lot of heating systems that year.

I care less about the cooling needs than the heating needs. The average subdivision owner(ie: crap house) wants to keep the t-stat at 20 in the summer and 23 in the winter so it is no wonder we are the worst per capita user of energy in the world.
The large temp swing is important but my market is more focused on the heating so I think I should concentrate on that.

Given the above, should i still look at R404? I haven't done so yet.

[mad fridgie]

Hi Fridgie,
the 120w is a design load, not an average. I come from the boiler world and we have a lot easier time designing the system. Typically, find the worst load, add a bit for pick up, know your supply temp and pop in the boiler. At least for me it is that easy.

HPs are different in many parts of Canada. From a greenhouse gas point of view (BTW, the current Canadian govt does not believe in global warming...I'm not kidding ) , it is better for me to use electricity which is currently made up of 25-30% hydraulic, 40% nuc (and that won't change) and most of the balance is NG. Coal is being phased out and we have the best FITs for solar PV at $.80/kwh. I want to get off gas.

Anyway. Electrical prices in Ontario are about $.09 for peak hours and $.05 for off peak 9pm to 7am. Gas right now is the equiv of about $.03/kwh

From a pricing point of view, the HP makes sense. I already build basements with 200mm of mass in the floor and store heat during the night for radiation during the day but I don't always have that opportunity.

Anyway, for my house, the peak at -20 is probably about 60w/m2 at this point. The average in January here would be a -15 night temp and a -7 or so daytime temp. The last time i recall -20, 24 hrs a day was in 1994 when we had it for 30 days straight. We were out fixing a lot of heating systems that year.

I care less about the cooling needs than the heating needs. The average subdivision owner(ie: crap house) wants to keep the t-stat at 20 in the summer and 23 in the winter so it is no wonder we are the worst per capita user of energy in the world.
The large temp swing is important but my market is more focused on the heating so I think I should concentrate on that.

Given the above, should i still look at R404? I haven't done so yet.

Yes i would still consider R404a for your application, somes comps are rated to a SST -40C.

[desA]

There are many commercial heat-pumps available. Doesn't make too much sense to re-invent these, unless it is for a hobby. It will take quite some effort to generate the kind of experience required, if you want to become a heat-pump designer.

[Bigfreeze]

You should really bite the bullet and install a ground loop system. Canada is far too cold for Air source units to be efficient.

You'll get two benefits from a ground loop. You should get COP's of 4.5 and up and you could also install an intermediary heat exchanger and passive cooling in the summer.

In a climate like yours this makes the most sense.

[MikeHolm]

There have been a number of studies completed here that show that GSHPs sold in north america do not have the COPs that are claimed. That said, unless you are in a climate with 60 days of -20C, an ASHP will outperform a GS on an annual basis especially if it is a system such as the Mits.
We used to have some rebates from the Federal government but they have removed them all and most people cannot afford the $30k+ to put the ground loops and HP in to meet the demand. We do have a lot of stupidly large houses over here so the average size of system is 5 ton and sometimes multiples of them. That's a lot of trenching or drilling.

[Bigfreeze]

There have been a number of studies completed here that show that GSHPs sold in north america do not have the COPs that are claimed. That said, unless you are in a climate with 60 days of -20C, an ASHP will outperform a GS on an annual basis especially if it is a system such as the Mits.
We used to have some rebates from the Federal government but they have removed them all and most people cannot afford the $30k+ to put the ground loops and HP in to meet the demand. We do have a lot of stupidly large houses over here so the average size of system is 5 ton and sometimes multiples of them. That's a lot of trenching or drilling.

I would completely disagree with you on the above. An ASHP will never outperform a ground loop system once it is installed properly. You cannot take the heatpump in isolation either. You must design the UFh to the lowest temp possible. We design to 30C.

We have Groundsource heatpumps running COP's of 7.6 and SPF's of 6.2 (recorded on a high performance monitoring system). You won't touch that with a ASHP, especially a Mits or Daikin who are not, in all practical terms, heatpumps which are suited to UFH systems.

You

[mad fridgie]

I would completely disagree with you on the above. An ASHP will never outperform a ground loop system once it is installed properly. You cannot take the heatpump in isolation either. You must design the UFh to the lowest temp possible. We design to 30C.

We have Groundsource heatpumps running COP's of 7.6 and SPF's of 6.2 (recorded on a high performance monitoring system). You won't touch that with a ASHP, especially a Mits or Daikin who are not, in all practical terms, heatpumps which are suited to UFH systems.

You

COP 7.6 for an installed system, this has to be the exception to the rule.
Is this DX system or with a secondary loop?
What temp is the ground.
You say you design 30C, supply or return?
Please indicate SST and SCT of these systems, when they have been running for an hour or two.

[Bigfreeze]

COP 7.6 for an installed system, this has to be the exception to the rule.
Is this DX system or with a secondary loop?
What temp is the ground.
You say you design 30C, supply or return?
Please indicate SST and SCT of these systems, when they have been running for an hour or two.

No, we have quite a few units running an SPF of 6+, while the majority of the rest would be 5.5+.
They are mostly DX systems.
The design of 30C refers to the flow temp into the ufh. The ground temp would average between 6 and 8 degrees at 1m deep.

[mad fridgie]

No, we have quite a few units running an SPF of 6+, while the majority of the rest would be 5.5+.
They are mostly DX systems.
The design of 30C refers to the flow temp into the ufh. The ground temp would average between 6 and 8 degrees at 1m deep.

Forgive my ignorance what is SPF?

[MikeHolm]

All this is fine with the equipment you have in Europe but in many ways you are way ahead of North Americans. For one thing, you may be able to build your own HP and get it passed by the local authority but here, we have no such thing as a CE designation so with few exceptions almost all installations need to have CSA or UL approvals which cost many $1000 and cannot be changed easily. The option, of course, is to install without permits. This is a much longer discussion anyway and not for this post.

I design heating systems for houses that can run with output temps of no more than 35C, DHW excepted, and the houses are typically 100-200m2 but look at the average system here where the heat loss is much higher (some provincial governments in Canada have actually reduced insulation requirements within the past 20 years due to builder pressure) and most houses in Canada are low mass houses (stick frame and insulation) so they are harder to retrofit for low temp systems.

Other than the houses with lots of land area which are in the country, most of my work is in the city where we would have to drill down at a cost of $50/m and the average size of system (monster homes excluded) would require 4-5 80m bore holes. Most people cannot afford this and there is little in subsidy and in the city there is no room anyway.

COPs here of 6-7 for GSHPs are unheard of. We don't have access to the equipment you have. Inverter compressors are not available (Danfoss and others won't sell theme for some reason).

More to come but I've got to walk the dogs.

[Bigfreeze]

All this is fine with the equipment you have in Europe but in many ways you are way ahead of North Americans. For one thing, you may be able to build your own HP and get it passed by the local authority but here, we have no such thing as a CE designation so with few exceptions almost all installations need to have CSA or UL approvals which cost many $1000 and cannot be changed easily. The option, of course, is to install without permits. This is a much longer discussion anyway and not for this post.

I design heating systems for houses that can run with output temps of no more than 35C, DHW excepted, and the houses are typically 100-200m2 but look at the average system here where the heat loss is much higher (some provincial governments in Canada have actually reduced insulation requirements within the past 20 years due to builder pressure) and most houses in Canada are low mass houses (stick frame and insulation) so they are harder to retrofit for low temp systems.

Other than the houses with lots of land area which are in the country, most of my work is in the city where we would have to drill down at a cost of $50/m and the average size of system (monster homes excluded) would require 4-5 80m bore holes. Most people cannot afford this and there is little in subsidy and in the city there is no room anyway.

COPs here of 6-7 for GSHPs are unheard of. We don't have access to the equipment you have. Inverter compressors are not available (Danfoss and others won't sell theme for some reason).

More to come but I've got to walk the dogs.

If you need 400m of bore that means the heatpump would be in the region of 23kw. Thats a crazy figure. You need to seriously look at your insulation levels if thats the heatloss on a 200sqm build. In Austria, where they have a similar climate to yours. A house of that size would normally be heated by a 8kw HP.
The unit I referred to was not an invertor unit. It was single phase capacitor start. Just a very well designed HP with a very well designed UFH and DHW system

[MikeHolm]

It has always bugged me that we have such big houses here and the average construction for a house built in the 70s will have a heat load of 120w/m2. The houses I work on now are far better than that but the housing stock built since the 60s has a life expectancy of less than 80 years and much of it will be less than 50 years. Most people here have no idea what well built house looks like.

None the less we typically size the GSHP systems at 4kw/hole as I said above and 16kw+ would be appropriate for a lot of stick frame homes (no one likes much electric back up but it is going to happen anyway). My house is a 1918 clay brick with no insulation in the walls (160m2) and I heat for a lesser cost (using the same heating system) than a 1990 200 m2 stick frame home (150mm of fibreglas bat insulation, vapour barrier and an air barrier on the outside and then a brick veneer or siding.

I use my house as a test bed for all my solar systems as well as boiler work etc. Now I want to reclaim the waste heat going down drain, store it, and use it as a supplementary evaporator on cold days but I don't know how to size coils in tanks. Anyone want to chime in on this one? The goal is to keep a minimum 5-10c water in a 500l tank and either use it to 1] help defrost the outside coil or 2] add some extra heat coming back to the system on really cold days (-15 or colder). I will use a R410 and a Copeland scroll. I like brain teasers.

[desA]

Coolpack predictions for R-134a.



Ideal case. Zero approaches for both evaporator & condenser. COP,hp = 8.510.



Perhaps a little more realistic. With approaches of 5K & 3K for evap & condenser respectively. COP,hp = 6.215.

[desA]

Refrigerant R134a. Source (ground/air/other) at 0'C. Approaches of 5K & 3K on evap & cond respectively. COP,hp = 4.862.



Refrigerant R410A. Source (ground/air/other) at 0'C. Approaches of 5K & 3K on evap & cond respectively. COP,hp = 4.691.



Are you really sure that you want to use R410A in this application?

[mad fridgie]

Air source Des, not ground source, for Mike.

[desA]

Air source Des, not ground source, for Mike.

Therefore, I'd think it should get worse...

Corrections made to above post.

[MikeHolm]

Thanks guys,
There seem to be good and bad for 410a over 134 (with 134 slightly winning) but this is at 0c. What will happen at -5, -10, etc. Yes I am going to try to save up some waste heat from elsewhere but I will have to figure out when to start using it and how. Is the the dip on the chart after the evap just before compression due to sub cooling? It appears that the assumption is that enough supplemental heat will be inputted to keep suction at 6 bar. How much room is ther to play with?

Mike

[desA]

The state information for R134a, for the case at 0'C. Compare these operating pressures to those for R410A.

Don't these R410A pressures concern you?

[MikeHolm]

The range is less but is 2000+kpa an issue for 410? Enthalpy is is lower with 134 but COP is higher. Is there enough range for a lower temperature operation? I assume that at some point low pressure will fails a lockout.

So much to learn and understand. I appreciate the comments.

[desA]

The range is less but is 2000+kpa an issue for 410?

You will need to think a bit more about the condenser you'd like to use for the ufh. What happens if the refrigerant side lets go at 20+ bar?

[Bigfreeze]

You will need to think a bit more about the condenser you'd like to use for the ufh. What happens if the refrigerant side lets go at 20+ bar?

Its quite normal for 410 heatpumps to be running head pressures of between 20 and 30 bar for hot water production. You'll be running 20 bar for heating alone.

[desA]

What kind of condensers are they using?

[Bigfreeze]

What kind of condensers are they using?

Plate heat exchangers

[desA]

Plate heat exchangers

Standard compact brazed heat-exchangers?

[Bigfreeze]

Standard compact brazed heat-exchangers?

BPHE but specially designed for heat pump applications

Check out the swep website or other major manufacturers, you'll find them there

[Bigfreeze]

SWEP's NHP high pressure HX's are rated up to 45bar

[MikeHolm]

Yes, swep or alfa laval etc. But for the dhw I would like to find a way to go directly to an internal coil in a tank but for this particular design I will go with a BPHE. The more I can eliminate pumps the better the system COP.

I will be going to a minimum 300l buffer tank so I can have a close deltaT on the refrig side and a 10C deltaT on the UfH.

When I get home I will get the specs for the BPHE and let you know. I'm working off the iPad and have no access to my info. What I remember is that the system is about 14kw and the HE is 26 plate, 10cm by 40cm(approx) by memory so I could be off a bit.

[MikeHolm]

The particular HEs have braze joints for the refrigerant and threaded ones for the secondary loops.

None the less, I agree that max pressure limits should be observed but what are they really?

All the NIBE, Danfoss etc. hPs use plates so what are they doing right?

[desA]

A standard brazed compact HE - SWEP, Alfa Laval et al, are NOT safe in applications where excessive, sudden venting can occur. Be warned well in advance.

Besides filling the water system with oil, a very nasty explosion can occur. This is irrespective of the refrigerant type involved. Pressure relief valves are generally designed to cope with gradual pressure rise - not a sudden pressure release. Sudden pressure releases generally require a few more safety aspects to be engineered into the system.

[Bigfreeze]

A standard brazed compact HE - SWEP, Alfa Laval et al, are NOT safe in applications where excessive, sudden venting can occur. Be warned well in advance.

Besides filling the water system with oil, a very nasty explosion can occur. This is irrespective of the refrigerant type involved. Pressure relief valves are generally designed to cope with gradual pressure rise - not a sudden pressure release. Sudden pressure releases generally require a few more safety aspects to be engineered into the system.

People far more qualified than myself have decided they are fine for their applications. I just purchase their products, as do hundreds of other installers across europe.
Besides the condenser sides of these systems are not subject to temperature shock. The temperature invariably gradually increases as the system comes up slowly in temp.
The evap side is far more likely to fatigue due to temp shock than the condenser side is.

[desA]

People far more qualified than myself have decided they are fine for their applications. I just purchase their products, as do hundreds of other installers across europe.
Besides the condenser sides of these systems are not subject to temperature shock. The temperature invariably gradually increases as the system comes up slowly in temp.
The evap side is far more likely to fatigue due to temp shock than the condenser side is.

You may want to research into vented condensers. You may get away with standard condensers most of the time, but there will be the time when they will fail you. Yes, I am fully aware of the types of systems supplied into Europe.

[MikeHolm]

The big question remains, what is the proper way to inject the waste heat near the suction side of the compressor? A long HE will induce a pressure drop but How will the added heat affect it and what temp water should the water side of the injector HE be set to? The goal bing to fool the compressor to think it is -5c not -15c outside.

[desA]

The big question remains, what is the proper way to inject the waste heat near the suction side of the compressor? A long HE will induce a pressure drop but How will the added heat affect it and what temp water should the water side of the injector HE be set to? The goal bing to fool the compressor to think it is -5c not -15c outside.

I suspect that you may well need to talk to MadFridgie in regards to his new 'Wonder Widget'...

[Bigfreeze]

DesA,

You're simplifying what would happen too much. In the event of a failure the refrigerant would first dissipate into the water side of the HX, then have to travel through the pipework out of the heatpump, then through any pipework linking the HP to the UFH.
The gas in the HP is only at 20bar because of the area it is confined in. Increase the area and the pressure will drop very quickly.
If i was to bring the water in a system containing 3-400 litres. How man litres would I need to introduce to bring that to 10 bar? The same principle applies to the gas. Then you have air valves and pressure relief valves which are all points of escape.

I'm not disagreeing that there is potential for failure. I'm merely pointing out that failure wouldn't be as disasterous as you imagine. It would be quite harmless actually.

[MikeHolm]

In the case of such a failure, assuming the pressure went into the water, which is hydraulically connected to the tubing, the first line of defense would be compressing any entrained air (assuming there is any) then the expansion tank, which would probably burst a bladder if it was too small but with acceptance volumes of 25-50L and the sudden condensing of the gasses, I think Bigfreeze is right and the most that would happen outside the system would be a PRV going off.

Des, I just downloaded Coolpack and am starting the tutorials (I learn better hands on). In your diagrams, what isentropic efficiency did you use for the compressor? Given is 70% but I know this changes for different units and refrigerants. I think i am getting a better understanding of the issues but, man there is so much to learn.

[desA]

65% is a reasonable number for isentropic efficiency. Will vary depending on position in the heat-up phase.

I'm not buying the pressure-diffusing arguments at all - doesn't follow the laws of physics, as far as I'm aware. Never know though, with today's 'new physics'.

Ever seen a geyser go bang? (The roof-mounted geysers blow the whole jolly roof to bits).

[Bigfreeze]

65% is a reasonable number for isentropic efficiency. Will vary depending on position in the heat-up phase.

I'm not buying the pressure-diffusing arguments at all - doesn't follow the laws of physics, as far as I'm aware. Never know though, with today's 'new physics'.

Ever seen a geyser go bang? (The roof-mounted geysers blow the whole jolly roof to bits).

Which law of physics would be broken in the scenario I just mentioned?
Are you telling me that 4 or 5kg of refrigerant will be unloaded instantaneously into the system?
Put your gauge line on a running system and crack the valve. What would happen? The answer, besides a cloud of gas, is not much.
Put that same gauge line straight onto a bottle and crack the valve. Again, not much happens, no explosions, no dramatics.
This isn't die hard, things don't go flying through the air anymore than a guy goes flying through the air if he gets shot.

Every action has an equal an opposite reaction does not mean that said reaction must manifest itself in as dramatic a manner as the first.

[MikeHolm]

gasses are different than liquids for sure. I have absolutely no issue working with liquids at high temps and pressures and I have seen the results of gas water tanks with faulty controls AND faulty pressure relief valves. Hole in the building roof can result!!!!!
Gases are a bit different and more explosive than water which more often than not, just leaks, but you have a limited volume to work with and the water acts as a big cushion to spread out and equalize the pressure.

Regarding the efficiency, if it is around 65% peak, what would the average reduction be at the limits? 45%, 35%? I assume this will be reflected in the COP and I may have answered my own question?

[desA]

As far as isentropic efficiency goes, you may want to consult the various compressor manufacturer's tables, for the type of unit you use.

Gas ruptures can be incredibly nasty - especially if sudden. Anyway, we digress.

[MikeHolm]

Are discharge pressures of 20-30bar unusual? I cannot see pressures above that being healthy. Also, I assume the p2-p1 is dependent on the compressor and the temps at the time but are there accepted min/max values?

Just soaking it up and learning. I appreciate all the chat.

[desA]

R410A.



R134a.

Which one would you consider to be the safer of the two options? For operating condition & off condition (settling pressure).

[MikeHolm]

the r134 appears better due to the ratio in pressures but does the compressor not like a higher ratio to be closer to full loading? I may have this concept wrong.

[MikeHolm]

actually by pressure ratio only the 410 looks better but again, I am not sure if the high pressure value is an issue for 410. COP is slightly better for 134