AWHP superheat & sub-cooling

[desA]

Originally Posted by desA
Now, where would you place that in the system? In place of the TXV?

Gary:
Yes. Generally these are used where the load is constant. It is probably not something we would use, but it is an alternative to be explored.

Ok - thanks for that. A nice valve. Looks to be pretty compact.

I'd imagine that the valve would be set in the factory & then left well alone, unless there was a very good reason to modify its setting.

[desA]

Alternatives to KVL?

The Danfoss KVL is proposed for small systems. Sporlan & Alco have equivalents.

For larger systems, Danfoss recommend "using an ICS (or PM) main valve, with a screwed-on pilot valve (type CVC). A branch line must also be routed to the CVC pilot valve from the main line, downstream of the main valve."

It looks like there are a number of different combinations around. I'd like to know more about how these work & how perhaps an alternative method could be developed for controlling, managing the evaporator pressure to remain below Te,sat=13'C.

Danfoss also mention "(electronically) adjustable MOP points in systems with electronic injection control."

I'd imagine that the last point is probably for refrigeration systems.

[Gary]

Alternatives to KVL?

The Danfoss KVL is proposed for small systems. Sporlan & Alco have equivalents.

For larger systems, Danfoss recommend "using an ICS (or PM) main valve, with a screwed-on pilot valve (type CVC). A branch line must also be routed to the CVC pilot valve from the main line, downstream of the main valve."

It looks like there are a number of different combinations around. I'd like to know more about how these work & how perhaps an alternative method could be developed for controlling, managing the evaporator pressure to remain below Te,sat=13'C.

I'm thinking you meant to say Te,sat=15'C.

[desA]

^ Yes... true. My brain's stuck in reverse.

Te,sat=15'C it is, indeed.

[Gary]

As I see it, our goal is to extract sufficient heat from the air to drive the Te,sat up to 15C given the coldest expected ambient temp, and then control it down to 15C at higher ambients.

[Gary]

As I see it, our goal is to extract sufficient heat from the air to drive the Te,sat up to 15C given the coldest expected ambient temp, and then control it down to 15C at higher ambients.

Driving the Te,sat up to 15C is primarily a matter of evap surface area and air flow. Obviously, the Te,sat cannot be driven up to 15C if the incoming air temp is 15C or lower, and in practice the incoming air would need to be at least 20C. Thus far we have been assuming a minimum air in temp of 25C. Do we need to re-think this, or is air temp below 25C a rare occurence in your target area?

[desA]

As I see it, our goal is to extract sufficient heat from the air to drive the Te,sat up to 15C given the coldest expected ambient temp, and then control it down to 15C at higher ambients.

I have to agree with this.

Driving the Te,sat up to 15C is primarily a matter of evap surface area and air flow. Obviously, the Te,sat cannot be driven up to 15C if the incoming air temp is 15C or lower, and in practice the incoming air would need to be at least 20C. Thus far we have been assuming a minimum air in temp of 25C. Do we need to re-think this, or is air temp below 25C a rare occurence in your target area?

You are quite correct. In my region perse' the air seldom goes below say 20-25'C, but, for the rest of SE Asia, this is not always the case. Some parts can actually go slightly sub-zero - not talking about NE China, where the winter temps can approach those of Siberia.

The lower end, up-push must be considered, to provide a complete design case. Thanks so much for this.

(Sidebar:
Over here, when the air temp hits around 25'C, out come the winter woolies, scarves, hats, coats - you'd think you were in Siberia... )

[Gary]

I'm wondering if we might want to lock out the tank temp maintenance mode at night.

The feedwater mode would fill the tank with minimum temp water after peak usage, but the rest of the heating would wait until morning, when the outdoor temp is warmer.

What do you think?

[desA]

For some countries, in the region, off-peak electrical rates (22h00-09h00) are can go down as low as around half of the typical daily rates. This is a major draw-card for the hotels, guest-houses & so forth.

So, it pays handsomely to make hot water at night, if possible - at least for these industries. It makes it tougher for the heat-pump, though.

If running hard at night, one has to also consider the possibility of freezing, in some parts. This can be handled inexpensively, though, by switching off the system during these times & moving over to electrical element heating-backup.

Some storage tanks are combination input-ready e.g. element heating, heat-pump & solar.

[Gary]

Hmmm... then perhaps we need to design for night time temperatures?

[desA]

Hmmm... then perhaps we need to design for night time temperatures?

This will be the safest way to go, as you can be sure that someone will always decide to run at night - especially with low-cost off-peak rates available.

[desA]

http://www.ra.danfoss.com/TechnicalI...imitations.pdf

An interesting article explaining pro's & con's of TXV's.

Found the discussion on minimum safe superheats to be very interesting.

[desA]

Liquid line air-cooler sub-cooler (revisited)

Do you perhaps have any information on liquid line air-cooled sub-coolers?

I'm currently working on the main reasons for the evaporator Te,sat drift over the duration of a water heating cycle. It is beginning to look like the effect of evaporator inlet vapour fraction (x) is very strong.

To compensate, it is proposed that a suitable air-cooled liquid-line cooler, that increases performance with Tc,sat, could be a useful feature.

[Gary]

Liquid line air-cooler sub-cooler (revisited)

Do you perhaps have any information on liquid line air-cooled sub-coolers?

I'm currently working on the main reasons for the evaporator Te,sat drift over the duration of a water heating cycle. It is beginning to look like the effect of evaporator inlet vapour fraction (x) is very strong.

To compensate, it is proposed that a suitable air-cooled liquid-line cooler, that increases performance with Tc,sat, could be a useful feature.

That's what the iced drier test was designed to demonstrate.

[desA]

That's what the iced drier test was designed to demonstrate.

Yep, I guessed so.

Do you have a suggestion on how to pack ice around the filter-drier so it stays in place for some time? I'm struggling here.

Today, I ran wet rags around the filter-drier & used wicking effect to draw water up from the evap pan - with a pre-soak of water.
I could get the rags to go pretty cold, & could actually see a drop of around 0.5-1'C on the Te,sat side during a 5-10 minute run.
The problem was that the water evaporated too quickly & the wick didn't replenish fast-enough...

[Gary]

Today, I ran wet rags around the filter-drier & used wicking effect to draw water up from the evap pan - with a pre-soak of water.
I could get the rags to go pretty cold, & could actually see a drop of around 0.5-1'C on the Te,sat side during a 5-10 minute run.
The problem was that the water evaporated too quickly & the wick didn't replenish fast-enough...

Seems you need a bigger pan of water and it needs to be much closer to the active portion of the rags.

[Gary]

Check this out:

http://www.swep.net/index.php?tpl=page0&lang=en&id=313

The economizer accomplishes the same effects as the VIC, but is far more complex.

[Gary]

Check this out:

http://www.swep.net/index.php?tpl=page0&lang=en&id=313

The economizer accomplishes the same effects as the VIC, but is far more complex.

Okay... upon further reading it doesn't have the same effects, because the economizer pumps the recirculated heat back to the condenser in the middle of the compression, thus more efficiently than dropping the pressure all the way down and then pumping it all the way back up.

The effect of the VIC will be more to extend the effective surface of the evaporator as opposed to increasing the efficiency of the compressor.

[Gary]

Today, I ran wet rags around the filter-drier & used wicking effect to draw water up from the evap pan...

Maybe the better way would be to place the liquid line in the pan. This would help get rid of the condensate, too.

[desA]

^ Would the extraction fan would like the high humidity?

So, saying, it's probably no worse than the current inlet air humidity.

I'm thinking to try the ice today, using a modified beverage bottle (PET), or two , wrapped around the filter & pipe.

If this works, then the general idea will be to move towards a properly designed liquid line cooler. I wonder if any commercial products exist for this. If not, then I'll design one up & have it made.

[Gary]

If this works, then the general idea will be to move towards a properly designed liquid line cooler. I wonder if any commercial products exist for this. If not, then I'll design one up & have it made.

Condensate runs down the coil into the pan, right?

What if the coil were oversized and the bottom two tubes were a liquid line subcooling loop?

[desA]

Condensate runs down the coil into the pan, right?

What if the coil were oversized and the bottom two tubes were a liquid line subcooling loop?

That's an idea - submerse the lower end of the coil in condensate. Mmmhhh...

Actually, I think that the idea of just running the liquid line through a flooded condensate pan would be a good one. No real need for fins, I'd think. Basically, all that has to be done is move modify the pan outlet to project up a height, to retain liquid level.

I'd also think that the hot discharge line (Tcon,exit > 65'C) at max operating condition would knock Legionaires disease on the head, in the evap pan. This is a very interesting development, I must say. I'm going to test the ice effect today & see how much info we can get from this.

[desA]

Evap Te,sat lift mechanism - described

Fairly straightforward:
1. TXV m'g [kh/h] characteristic different to compressor characteristic. One has increasing slope & the other negative. They intersect at a nominal point.
2. Below nominal point, TXV under-feeds. Tends to depress Te,sat.
3. Above nominal point, TXV over-feeds. Tends to raise Te,sat.
4. (3) gives rise to the observed 'Te,sat drift phenomenon'.

Now that the drift mechanism is understood, how to correct it?

Points to consider:
a. Valve characteristic slope;
b. Te,sat at start of process can vary on the day due to Ta,in (air inlet temp), RH% (relative humidity).
c. Te,sat (op) = Te,sat (start) + dTe,drift

Item (c) can force the Te,sat at entry into the compressor into an unsupported range, as per compressor manufacturer.

In light of the above, what effective control mechanisms are there?

[desA]

desA:
In light of the above, what effective control mechanisms are there?

So far, we have:
1. CPR pressure regulation at compressor suction;
2. Fan speed control;
3. Liquid line sub-cooling.

Is it possible to obtain a TXV, or electronic expansion valve, with a flatter massflow response characteristic?
How would evaporator over-size affect the situation?

Are there any other control concepts in the armoury, we can call on?

[Gary]

Any data on the iced drier test yet?

[Gary]

So far, we have:
1. CPR pressure regulation at compressor suction;
2. Fan speed control;
3. Liquid line sub-cooling.

Is it possible to obtain a TXV, or electronic expansion valve, with a flatter massflow response characteristic?
How would evaporator over-size affect the situation?

Are there any other control concepts in the armoury, we can call on?

Actually, I'm expecting liquid line subcooling to prove counter-productive. It will improve the evap performance, but our end product is hot water, not cold air.

Why do you believe the evap is oversized?

[desA]

Any data on the iced drier test yet?

Just completed the run. It was exceptionally difficult to keep the ice in place - cut open a pet bottle & wrapped it around the filter-drier. Kept opening the lid (machine running) & filling with ice - allowed to stabilise, observe, add more ice... The results are more qualitative, then quantitative in nature..

Basically, it seemed that the ice was able to drop the Te,sat by around 1-1.5'C. It held it quite nicely while the ice was melting. Practically, this would show that sub-cooling the liquid line does influence the evaporator, as expected. Not quite sure how much though. Impossible to estimate the overall effect on condenser output, though, since need a full heat-run to see the effect of the changes.

From my perspective, a combination of reasonable sub-cooling (TXV inlet control ~ 8.5K),(not sure if extra sub-cooling will asist the heat-pump hot-water delivery cause, however), combined with fan control, works at this stage. The fan gives the easiest response at this stage. If an air-cooled sub-cooler were integrated, then the sub-cooling effect could be more controlled.

The fan control basically can allow for the evaporator over-surface to be righted.

[Gary]

Just completed the run. It was exceptionally difficult to keep the ice in place - cut open a pet bottle & wrapped it around the filter-drier. Kept opening the lid (machine running) & filling with ice - allowed to stabilise, observe, add more ice... The results are more qualitative, then quantitative in nature..

Basically, it seemed that the ice was able to drop the Te,sat by around 1-1.5'C. It held it quite nicely while the ice was melting. Practically, this would show that sub-cooling the liquid line does influence the evaporator, as expected. Not quite sure how much though. Impossible to estimate the overall effect on condenser output, though, since need a full heat-run to see the effect of the changes.

From my perspective, a combination of reasonable sub-cooling (TXV inlet control ~ 8.5K),(not sure if extra sub-cooling will asist the heat-pump hot-water delivery cause, however), combined with fan control, works at this stage. The fan gives the easiest response at this stage. If an air-cooled sub-cooler were integrated, then the sub-cooling effect could be more controlled.

The fan control basically can allow for the evaporator over-surface to be righted.

At best, the subcooler is a trade-off. The improvement in evap performance makes up for the heat that is rejected to the waste air by the subcooler.

We need to recover that heat and have the evap improvement. We need the VIC.

[desA]

Why do you believe the evap is oversized?

Ok, here goes. (I think we have this some pages back, but I'll repeat it here, with some additions. Don't ask me how I know - but, I know - it's a long story ).

Lab machine evap spec:
610x384x76.2mm
4 row, 10 fpi, copper fin
Tubeside : 5 parallel passes, 12 tubes per pass
Q'e = 13.025 kW (R-134a)
va,in = 2.03 m/s

Measured : Va,in~ 3.3 - 3.6 m/s (based on fan selection).

Compressor needs matching as follows (subject to TXV):
Q'e = 7.0 kW @ Tc,sat=40'C, Te,sat=15'C
Q'e = 5.9 kW @ Tc,sat=55'C, Te,sat=15'C
Q'e = 4.3 kW @ Tc,sat=75'C, Te,sat=15'C

----------------

So, basically, the evap is selected too large in the first place - actually, all the way though the range. On top of that the fan runs between +62.6% & +77.3% of design face velocity.

This beast adds to the Te,sat drift problem, in my view.

[desA]

At best, the subcooler is a trade-off. The improvement in evap performance makes up for the heat that is rejected to the waste air by the subcooler.

True. The sub-cooler does serve a useful purpose, though, in ensuring that the liquid at entry to the TXV is adequately cooled - for control purposes. I'd say that where the condenser sub-cooling is marginal, that an air-cooled sub-cooler (ACSC) will be a useful ally.

We need to recover that heat and have the evap improvement. We need the VIC.

I'll continue on the VIC development. I am concerned about why my simulations are kicking me back with such force on this one. I'll set up a full spreadsheet on this, to get around the numeric shenanigans. It may be just that... something to continue with.

[Gary]

Ok, here goes. (I think we have this some pages back, but I'll repeat it here, with some additions. Don't ask me how I know - but, I know - it's a long story ).

Lab machine evap spec:
610x384x76.2mm
4 row, 10 fpi, copper fin
Tubeide : 5 parallel passes, 12 tubes per pass
Q'e = 13.025 kW (R-134a)
va,in = 2.03 m/s

Measured : Va,in~ 3.3 - 3.6 m/s (based on fan selection).

Compressor needs matching as follows (subject to TXV):
Q'e = 7.0 kW @ Tc,sat=40'C, Te,sat=15'C
Q'e = 5.9 kW @ Tc,sat=55'C, Te,sat=15'C
Q'e = 4.3 kW @ Tc,sat=75'C, Te,sat=15'C

----------------

So, basically, the evap is selected too large in the first place - actually, all the way though the range. On top of that the fan runs between +62.6% & +77.3% of design face velocity.

This beast adds to the Te,sat drift problem, in my view.

Where does Ta,in factor in to all this?

Q'e = 13.025 kW (R-134a)

This would depend very much upon the temperature of the air entering the coil.

[Gary]

An example:
Take an establishment which only uses most off its water (21 m3) during a peak time of 19h00 - 21h30. Hot water storage tanks are extremely pricey in this region - more than the heat-pumps, for instance.

Current system as follows:
1. Small inventory of hot water, via small fuel-fired boiler - pump-around. Start of cycle 30'C - end ~65'C - hold when hot. Enough for off peak usage.
2. At peak usage, hot water storage not enough, so heat mains water directly from 30'C to 65'C on the fly - for use during peak time only.
3. Cost of fuel incredibly high, need to go to alternative energy source.

Proposal (by consultant):
4. To install a heat-pump in either of two configurations:
4.1 Direct heating of small volume flow of water, through heat-pump (direct), into hot-water storage tanks - sufficient for peak usage. Heat-pumps operate off-peak, taking advantage of off-peak electrical rates. Fuel-boiler on make-up call if water storage begins to run short.
4.2 Pump-around heating of larger volume flow of water, through heat-pump, into hot-water storage tanks - sufficient for peak usage. Heat-pumps operate off-peak, taking advantage of off-peak electrical rates. Fuel-boiler on make-up call if water storage begins to run short.

If I were custom designing for this particular customer, I would want to:

1. Select a condenser/pump combination capable of heating 21 m3 of feedwater/tankwater from X'C to Y'C during the period of off peak electrical rates (22h00-09h00). (Ideally the storage tank capacity would exceed 21 m3 so that the heating could be delayed until 22h00)

2. Select a compressor capable of supplying sufficient heat to the condenser to achieve #1.

3. Select an evaporator capable of supplying sufficient heat to the compressor to supply to the condenser to achieve #1, given the average Ta,in during the period of off peak electrical rates during the coldest season of the year.

By far, the most difficult selection is #3.

From one point of view, the "correct" selection is that which does not require load limiting control (MOP, CPR, Fan Speed, etc.). It reaches the upper limits of the compressor design envelope at the highest expected ambient temp. In this case Te,sat=15C @ Ta,in=35C. But this means that the compressor will be pumping lower capacity (lower Te,sat) at any ambient that is less than 35C. Clearly this is not going to do the job given night temps during the cold season. For this we need a larger evap and load limiting control.

[mad fridgie]

desA, what is the difference between normal and off peak rates ($), and what is the difference between day ambient and night ambient, and difference in seasons and water use in seasons

[desA]

desA, what is the difference between normal and off peak rates ($), and what is the difference between day ambient and night ambient, and difference in seasons and water use in seasons

Very good questions. A lot to answer in one go.

I'll pull up the relevant info, check the latest off-peak rates & post it.

If memory serves, the off-peak rates varied from industry-to-industry, & for major metropolitan centres compared to less developed. At one point, I seem to recall almost 50% reduction from 2100 to 09h00 in BKK, for instance - I'll need to update that.

[mad fridgie]

One would presume that this is a hotel in Bankock? The water that you are heating itself is not sanitary? (HX elsewhere) I have a product which could do this "effective COP" approx "10" But this not the place to share the design "sorry", as far as storage, this is a fair amount of water (weight volume) I would suggest you store the energy in a phase change material, this would also keep your Ct very constant (time basis) This would reduce your tank requirement and also keep outlet temps constant

[desA]

Lab machine evap spec:
610x384x76.2mm
4 row, 10 fpi, copper fin
Tubeside : 5 parallel passes, 12 tubes per pass
Ta,i = 26.67'C (DB) / 19.44'C (WB)
Ta,o = 12.46'C (DB) / 11.40'C (WB)

Q'e = 13.025 kW (R-134a)
va,in = 2.03 m/s

Measured : Va,in~ 3.3 - 3.6 m/s (based on fan selection).

Compressor needs matching as follows (subject to TXV):
Q'e = 7.0 kW @ Tc,sat=40'C, Te,sat=15'C
Q'e = 5.9 kW @ Tc,sat=55'C, Te,sat=15'C
Q'e = 4.3 kW @ Tc,sat=75'C, Te,sat=15'C

I have added in the design air in/out temps.

[desA]

One would presume that this is a hotel in Bankock? The water that you are heating itself is not sanitary? (HX elsewhere) I have a product which could do this "effective COP" approx "10" But this not the place to share the design "sorry",

This is correct - this is a thread about "AWHP superheat & sub-cooling". You are most welcome to discuss marketing of your high-performance product in SE Asia, with me per private mail, or my e-mail.

as far as storage, this is a fair amount of water (weight volume) I would suggest you store the energy in a phase change material, this would also keep your Ct very constant (time basis) This would reduce your tank requirement and also keep outlet temps constant

A very good idea &, as it happens, I have developed just the cost-effective PCM to serve this purpose.

The thing with these energy-storage systems is always the initial cost, unfortunately. We can open up a separate thread for these, if you like - an idea well worth discussing.

[mad fridgie]

Hi, yes please open a seperate thread regarding phase change

[Gary]

I have added in the design air in/out temps.

The point is the larger the coil the more heat it absorbs at low temperatures. The coil that is the best match at higher temp is not necessarily the best match at lower temp.

[desA]

The point is the larger the coil the more heat it absorbs at low temperatures. The coil that is the best match at higher temp is not necessarily the best match at lower temp.

This is very true & the coil must be selected to suit the operating condition.

[desA]

I'm going to throw a spanner in the works here.

I've been 'reverse-simulating' from the measured power input, less the fan = compressor input power, to compute the system operating point.

This technique shows up some very interesting details, for this particular heat-pump:

1. The operating m'g [g/s] (refrigerant mass flow) always drags the theoretical maximum TXV orifice capability, by a long margin & seems to stay reasonably constant in the range Tc,sat=50'C to 65'C.
2. The theoretical maximum output of this unit is around 6.67 kW at the 'sweet spot' of orifice & compressor curves.
2A. This sweet spot is the same as my tank heat-up measurements.
3. My theoretical calculations for the coil used in this heat-pump, predict a maximum Qc of ~ 7kW (best-case scenario).

Tentative diagnostic:
1. Condenser coil is undersize;
2. This causes the TXV to throttle-back to a low m'g;
3. System bottleneck is currently the condenser;
4. Evap merely follows in what the condenser can manage.

Now, if there is agreement with this, here's what I propose to do - unless we have more business to do on this particular configuration:
a. Swap in a new larger tube condenser. I have a new 3-coil unit on hand (Qc ~ 10.51 kW);
b. Re-balance system & move forwards.

This will take some time, as I will be out of office most of next week. I expect to be able to push the unit up a few notches with such a modification.

[desA]

Concept - alternative for CPR

A thought, as an alternative to a CPR, in a budget heat-pump system. Could a suitably-sized (over-sized) service valve be used?

Lock the valve in a setting which reduces Te,sat until equals 15'C (pressure 0.387 MPa(g)), during set-up, or on-site installation. Servicing by trained, qualified personnel only.

[Gary]

I'm going to throw a spanner in the works here.

I've been 'reverse-simulating' from the measured power input, less the fan = compressor input power, to compute the system operating point.

This technique shows up some very interesting details, for this particular heat-pump:

1. The operating m'g [g/s] (refrigerant mass flow) always drags the theoretical maximum TXV orifice capability, by a long margin & seems to stay reasonably constant in the range Tc,sat=50'C to 65'C.
2. The theoretical maximum output of this unit is around 6.67 kW at the 'sweet spot' of orifice & compressor curves.
2A. This sweet spot is the same as my tank heat-up measurements.
3. My theoretical calculations for the coil used in this heat-pump, predict a maximum Qc of ~ 7kW (best-case scenario).

Tentative diagnostic:
1. Condenser coil is undersize;
2. This causes the TXV to throttle-back to a low m'g;
3. System bottleneck is currently the condenser;
4. Evap merely follows in what the condenser can manage.

Now, if there is agreement with this, here's what I propose to do - unless we have more business to do on this particular configuration:
a. Swap in a new larger tube condenser. I have a new 3-coil unit on hand (Qc ~ 10.51 kW);
b. Re-balance system & move forwards.

This will take some time, as I will be out of office most of next week. I expect to be able to push the unit up a few notches with such a modification.

The compressor is rated at 7kW at Te,sat=15C and Tc,sat=40C.

The chart for run #7 doesn't show Tc,sat=40C, but it appears that if it did the Te,sat would have been at least 15C.

So... there doesn't appear to be a bottleneck.

However... with the larger condenser we could drop the cond approach down closer to 5K for improved performance.

In any case, a larger condenser won't hurt.

[Gary]

Concept - alternative for CPR

A thought, as an alternative to a CPR, in a budget heat-pump system. Could a suitably-sized (over-sized) service valve be used?

Lock the valve in a setting which reduces Te,sat until equals 15'C (pressure 0.387 MPa(g)), during set-up, or on-site installation. Servicing by trained, qualified personnel only.

The lower cost alternative to the CPR is the MOP. Although the MOP raises superheat, the fan speed control will compensate for this and bring the superheat back down.

[desA]

The compressor is rated at 7kW at Te,sat=15C and Tc,sat=40C.

The chart for run #7 doesn't show Tc,sat=40C, but it appears that if it did the Te,sat would have been at least 15C.

So... there doesn't appear to be a bottleneck.

However... with the larger condenser we could drop the cond approach down closer to 5K.

In any case, a larger condenser won't hurt.

Just checking on the compressor rating again at Te,sat15'C, Tc,sat=40'C - using suppler software:

Sub-cooling SC=3K:
Q'e = 7.0 kW
W'in = 1.1 kW
Q'c = 8.1 kW

Sub-cooling SC=7K:
Q'e = 7.3 kW
W'in = 1.1 kW
Q'c = 8.4 kW

The reason for the showing the sub-cooling option is that the tube-in-tube coils seem quite happy to operate with a sub-cooling of 7-8.5K, whereas the best that a plate will give is around 0.5-4K (3K typical).

In this way, I must say, the tube-coil condensers seem to have their place as a combination condenser/sub-cooler. They are incredibly bulky, however.

[desA]

The lower cost alternative to the CPR is the MOP. Although the MOP raises superheat, the fan speed control will compensate for this and bring the superheat back down.

Just some brief feedback on a CPR trial on another machine, is that these work incredibly well. I am most impressed indeed.

The 'boys' got onto the spanners & were trying to get the suction gas temp down to 15'C, instead of Te,sat=15'C. Frantic e-mail shows that Te,sat=-0.5'C was reached & the only reason for stoppage was that the Tcomp,disc rose to around 105'C.

Lol...

Training time tomorrow morning...

[desA]

I'm hoping to have a better idea of just how well these CPR's operate tomorrow. I have a sneaky feeling that this may be a super solution.

The trick then is surely to push the evap a little harder & push up the Te,sat to 15'C & hold it there. Perhaps this could assist in early startup performance?

So, basically - push the evap a little harder at startup, then back off as load climbs.

[Gary]

However... with the larger condenser we could drop the cond approach down closer to 5K for improved performance.

To expand on this:

If our approach is 10K, then we need Tc,sat=75C to get 65C leaving water.

If our approach is 5K, then we need Tc,sat=70C to get 65C leaving water.

So... reducing the approach will raise the COP.

[desA]

Fan-speed control system - re-visited

So far:
1. Input signal from compressor discharge temp;
2. Fan speed curve set to lower fan speed as heating cycle climbs;
3. Curve for (2) still to be determined;
4. Fan speed modulation mechanism to be discussed.

To address:
a. What should fan do when max Tcomp,d reached? Should it drop to say 50% to limit heat-generation?
b. Can a second input signal be used to trim the fan profile;
b.1 Target Tevap,exit; or
b.2 Pressure P,evap;
c. Could a simple 2-speed motor be used, with pressure switches controlling the speeds? (Much like the option used to stop 1-fan for a 2-fan system)

I think that a few other options have been discussed along the way, & can be added (slip my mind at the moment).

[desA]

To expand on this:

If our approach is 10K, then we need Tc,sat=75C to get 65C leaving water.
If our approach is 5K, then we need Tc,sat=70C to get 65C leaving water.
So... reducing the approach will raise the COP.

Yes, very much so.

The larger coil I have will just need to be checked for optimum water flow circuiting as I seem to remember that all passes are in parallel. I'll check the 3x parallel refrigerant, 3x series water side as an option to 3x parallel refrigerant, 3x parallel water side.

One concern with the series water arrangement may be a slight condensing imbalance between the 3x parallel refrigerant coils... not overly sure about the inter-coil dynamics, for the moment.