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Re: AWHP superheat & sub-cooling
Thanks very much - now clear.
Quote:
We could improve upon this with a two stage temp control. With a two stage temp control the second set of contacts could shut off the fan if for any reason the discharge temp continues to rise above maximum.
I'd like to explore this option a little further, if possible.
A further alternative to consider - a simple 2-step:
What if a condenser fan speed controller is used - with a HP take-off, instead of temp at compressor discharge? The control signal would have to be modified to give reducing fan speeds with rising HP (the condenser control signal speeds up the fan with increasing HP).
This modified signal can then be used to adjust fan speed.
A variation on this is to have 2 fans, then stop one when the HP reaches a pre-determined value.
Under these options, a correlation between the thermodynamic Q'e (evap load) & the Tc,sat (or HP value) can easily be developed. This then allows the HP circuit to be used to control the LP circuit temp.
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Re: AWHP superheat & sub-cooling
Have you tested the iced drier yet?
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Re: AWHP superheat & sub-cooling
Quote:
Originally Posted by
Gary
Have you tested the iced drier yet?
Was trying to figure out a way to set up a holding tub around the drier, so that the ice stays in place. I'll play around a little later today...
I had a further idea - to use evaporative cooling effect by wrapping the drier in a rag & then soaking it in water. The evap airstream should evaporate the water & drop the local temp around the drier.
I'll get onto that... :D
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Re: AWHP superheat & sub-cooling
Quote:
Originally Posted by
desA
Revised plan of action:
1. Modify partition separating condenser from compressor.
2. Properly insulate condenser & partition walls.
3. Test system response to these changes.
4. Re-position filter drier to vertical orientation.
5. Test system response to this change.
6. Install water regulating valve.
7. Test system response to this change.
8. Adjust WRV & iterate adjustment/testing.
How is the plan going?
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Re: AWHP superheat & sub-cooling
^ I only arrived back yesterday... 10h overnight bus trips in Asia are a killer... Add to that a border crossing. :eek:
I'm off to sort out a few bits for the system, today. I have the tools to modify the panel, but still need some decent insulation - it was too heavy to lug up from BKK.
:D
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Re: AWHP superheat & sub-cooling
Quote:
Originally Posted by
desA
^ I only arrived back yesterday... 10h overnight bus trips in Asia are a killer... Add to that a border crossing. :eek:
I'm off to sort out a few bits for the system, today. I have the tools to modify the panel, but still need some decent insulation - it was too heavy to lug up from BKK.
:D
I suppose next you'll be claiming that you require sleep... lol
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Re: AWHP superheat & sub-cooling
Quote:
Originally Posted by
desA
A further alternative to consider - a simple 2-step:
What if a condenser fan speed controller is used - with a HP take-off, instead of temp at compressor discharge? The control signal would have to be modified to give reducing fan speeds with rising HP (the condenser control signal speeds up the fan with increasing HP).
This modified signal can then be used to adjust fan speed.
A variation on this is to have 2 fans, then stop one when the HP reaches a pre-determined value.
Under these options, a correlation between the thermodynamic Q'e (evap load) & the Tc,sat (or HP value) can easily be developed. This then allows the HP circuit to be used to control the LP circuit temp.
There are a great many control strategies which can be made to work. To my mind, the key temperature is the discharge temperature.
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Re: AWHP superheat & sub-cooling
Quote:
Originally Posted by
desA
To be honest, I'm currently more worried about the Te,sat drift upwards to 19'C, during the operational phase. In my view, the inward 'energy pump' via the evap, in excess of what the condenser can/should deliver (from a thermodynamic balance perspective), causes the cycle to lift, since the heat enters faster than it can be extracted.
The heat-pump cycle seems to be inherently unstable - distinctly different to an aircon system which is inherently stable, if managed correctly. We can talk more about this at a later stage. The evap control provides the necessary decelerator effect.
To my mind, Te,sat is not a concern so long as the compressor is kept cool. In fact, higher Te,sat reduces the compression ratio.
Reducing the airflow reduces the efficiency of the system, and is thus counter-productive. As I see it, the discharge temp/fan control strategy is purely for limiting the compressor heat while we find more ways to load the evaporator right up to the max.
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Re: AWHP superheat & sub-cooling
Quote:
Quote:
Originally Posted by desA
To be honest, I'm currently more worried about the Te,sat drift upwards to 19'C, during the operational phase. In my view, the inward 'energy pump' via the evap, in excess of what the condenser can/should deliver (from a thermodynamic balance perspective), causes the cycle to lift, since the heat enters faster than it can be extracted.
The heat-pump cycle seems to be inherently unstable - distinctly different to an aircon system which is inherently stable, if managed correctly. We can talk more about this at a later stage. The evap control provides the necessary decelerator effect.
Gary:
To my mind, Te,sat is not a concern so long as the compressor is kept cool. In fact, the higher Te,sat reduces the compression ratio.
If the condenser can't handle the output of the evaporator, then a bigger water pump is needed... or perhaps a bigger condenser.
Reducing the airflow reduces the efficiency of the system, and is thus counter-productive. As I see it, the discharge temp/fan control strategy is purely for limiting the compressor heat.
Heat-pump philosophy
The compressor is the heart of the system. To obtain a high COP, the evap & condenser must move in synchronous with the compressor, since we have an interacting system.
As I see it, pushing in excess heat through the evaporator, must be extracted by the condenser, without forcing up the compressor motor power. The system starts at maximum COP & gradually slides down, as the system moves towards the hot condition.
The evap fan is the input throttle, the condenser water flow is the condenser accelerator. Managing these two ends carefully, will keep the system on the correct thermodynamic balance & optimum COP curve. Increasing the condenser water flow will lower Tc,sat slightly, also increasing COP slightly. So, essentially, anything that forces a system imbalance may end up pulling the system away from the optimum thermodynamics.
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Re: AWHP superheat & sub-cooling
Fan speed regulation via dimmer switch
How far can the motor speed safely be reduced?
The dimmer switch introduces a slight motor hum.
I've read about this hum & the reasons for it. Some folks suggest to rather use a fan speed controller - as per ceiling fan adjustment. Well, I did try that option today, without much success - the arcing between switch settings concerned me - as did the rapid fall-off in speed from 100% to supposed 80% position (turns out was more like 20%). Went back to the dimmer switch - hum & all - in combination with my sliding panel tricks. :)
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Re: AWHP superheat & sub-cooling
Lab machine retrofit
I'll get onto the lab machine modifications tomorrow. The first work will be to open up the barrier around the compressor & to insulate the condenser with decent insulation (have three rolls on hand, plus some other materials). Didn't want to spend too much on this machine, as it is only for testing & evaluation purposes.
The water regulating valve has still eluded me, but, for now, I propose to manually set the flow via an inline gate valve - until a suitable valve can be sourced. May have to drive a valve through an actuator & controller, if pressure/flow valve cannot be sourced. Any ideas on this?
My HVAC supplier has my filter-drier on order, & so I'm loathe to cut into the refrigeration line until all parts are in-house. I can find alternative filter-driers, but did not want to change too many things at once - firm base to firm base.
(Sidebar: This filter-drier oder thing is a direct consequence of the world economic slowdown. I'm talking about one of Thailand's main parts suppliers here, not some fly-by-night. The stock imports are slowing down - it's clear).
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Re: AWHP superheat & sub-cooling
If you can simply tilt the drier downward in the direction of flow, it will help.
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Re: AWHP superheat & sub-cooling
Quote:
Originally Posted by
desA
Heat-pump philosophy
The compressor is the heart of the system. To obtain a high COP, the evap & condenser must move in synchronous with the compressor, since we have an interacting system.
I agree.
Quote:
Originally Posted by
desA
As I see it, pushing in excess heat through the evaporator, must be extracted by the condenser, without forcing up the compressor motor power. The system starts at maximum COP & gradually slides down, as the system moves towards the hot condition.
The excess heat is being returned to the evaporator in the form of hot liquid entering the TXV. That's what the iced drier test is going to demonstrate.
Quote:
Originally Posted by
desA
The evap fan is the input throttle, the condenser water flow is the condenser accelerator. Managing these two ends carefully, will keep the system on the correct thermodynamic balance & optimum COP curve. Increasing the condenser water flow will lower Tc,sat slightly, also increasing COP slightly. So, essentially, anything that forces a system imbalance may end up pulling the system away from the optimum thermodynamics.
If the condenser is the weak link, and I'm not certain that it is, then why decelerate the rest of the system to match the weak link? Let's find ways to strengthen that link.
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Re: AWHP superheat & sub-cooling
Quote:
Originally Posted by
Gary
If you can simply tilt the drier downward in the direction of flow, it will help.
Ok, great. That can certainly be done. I've got a decent pipe-bender here. I'll work on getting that set tomorrow.
Thanks for that advice.
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Re: AWHP superheat & sub-cooling
Quote:
Quote:
Originally Posted by desA
As I see it, pushing in excess heat through the evaporator, must be extracted by the condenser, without forcing up the compressor motor power. The system starts at maximum COP & gradually slides down, as the system moves towards the hot condition.
Gary:
The excess heat is being returned to the evaporator in the form of hot liquid entering the TXV. That's what the iced drier test is going to demonstrate.
This makes absolute sense, in that the evap/condenser imbalance will force heat-recirculation within the refrigerant loop itself. This is obviously something we really don't need.
I'll get that test done early tomorrow, before modifying the machine. Then at least we only have one change at a time.
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Re: AWHP superheat & sub-cooling
Quote:
Quote:
Originally Posted by desA
The evap fan is the input throttle, the condenser water flow is the condenser accelerator. Managing these two ends carefully, will keep the system on the correct thermodynamic balance & optimum COP curve. Increasing the condenser water flow will lower Tc,sat slightly, also increasing COP slightly. So, essentially, anything that forces a system imbalance may end up pulling the system away from the optimum thermodynamics.
Gary:
The condenser is the weak link. Why decelerate the rest of the system to match the weak link? Let's find ways to strengthen that link.
The condenser will always be the bottle-neck, if the system is sized on what appears to be an air-conditioning philosophy. "Design evap oversize, blast-cool condenser if P,hi goes up too far".
For the heat-pump, it seems that the condenser must be sized properly to pull out the heat correctly. The evap throttle is there to lag the condenser pull, as I see it.
Now, how to push the condenser further?
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Re: AWHP superheat & sub-cooling
I'm not certain that the condenser is indeed the weak link. Is the condenser unable to handle that heat?... or is that heat being stolen away from the condenser?
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Re: AWHP superheat & sub-cooling
Quote:
Quote:
Originally Posted by Gary
If you can simply tilt the drier downward in the direction of flow, it will help.
desA:
Ok, great. That can certainly be done. I've got a decent pipe-bender here. I'll work on getting that set tomorrow.
Thanks for that advice.
All done. The downward slope is now around 30' off the horizontal, facing downwards in direction of flow.
What reaction are we expecting to see from the system?
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Re: AWHP superheat & sub-cooling
Quote:
Originally Posted by
desA
All done. The downward slope is now around 30' off the horizontal, facing downwards in direction of flow.
What reaction are we expecting to see from the system?
If the system is sufficiently charged, then it probably won't make any difference. If the system were a little undercharged, then it would make a BIG difference. Liquid at the TXV inlet as opposed to vapor at the TXV inlet.
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Re: AWHP superheat & sub-cooling
Quote:
Originally Posted by
Gary
I'm not certain that the condenser is indeed the weak link. Is the condenser unable to handle that heat?... or is that heat being stolen away from the condenser?
Condenser review
1. Tube-in-tube coil;
2. Estimated heat-transfer = 7kW (heat-transfer simulation - internal procedure)
3. Compressor calculations - Te,sat=12.5'C:
3.1 At Tc,sat=40'C : Qc=7.8kW;
3.1 At Tc,sat=50'C : Qc=7.5kW;
3.3 At Tc,sat=70'C : Qc=6.8kW;
4. Condenser originally un-insulated (lab m/c), located inside condenser/compressor compartment.
5. Condenser response to rough insulation fit-up was increase of around 10% on overall heat duty.
So, I'd say that the condenser is marginally sized - with no over-surface margin to play with. The lack of insulation is daft.
I'll have the insulation looking pretty good tomorrow - we can then see what it's effect is - before I remove the partition.
When running this system, it always feels like the condenser is under-performing.
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Re: AWHP superheat & sub-cooling
Quote:
Quote:
Originally Posted by desA
All done. The downward slope is now around 30' off the horizontal, facing downwards in direction of flow.
What reaction are we expecting to see from the system?
Gary:
If the system is sufficiently charged, then it probably won't make any difference. If the system were a little undercharged, then it would make a BIG difference.
For this condenser, when we insulated it - the approach temps reduced from 8.1K to 7.75K (at Tc,sat=75'C), & SC reduced dramatically from 8.5K to 5.8K.
Before that point, the data seemed to point to a slightly overcharged system. After the insulation addition, it may in fact be close to correct charge, or marginally under-charged.
What big changes could be expected for an under-charged system?
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Re: AWHP superheat & sub-cooling
Quote:
Originally Posted by
desA
Condenser review
1. Tube-in-tube coil;
2. Estimated heat-transfer = 7kW (heat-transfer simulation - internal procedure)
3. Compressor calculations - Te,sat=12.5'C:
3.1 At Tc,sat=40'C : Qc=7.8kW;
3.1 At Tc,sat=50'C : Qc=7.5kW;
3.3 At Tc,sat=70'C : Qc=6.8kW;
4. Condenser originally un-insulated (lab m/c), located inside condenser/compressor compartment.
5. Condenser response to rough insulation fit-up was increase of around 10% on overall heat duty.
So, I'd say that the condenser is marginally sized - with no over-surface margin to play with. The lack of insulation is daft.
I'll have the insulation looking pretty good tomorrow - we can then see what it's effect is - before I remove the partition.
When running this system, it always feels like the condenser is under-performing.
And yet the approach keeps improving.
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Re: AWHP superheat & sub-cooling
^ Very true.
I'll get the insulation as tight as possible & probably close in the modified compartment to seal the coil end from the compressor. With all the piping in the way, this divide will probably be some sheet insulation, sealed along the edges with insulation.
What would you expect the temperature of the outside surface of a well insulated condenser to be?
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Re: AWHP superheat & sub-cooling
Quote:
Originally Posted by
desA
For this condenser, when we insulated it - the approach temps reduced from 8.1K to 7.75K (at Tc,sat=75'C), & SC reduced dramatically from 8.5K to 5.8K.
Before that point, the data seemed to point to a slightly overcharged system. After the insulation addition, it may in fact be close to correct charge, or marginally under-charged.
What big changes could be expected for an under-charged system?
If the system were undercharged we would have seen high superheat. I would not expect any changes from tilting the drier on this system.
However... tilting the drier gives you a lot more wiggle room on the charge, which can make a big difference when these systems are serviced in the field.
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Re: AWHP superheat & sub-cooling
^ Agreed. Angling the filter-drier with the exit towards the bottom makes for sound engineering.
I think that many of us never really thought about it as if it were a funnel system - managing full liquid to the TXV.
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Re: AWHP superheat & sub-cooling
Quote:
Originally Posted by
desA
^ Agreed. Angling the filter-drier with the exit towards the bottom makes for sound engineering.
I think that many of us never really thought about it as if it were a funnel system - managing full liquid to the TXV.
This is extremely important on cap tube systems as the charge is critical.
It is less important on TXV systems because they either have a receiver for extra refrigerant or they back extra refrigerant up into the condenser, thus assuring a full flow of solid liquid through the drier.
Having neither of these options, the charge becomes critical on this system and the drier, such as it is, is the closest thing we have to a receiver... if it is tilted downwards.
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Re: AWHP superheat & sub-cooling
Quote:
Originally Posted by
Gary
It is less important on TXV systems because they either have a receiver for extra refrigerant or they back extra refrigerant up into the condenser, thus assuring a full flow of solid liquid through the drier.
Having neither of these options, the charge becomes critical on this system and the drier, such as it is, is the closest thing we have to a receiver... if it is tilted downwards.
That is pretty cunning, I must say... :)
Now, that said, would it then be a useful design strategy to perhaps use large-size (larger than for reasonable water extraction) filter-driers for these systems?
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Re: AWHP superheat & sub-cooling
Quote:
Originally Posted by
desA
That is pretty cunning, I must say... :)
Now, that said, would it then be a useful design strategy to perhaps use large-size (larger than for reasonable water extraction) filter-driers for these systems?
Throughout our tests the superheat has remained impressively stable, even with the drier horizontal. I see no particular need for more surplus refrigerant, so no need for a larger drier.
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Re: AWHP superheat & sub-cooling
Quote:
Originally Posted by
desA
When running this system, it always feels like the condenser is under-performing.
No matter how well the condenser performs, the leaving refrigerant temp can be no lower than the incoming water temp. As the incoming water temp rises, the leaving refrigerant temp must rise and the temp at the TXV inlet must rise and the Te,sat must rise... unless the liquid line is cooled.
VIC to the rescue.
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Re: AWHP superheat & sub-cooling
Quote:
Originally Posted by
Gary
Throughout our tests the superheat has remained impressively stable, even with the drier horizontal. I see no particular need for more surplus refrigerant, so no need for a larger drier.
Thanks Gary.
The driers typically used, contain around 207 to 257g of refrigerant. This is already around 20-25% of the total refrigerant circuit mass charge, so if mounted vertically, the filter-drier would provide an excellent storage & feed buffer for the HP circuit.
I'd imagine that, for this kind of device, a taller-slimmer drier may feed better than a shorter-fatter option. A few things for me to think about down the track.
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Re: AWHP superheat & sub-cooling
Initial feedback after today's system retrofit
1. Create air opening in partition - to allow flow over the compressor;
2. Insulate condenser coil thoroughly;
3. Tilt filter-drier (last evening).
Initial runs are showing SC ~ 1.6K... lol... :)
I'll do a complete proving run tomorrow, then work out how much refrigerant to add back in...
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Re: AWHP superheat & sub-cooling
Quote:
Originally Posted by
desA
Initial feedback after today's system retrofit
1. Create air opening in partition - to allow flow over the compressor;
2. Insulate condenser coil thoroughly;
3. Tilt filter-drier (last evening).
Initial runs are showing SC ~ 1.6K... lol... :)
I'll do a complete proving run tomorrow, then work out how much refrigerant to add back in...
How is the superheat looking?
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Re: AWHP superheat & sub-cooling
Quote:
Originally Posted by
Gary
How is the superheat looking?
I did look at that, but will do a full set of runs tomorrow. SH was around the 7-7.5K mark, much like in the past. That part seems to be behaving itself.
What was interesting is that the SC at TXV inlet is around 7.5K (added this sensor today). This means that the bare liquid line, with filter-drier in the evap off stream, is cooling fairly well.
I hope to have a full set of readings tomorrow. I'll do the run at 100% fan speed, so as not to distort the picture. We can then compare like-with-like.
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Re: AWHP superheat & sub-cooling
Have you added a Tw,in sensor? This is a crucial measurement that has been missing from past runs.
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Re: AWHP superheat & sub-cooling
Quote:
Originally Posted by
desA
I did look at that, but will do a full set of runs tomorrow. SH was around the 7-7.5K mark, much like in the past. That part seems to be behaving itself.
The refrigerant charge is sufficient to feed the coil, then.
So the question at this point is, how much refrigerant charge works best for the condenser?
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Re: AWHP superheat & sub-cooling
Quote:
Originally Posted by
desA
The water regulating valve has still eluded me, but, for now, I propose to manually set the flow via an inline gate valve - until a suitable valve can be sourced. May have to drive a valve through an actuator & controller, if pressure/flow valve cannot be sourced. Any ideas on this?
Maybe this will help:
http://www.grainger.com/Grainger/sta..._reseller.html
http://www.grainger.com/Grainger/items/4LZ98
http://www.grainger.com/Grainger/wwg...=27&submit.y=3
Hmmmm... these seem to be limited to 260psi (Tc,sat = 65C), which may not be high enough for our purposes.
On the other hand, modifying it may be as simple as adding a spacer under the spring.
http://www.grainger.com/Grainger/rp_...&iw=713&ih=600
And here a manufacturers tech bulletin:
http://cgproducts.johnsoncontrols.co...PDF/125687.PDF
High pressure valves:
http://cgproducts.johnsoncontrols.co...DF/1900575.PDF
http://cgproducts.johnsoncontrols.co...F/12011514.PDF
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Re: AWHP superheat & sub-cooling
Here is another small detail that can make a big difference:
When installing a device which uses a sensing bulb (TXV, temp control, etc.) the bulb should be mounted with the cap tube upwards.
If the cap tube is on the bottom, when the bulb gets warm the liquid portion of the charge can be pushed through the cap tube to the power element. Then the power element temp is sensed instead of the bulb temp.
It is also a good idea for the power element to be above the bulb with its cap tube on the bottom, so that any liquid can drain back to the sensing bulb.
Like the fine print in a contract, the devil is in the details.
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Re: AWHP superheat & sub-cooling
Filter-drier retention time
Quote:
Originally Posted by
desA
The driers typically used, contain around 207 to 257g of refrigerant. This is already around 20-25% of the total refrigerant circuit mass charge, so if mounted vertically, the filter-drier would provide an excellent storage & feed buffer for the HP circuit.
Based on the compressor currently in the lab heat-pump:
m'g = 38.4 g/s (refrigerant mass flow at nominal operating point)
Retention time in F-D = 257/38.4 = 6.7 sec
This provides a very handy accumulator, gas-liquid separator effect. :)
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Re: AWHP superheat & sub-cooling
Quote:
Originally Posted by
Gary
Have you added a Tw,in sensor? This is a crucial measurement that has been missing from past runs.
I've been 'sensor challenged' up here & will have to get a few more thermocouples sent up. (Tried last trip to BKK, but they were incredibly slow in dealing :( )
In the meantime, I can rig up a spare tank sensor, via a controller readout, onto the outside of the water inlet pipe. This may give a little thermal lag, but should be reasonable, if I lag it well-enough.
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Re: AWHP superheat & sub-cooling
Quote:
Quote:
Originally Posted by desA
I did look at that, but will do a full set of runs tomorrow. SH was around the 7-7.5K mark, much like in the past. That part seems to be behaving itself.
Gary:
The refrigerant charge is sufficient to feed the coil, then.
So the question at this point is, how much refrigerant charge works best for the condenser?
Agreed. It seems that the combination of unlagged liquid line & filter-drier tilting, have assisted the TXV in terms of feed temperature & liquid feed consistency. Excellent.
After today's test runs, we'll have an idea of how much refrigerant charge to add in to satisfy the condenser sub-cooling.