New system tripping on low pressure??

[DTLarca]

Am I sure I don't see it? Yes.

Am I sure you are wrong? No.

Healthy scepticism

See the attached, Gary.

[mad fridgie]

I know I am very stupid (my wife tells me all of the time, and she is never wrong)
But what is the meaning of
BPF
ADP
??????????????????????

[DTLarca]

But what is the meaning of BPF ADP

If you had an air-on dry bulb temperature of 20°C and a cooling coil temperature of 0°C but the air came out the other side of the coil at 10°C then we would say that 50% of the air came into contact with the coil and cooled to 0°C and 50% made no contact with the coil and so left the other side still at 20°C. The two air streams mix giving a mixed temperature of 10°C. Thus the by pass factor is 0.5 and so too the contact factor is 0.5. The percentages thus have to be by mass - not volume - since the densities change.

ADP is the Apparatus dew point temperature. It has to be below the return air dew point temperature in order for there to be moisture removal. The ADP is perhaps 3 or more degrees above saturated suction temperature - sometimes as much as 12K above. The edge of the fins on the coil facing the return air will be close to room temperature while the inner tubing walls will be close to the refrigerant temperature - the ADP is the average between those two extremes and it depends on the design of the coil, the air velocity and the enthalpy of the return air.

But for a given return air condition, dry bulb and wet bulb, for any desired leaving air condition, dry bulb and wet bulb, there is only one possible ADP to do the job. That's when the process is a simple one step happening. It is different if your extended process line cannot meet the saturation curve - that's when you need reheat or heat pipes or a run-around but for a multi step process you are not stuck to one ADP for the cooling coil process.

[mad fridgie]

ADP what type of average, mid point,?
ADP very similar to LMTD?
Your chart show 2 different ADP with the same result, and it is the same coil, with the same air flow, so then your are playing with fluid temp, to get 2 different ADP, and allowing for BPF. Is this just a way of playing with numbers to prove a point.
So has no practical use unless you have the full coil specifications, including design BPF and ADP,
OK understand, so when I design or get designed a coil, we give inlet conditions, oulet condtions, fluid medium temps and flow rates for both.
If we have a variation one process variable we must have variation on another process variable. is this not the same thing, without giving going through another layer of steps.?

[Hans008]

Hey mate i'm not much of an air con expert but had the same sort of situation with 2 systems running on 410a.
on one of them the return air ducting was too small creating lake of air intake & causing it to trip on hp
the other one was overcharged it was tripping on hp @ 600 psi on heating. this was due to a shorter pipe run than what it was designed for.

cheers

[Willsmithiraq]

Especially for Gary is the fan picture. The direction of rotation in this shot is anti-clockwise as the air flow is moving right to left in the picture.

The other picture is inside the suction header of the evaporator showing that there is no restriction there (was playing on my mind so I double checked)

[Gary]

Healthy scepticism

See the attached, Gary.

Maybe after a few more cups of coffee it will sink in.

In any case, would you agree that a volume of air at 11C WB when warmed to 25C DB will be at ~40% RH?

[Gary]

Especially for Gary is the fan picture. The direction of rotation in this shot is anti-clockwise as the air flow is moving right to left in the picture.

The other picture is inside the suction header of the evaporator showing that there is no restriction there (was playing on my mind so I double checked)

Hmmm... seems the rotation is correct, then.

[Gary]

We need to rethink this:

On a recirculating A/C system, we would normally drop the temp of the air about 11K, but on a fresh air system where we are conditioning very warm outdoor air we may need to drop the temp twice as far, so we would need twice the compressor and twice the condenser. IOW, the evap would be undersized in relation to the condensing unit.

Given this relative sizing, we might expect the low side pressure to run very low in mild weather and we would need a means of unloading our compressor, such as hot gas bypass.

What is surprising is the evap leaving air temp not being low also, but as DTLarca points out, this could be explained by a high bypass factor coil.

This being a reverse cycle machine adds to the complications, because in heat mode the evap becomes the condenser, which is undersized. In heat mode we will need to reduce the load to match the undersized condenser.

Possibly we can control the load in heat mode by cycling the outdoor fan.

An electrical diagram would be helpful.

[Willsmithiraq]

I have an electrical diagram on pdf but it's too large to upload. Any suggestions??

[DTLarca]

This being a reverse cycle machine adds to the complications, because in heat mode the evap becomes the condenser, which is undersized. In heat mode we will need to reduce the load to match the undersized condenser.

Possibly we can control the load in heat mode by cycling the outdoor fan.

An electrical diagram would be helpful.

When this unit is in heating it will be heating outdoor air which, when it is in heating, will be cooler than normal return air. So, it is okay for the indoor coil to be smaller than normal.

[DTLarca]

Maybe after a few more cups of coffee it will sink in.

In any case, would you agree that a volume of air at 11C WB when warmed to 25C DB will be at ~40% RH?

No

I could give you a final RH, after a 100% sensible reheat to 25°C, anywhere from 42% to 23% using ADP's from 11°C to 2°C using different BPF's.

[TRASH101]

I could give you a final RH, after a 100% sensible reheat to 25°C, anywhere from 42% to 23% using ADP's from 11°C to 2°C using different BPF's.

I don't think you read Garys statement right DTLarca, I don't think I have ever seen a psychrometric chart that, for a given water content for a volume of air undergoing a gain in enthalpy brought about by sensible heating, could imply such diverse results. Could you elaborate?

[DTLarca]

I don't think you read Garys statement right DTLarca, I don't think I have ever seen a psychrometric chart that, for a given water content for a volume of air undergoing a gain in enthalpy brought about by sensible heating, could imply such diverse results. Could you elaborate?

But 11°C wet bulb does not imply any given dew point.

Along the 11°C wet bulb line we have specific moisture contents ranging from a maximum possible at 0.0082kg/kg to the minimum of 0.000kg/kg.

[Gary]

Life was so much simpler when I could just ignore BPF.

Seems I will have to start using air off dewpoint.

[TRASH101]

But 11°C wet bulb does not imply any given dew point.

Along the 11°C wet bulb line we have specific moisture contents ranging from a maximum possible at 0.0082kg/kg to the minimum of 0.000kg/kg.

Sorry DTL its me that can't read I was sure that it said at 100% rh.

[Gary]

What has worked well for me over the years is to simply manipulate the air off temp to get the humidity I want in the room.

[Gary]

Generally, I tell people that for comfort A/C, in order to achieve 50%RH we want the air off temp to be 11K/20F lower than the temp at the thermostat. This ignores bypass factor (BPF), and the BPF is generally low enough in recirculating systems to ignore and still get the humidity in the ballpark. Not so in fresh air systems where the BPF can be substantial.

Perhaps it would be more useful/accurate, albeit more cumbersome to say that for comfort A/C, in order to achieve 50%RH the air off dewpoint temp should be 11K/20F lower than the temp at the thermostat.

[DTLarca]

Life was so much simpler when I could just ignore BPF.

Seems I will have to start using air off dewpoint.

No

Not a good idea. I mean the ideas as far as ideas go are good ideas but this particular instances details do not seem very good.

But you have inspired in me an idea I shall experiment with later

[mad fridgie]

BPF,is to be applied to the design of a coil or are you looking at appling to commisioning and service.
A sealed coil under a set of fixed conditions will always perform the same (regardless of the design) and how you want to describe the design.
However when installed other factors come into play, assuming that airflow is as spec and air on is as per spec, but you may get air leakage for example air under the drip tray. this would mean that your actual coil design is comprimised. If though the air handler is not. Would this then be the only point at which you would use BPF as away of explaining variances in ADP (or I would call it different SST/superheat for dx, or difference fluid return temps for water or glycol) and effectrively out put performance ( and practically you would do regually without thinking about it, measure all variables and assimulate against precieved design, so to underdake correct calculations you must first have a completed design specification cover all a range of different condtions) then call any variants what ever you want, just a set of name.
in the same way some us dry/ wet bulb and others use dry bulb and RH, same result and does not apply that neither understand the principles of air even if they are not aware of each others method.

[DTLarca]

ADP what type of average, mid point,?

ADP very similar to LMTD?

Good question - I don't know how it would best be described if it was to be described in a predictive fashion. But I do expect it to suspended beneath LMTD and so lifted with LMTD. But would we use the logarithmic curve (Newton's cooling curve) to predict where it shall fall between RA°C and SST°C - I do not know. Some experimentation is called for...

On the next AHU job measure WB and DB on and off and also SST. Then determine ADP by extending the line between RA and SA conditions to the saturation curve then compare ADP to SST and then the ratios of them.

Because the heat flux is through a volume of material toward a centre the flux density will increase as the cube every half distance from the outer fins to the refrigerant and since heat flow is proportional to TD the TD between 1 mm and the next, heading toward the refrigerant, should increase at a cubed rate every half distance. So if the TD per mm is 2K at the fin edge it should be 8K per mm half way closer to the refrigerant etc. That's an exponential rise - not a logarithmic rise. Perhaps a discussion topic for another day.

Your chart show 2 different ADP with the same result, and it is the same coil, with the same air flow, so then your are playing with fluid temp, to get 2 different ADP, and allowing for BPF. Is this just a way of playing with numbers to prove a point.

No, for my examples they would have to be different coils for the same airflow for such differing BPF's. I was only showing how different two systems can be even though they share the same leaving WB temp.

[DTLarca]

BPF,is to be applied to the design of a coil or are you looking at appling to commisioning and service.
A sealed coil under a set of fixed conditions will always perform the same (regardless of the design) and how you want to describe the design.
However when installed other factors come into play, assuming that airflow is as spec and air on is as per spec, but you may get air leakage for example air under the drip tray. this would mean that your actual coil design is comprimised. If though the air handler is not. Would this then be the only point at which you would use BPF as away of explaining variances in ADP (or I would call it different SST/superheat for dx, or difference fluid return temps for water or glycol) and effectrively out put performance ( and practically you would do regually without thinking about it, measure all variables and assimulate against precieved design, so to underdake correct calculations you must first have a completed design specification cover all a range of different condtions) then call any variants what ever you want, just a set of name.
in the same way some us dry/ wet bulb and others use dry bulb and RH, same result and does not apply that neither understand the principles of air even if they are not aware of each others method.

Consider the case where the commissioning engineer see's his supply air dt seems insufficient - he slows the fan down expecting dt to increase in proportion. He will be right because both the contact factor increase and the dwell time increase thus dt increases - but the SHR would have taken a major dive.

Should the engineer simply commission to achieve design parameters? - Sure, but what if design parameters turn out to be contrary to design intent?

I have attached a powerpoint slide and its notes from one of my HND lectures I gave the guys on coil selection - there are about 12 slides to the process but this one tells a bunch

[Gary]

Consider the case where the commissioning engineer see's his supply air dt seems insufficient - he slows the fan down expecting dt to increase in proportion. He will be right because both the contact factor increase and the dwell time increase thus dt increases - but the SHR would have taken a major dive.

Assuming all else is working properly... if he cannot increase compressor capacity, he can only decrease airflow. Are there any other options?

But I'm not sure dT is the point here. The point would be high supply air dewpoint.

[DTLarca]

Assuming all else is working properly... if he cannot increase compressor capacity, he can only decrease airflow. Are there any other options?

Yes - in the scenario I intended to represent above the guy was not supposed to try alter the dt - he should have known it was designed to have a high BPF - so his other option was to do nothing

[mad fridgie]

Yes - in the scenario I intended to represent above the guy was not supposed to try alter the dt - he should have known it was designed to have a high BPF - so his other option was to do nothing

Why would he think that the dt is insufficient, if the design spec indicates a set of inlet and outlet conditions.
As with any set heat exchange designs your are able to get the same end result with various designs.
If what i believe you are trying to explain, is the difference between actual design (what ever method it is called) and a commissioning engineers rule of thumb.
If in the above example the commissioning engineer was given the range of design conditions, in, out, air and working fluid, why would he question the td, only if the td was somewhat different to what the designed asked for. If the coil had been designed with BPF as a term, no problem here if this is how it is expressed, but the design outlet condition then would be predetermined.

[DTLarca]

Why would he think that the dt is insufficient, if the design spec indicates a set of inlet and outlet conditions.
As with any set heat exchange designs your are able to get the same end result with various designs.
If what i believe you are trying to explain, is the difference between actual design (what ever method it is called) and a commissioning engineers rule of thumb.
If in the above example the commissioning engineer was given the range of design conditions, in, out, air and working fluid, why would he question the td, only if the td was somewhat different to what the designed asked for. If the coil had been designed with BPF as a term, no problem here if this is how it is expressed, but the design outlet condition then would be predetermined.

But I have never known a design to work precisely well - out in the field much of the commissioning engineers job is completing the design making alterations to the design parameters so that the system finally might meet the design intent and the design intent might be known on paper but in the real world how one gets there is often quite different from designed operating parameters.

And in this case the OP hasn't got even the design parameters.

[Gary]

To keep this in tune with the system under discussion:

Assuming that we want 25C@40%RH, we would need to adjust the HGBV to maintain 11C supply air dewpoint.

How are we to determine that the airflow is right? Just visual inspection?

[mad fridgie]

But I have never known a design to work precisely well - out in the field much of the commissioning engineers job is completing the design making alterations to the design parameters so that the system finally might meet the design intent and the design intent might be known on paper but in the real world how one gets there is often quite different from designed operating parameters.

And in this case the OP hasn't got even the design parameters.

Total agree, he can neither alter the main guts of the system. It is a little unfair that it does seem that he does not even have a starting point for the design! His goal is to make run and reliable first, would you not agree, then he is able to take detailed measurements,
So going onto the practical for the OP.
Lets just look at air flow first.
He needs to work out face velocity, this should less than lets say 2m/s (as starting point and with some safety margin) to ensure we do not get water carry over.
This would also give us a range of possible load scenerios (presume that the fan is fixed speed)
Then we can look at the compressor duty (at a range of operating conditions)
Leaving efficiency out of the equation, we will end up with a performance line(s) between comp duty and possible air duty.
We should then be able to see simply what region/regime the system is designed to work in.
From this we can wotk out the best method of keeping the machine performing.

[DTLarca]

To keep this in tune with the system under discussion:

Assuming that we want 25C@40%RH, we would need to adjust the HGBV to maintain 11C supply air dewpoint.

How are we to determine that the airflow is right? Just visual inspection?

We want 25°C and 50%RH in the kitchen - if we are lucky.

You will be lucky to get 25°C and 50% RH in your supply duct in peak summer.

The compressor has no capacity regulation other than a hotgas injection which is to be set purely for coil frost prevention.

I have attached a sheet on how to measure air volume flow with just a short piece of see through hose and water.

[DTLarca]

Total agree, he can neither alter the main guts of the system. It is a little unfair that it does seem that he does not even have a starting point for the design! His goal is to make run and reliable first, would you not agree, then he is able to take detailed measurements,
So going onto the practical for the OP.
Lets just look at air flow first.
He needs to work out face velocity, this should less than lets say 2m/s (as starting point and with some safety margin) to ensure we do not get water carry over.
This would also give us a range of possible load scenerios (presume that the fan is fixed speed)
Then we can look at the compressor duty (at a range of operating conditions)
Leaving efficiency out of the equation, we will end up with a performance line(s) between comp duty and possible air duty.
We should then be able to see simply what region/regime the system is designed to work in.
From this we can wotk out the best method of keeping the machine performing.

Indeed - he needs to start off with the TECH Method from which he'll get a starting snap shot, base data, to watch trends and from the trends (varying ambients) extrapolate to possible optimisations or controls adjustments or additions.

[Gary]

We want 25°C and 50%RH in the kitchen - if we are lucky.

You will be lucky to get 25°C and 50% RH in your supply duct in peak summer.

The compressor has no capacity regulation other than a hotgas injection which is to be set purely for coil frost prevention.

To get 25C@50%RH, we would need 14C supply air dewpoint temp. With a good HGBV, this could be maintained in mild weather.

But yes... at some point the outdoor conditions would raise the pressure off this lower limit and they would be getting (relatively cool) wind.

We have no idea what the current dewpoint is, so it would be difficult to guesstimate at what point this might occur.

We could reduce the airflow to raise the point at which this happens, but presumably the airflow is what it needs to be.