Can't suss strange paramaters on Denco downflow unit

[Coolie]

A brief history:

7 New Denco down-flow units serving two large server rooms. Units are outside the room and ducted in/out.
The units were commissioned by denco, who also maintained them for the 1 year warranty period. After that, my company took over the maintenance. On my first visit, in march, I found that all the fan-speed controllers were set so the Discharge pressure was 23Bar. I decided this is too high for R407c, so adjusted them down to 17Bar. The sub-cool/superheat were 10deg and 7deg respectively.

On my most recent visit I found a strange anomaly on all units. With the unit running at 100% capacity (two comps on a common circuit with two TEV's serving two evaps) there was considerable intermittent flashing in the sight-glass. (Readings taken on common suction line)

Discharge:17Bar; discharge temp 60deg; liquid 39deg
Suction: 4Bar; suction temp fluctuating between 5deg and 17deg.
This was the case on all 7 units (readings approx same for all units)
Now, the units were all commissioned in december, so very cold outside and thinking that they were not trimmed correctly to compensate for warmer ambients in the summer. Thus I deducted that the TEV was hunting for refrigerant, so on one of the units i charged liquid till the sight glass cleared, (4kg) with the aim to trim it to superheat of 7 and sub cool of 10. To my surprise the compressor started sweating and got extremely cold (7deg) indicating that liquid is returning to the comp. It was then that i noticed that the bulbs from the two TEV's were installed on a vertical section of pipe.
I took out the extra charge, again in liquid, and moved the bulbs to a horizontal section of pipe, but unfortunately this is about a meter away from the evaporator outlet and after the common suction pipe tee's off to each compressor, so it is effectively measuring the common suction temp.
The superheat settled down slightly, but still fluctuating. The sight glass was still flashing off, but not as much, and the comp was still cold, although not sweating as much. Thinking it might be overcharged I removed two kilo's, but still no joy apart from comp slightly warmer, but till cold.

I must add that the driers are not blocked, nor is the solenoid restricting flow as no TD.

Any suggestions will help me loads as I am going back on monday to try figure this out.

Thanks

:confused:

[nike123]

Intermittent flashing in sight glass is normal consequence of TEV actions (at least with R407C refrigerant).
You should charge by weight at nameplate and check subcooling.
Superheat will be controlled only when load at evaporator is near design conditions (in fact, at TEV control range of about 1/3 to full capacity). Otherwise it will be higher or lower than normal.
We need information's about air temperatures at exchangers (WB/DB) to evaluate functioning and you could explain how condensation is controlled (if it is controlled at all).

[Gary]

Hunting generally indicates that the orifice is too large for the heat load. This could mean that the orifice is too large or it could mean that the load is too small. Not enough heat load is usually an evaporator airflow problem.

Mount the bulbs back on the vertical lines where they belong.

[desA]

Mount the bulbs back on the vertical lines where they belong.

The Alco spec usually calls for bulb positioning on a horizontal line, near the evap discharge, before equaliser line tee-off.

Would it not be better to locate on a horizontal section of pipe nearer to each evap, than on the vertical lines?

Just an observation. :)

[Gary]

The Alco spec usually calls for bulb positioning on a horizontal line, near the evap discharge, before equaliser line tee-off.

Would it not be better to locate on a horizontal section of pipe nearer to each evap, than on the vertical lines?

Just an observation. :)


... and moved the bulbs to a horizontal section of pipe, but unfortunately this is about a meter away from the evaporator outlet and after the common suction pipe tee's off to each compressor, so it is effectively measuring the common suction temp.

Does this sound right?

[mad fridgie]

Firstly place TEV bulbs back to original place, ensure equalising pipe is after the bulb (there is a chance of liquid passing down this line)
Secondly I do not believe you have this much sub-cooling, I suspect lower liquid pressure, I think the liquid is being held up in the condensor.
The hunting and over feeding may be direcly related to your flash gas, the valve is struggling to control the swings, may be the system was designed to run at a head pressure.
I suspect coil design (evap and cond) are critical, not much room for variations (distribution, pressure drop etc)

[Grizzly]

A good post Coolie, well constructed with lots of info.
Just to add to the mix, which way up are the phials?
Have you considered that the power elements or orifices may be faulty?

Good luck with this one as like you it has me confused!
Grizzly

[desA]

Everyone is going to shout me down for this one, but here goes.

I would raise your superheat as per the Magoo rule (once the TXV bulbs are in the right place again):
SH = (0.6 to 0.7)*TD

The optimum point given as 0.65*TD.

If the system stabilises, we can discuss the possible reasons afterwards.

[nike123]

Everyone is going to shout me down for this one, but here goes.

I would raise your superheat as per the Magoo rule (once the TXV bulbs are in the right place again):
SH = (0.6 to 0.7)*TD

The optimum point given as 0.65*TD.

If the system stabilises, we can discuss the possible reasons afterwards.

This is not shooting down of you, just some observations.;)


In case of low load condition, there is no point in raising superheat when TXV is hunting.
It probably hunts because it cannot control such small load.
If it cant control that load how will you know when your superheat setting is OK except by adjusting it with number of turns from end position?
Yet again, it cannot control superheat if load is outside its control range.


Predicting TEV performance at reduced system capacities
is difficult due to the many influencing design factors present
in any system. These factors include: TEV sizing,
refrigerant distribution, TEV setting, evaporator coil
design, suction line piping, and bulb location. General recommendations
which address these factors are provided
below. By observing these recommendations, a conventional
TEV can be expected to operate satisfactorily down to
approximately 35 percent of its rated capacity. The Types
(E)BF, SBF, EBS, and O valves, featuring the balanced port
design, can be expected to operate satisfactorily down to
approximately 25 percent of its rated capacity.
Valve Size — The TEV should be sized as close as possible to
the system’s maximum designed heat load condition. A valve
with a capacity rating up to 10 percent below the full load conditions
may be selected if the system is to operate at reduced
loads for long periods of time, and if slightly higher than normal
superheats can be tolerated at full load conditionsThis supporting your approach:


Superheat Adjustment — The superheat setting of the TEV
should be set at the highest possible superheat that can be tolerated
at full load conditions. A high superheat setting will
reduce problems associated with mild TEV hunting at low load
conditions. High superheats are more acceptable on air conditioning
systems where the wide temperature difference
between the refrigerant and the air allows the TEV to operate
at higher superheats without a significant loss in coil capacity.Page 9-10
(http://www.icecoldcomputing.com/library/reflib/10-9.pdf)

[desA]

^ Excellent post, nike.

[Grizzly]

Yep I agree.
I nice post Nike!
That answers why we are getting repeat call back issues on a particular chiller.
As this runs continually at low load.
Given this outlook would it not be prudent in these situations to fit a smaller orifice? (where possible?)
Cheers Grizzly

[nike123]

Yep I agree.
I nice post Nike!
That answers why we are getting repeat call back issues on a particular chiller.
As this runs continually at low load.
Given this outlook would it not be prudent in these situations to fit a smaller orifice? (where possible?)
Cheers Grizzly


That coincide with underlined sentence from Sporlan paper!

[Jasper]

As this is a downflow unit check the amount of grilles serving the room!
If the air flow is compromised you will get low suction temps and sweating on the compressors

[TRASH101]

Coolie

I have had similar problems with Denco downflows and as "mad fridgie" has stated they seem to be very load sensitive.

You can trim it in by charge and less sub cooling and tweaking super heat but first check and double check that-

a) you get the rated air volume
b) that you have no short circuit from your floor grilles to the unit.

[Coolie]

Wow, I am flattered by the response. I was not expecting it to invoke as much interest....but it is a sticky one...

Well, an update after my visit today...

I found a place to mount the bulbs correctly on a horizontal section of pipe right at the exit from the evaporator, about 2" away from the header.
After allowing the system to run up at full load for about 10 minutes I checked the parameters again. There was still significant liquid slugging back to the comp's, so thought it prudent to remove the charge I added on friday. This then cause the flash gas to return to the sight glass, and the hunting to continue, but on a slightly more stable and slower basis.

Bear in mind that there are two evaporators on one refrigeration circuit. Each is fed by it's own TXV that is Tee'd from the common liquid line and the suction side of each evaporator then join to form the common suction, which then branches again to each comp.
My next step was to throttle down the TXV's in turn in an attempt to set the superheat for each individual evaporator. I actually lost count at how many times I turned the adjustment screw, but it was 15+ times. This was not enough to fully close the valve, so I am guessing that the valves were almost fully open.

Before making any of these adjustments, the suction temp was fluctuating between 5deg and 17deg constantly. (climbing to 17, then immediately dropping very quickly to 6deg. The suction pressure was pretty constant at 4.1 to 4.3 bar) There was also a lot of flashing in the sight glass.

After these adjustments, the suction temp was still fluctuating, but between 10deg and 16deg, with the suction pressure still between 4.1 and roughly 4.5.
The flashing in the sight glass had now stopped.
The sub-cooling is also better at 9deg

The main issue, liquid returning to the comp's, has now stopped. The crank case is nice and warm, and the oil equalising line, which was 5deg and sweating profusely :rolleyes:, is now resembling something normal.

Someone mentioned orfice size?
This crossed my mind, but I ruled it out, partly because my boss said it's unlikely, and partly because I think there is a fundamental design flaw with the unit.

Remember the set-up of the TXV's? Branching from the common liquid line?
Well, my best educated guess is (feel free to correct me if you think i am on the wrong track) that they are in a constant battle against each other for the correct amount of refrigerant.
Where TXV 1 needs a bit more, it opens up and starves TXV 2, resulting in the superheat dramatically increasing in evaporator 2. TXV 2 then opens up wider resulting in a massive rush of liquid to enter evaporator 2. As there is not a big enough load, and because there is too much liquid in evaporator 2, the compressor unfortunately gets it. Now that there is all this liquid in evaporator 2, the superheat shrinks dramatically, and TXV 2 closes fully, resulting in evaporator 2 to empty all that liquid to the comp's.
Now while all this is going on in Evaporator 2, TXV 1 has been dramatically starved, so it opens fully.
And so it carries on.

Solution 1: move the external equalising line onto the common suction line, and change the liquid filled power assemblies for vapour filled ones as they react faster.

Solution 2: As above, but also install two solenoid valves, one before each TXV, and connect them so it loads both when both comps are running, and loads one when only one comp is running.

TRash101: There is no short cycle of the air.

[Gary]

It's about heat load vs orifice size.

You can increase the heat load by increasing the airflow or you can downsize the orifice.

But wait... there is another option. You can raise the discharge pressure... which puts hot liquid at the TXV inlet... which puts a false heat load on the evaporator.

[Denco Technical]

I would suggest you have overcharged this system, i take it because the last post is in 2004 its sorted?

Rob

[mgtet]

i know this is an old post but i look after a site with a range of kit on site, i look after all but the denco equipment, denco maintain the denco equipment but i am due to inherit it soon. It sounds like the units mentioned are the same model and i know of problems where they have increased the head pressure to around 23-24bar to keep the suction pressure up and to stop the evaporators from freezing up. These units have a very long pipe run, 100m+, it has been suggested that the compressors were increased in capacity to allow for the long pipe run. In my humble opinion the compressors are oversized for the evaporators and the only way to decrease system capacity/duty without notifying the customer is to run at an increased head pressure. I personally would reccomend fitting hot gas bypass but it is never going to happen as my customer put the project in place and obviously the end user is never going to pay for this. Not systems i'm looking forward to inheriting!!

[Denco Technical]

i know this is an old post but i look after a site with a range of kit on site, i look after all but the denco equipment, denco maintain the denco equipment but i am due to inherit it soon. It sounds like the units mentioned are the same model and i know of problems where they have increased the head pressure to around 23-24bar to keep the suction pressure up and to stop the evaporators from freezing up. These units have a very long pipe run, 100m+, it has been suggested that the compressors were increased in capacity to allow for the long pipe run. In my humble opinion the compressors are oversized for the evaporators and the only way to decrease system capacity/duty without notifying the customer is to run at an increased head pressure. I personally would reccomend fitting hot gas bypass but it is never going to happen as my customer put the project in place and obviously the end user is never going to pay for this. Not systems i'm looking forward to inheriting!!



Okay first of all you are taking the machine out of its design parameters running the head at them kinds of pressures.


If the suction is low, this would be suggesting to me that either the pipe sizes are too small (for a 100mt pipe run) or the airflow is down on the unit. What are the pipe sizes

Is the unit a upflow or downflow?
What is the airflow (20 point traverse on the return air stream)

Whats the external resistance of the unit? (measure resistance between inlet and outlet in Pa)

What speed are the evap fans running

Whats your delta T

Superheat?

Liquid line temperature
What type of compressors are fitted?
What type of evaporator is fitted (plate on side)
If you can supply me them details with the model, serial and contract number then i can plot it on the system. and we can see why the suction is low.

[aeb200]

Okay first of all you are taking the machine out of its design parameters running the head at them kinds of pressures.

[LIST]
If the suction is low, this would be suggesting to me that either the pipe sizes are too small (for a 100mt pipe run) or the airflow is down on the unit. What are the pipe sizes

What is the airflow (20 point traverse on the return air stream)

What speed are the evap fans running



Steady on Rob!!
don't forget denco designed this system, as a technical support guy you can't suggest that pipe sizes are wrong or evaporator fans are running at wrong speed - top denco contracts engineers specified and commisioned this system!!!!

[Denco Technical]

Steady on Rob!!
don't forget denco designed this system, as a technical support guy you can't suggest that pipe sizes are wrong or evaporator fans are running at wrong speed - top denco contracts engineers specified and commisioned this system!!!!

I think without stood infront of the system i can't really make any comment.

But my money would be on either external static
or airflow, regardless of who designed it.