Centravac Chiller has wide condenser approach
Centravac Chiller has wide condenser approach
(OP)
We have a issue with one of our chillers and I am hoping that someone can shed some light on it. One of our R11 Chiller is showing low condenser leaving water temperature and high Condenser approach. Here are the specs taken yesterday.
Condenser entering water temp = 34.6 F
Condenser leaving water temp = 68.8 F
Sat Evaporator Temp = 37.8 F
Evap rfgt pressure = -7.9 psig
Saturated Condenser temp = 94.1 F
Condenser Pressure = 6.6 psig
Condenser approach = 25.5
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This Chiller has been out of service since 08/22/07 and we put it in service last week. All other indications are normal. Purge is 0 and unit seems to be working fine. We never had an issue with micro fouling, which can also cause this condition. Any thoughts?
Condenser entering water temp = 34.6 F
Condenser leaving water temp = 68.8 F
Sat Evaporator Temp = 37.8 F
Evap rfgt pressure = -7.9 psig
Saturated Condenser temp = 94.1 F
Condenser Pressure = 6.6 psig
Condenser approach = 25.5
.
This Chiller has been out of service since 08/22/07 and we put it in service last week. All other indications are normal. Purge is 0 and unit seems to be working fine. We never had an issue with micro fouling, which can also cause this condition. Any thoughts?





RE: Centravac Chiller has wide condenser approach
1) Condenser fouling (as you mentioned).
2) Non-condensibles in the condenser. I assume that what you meant by purge is 0 is that there are no non-condensibles.
3) Something is causing your condenser to flood (i.e. liquid is backing up and covering the heat exchange (condensing) surface).
It would be nice if you could get a reading of the liquid temperature as it comes off the condenser. Assuming that you don't have a subcooling section in your condenser the liquid temperature should be close to the SCT. Sometimes I use an IR thermometer on the trap of the condensate pipe after the condenser to get a measurement (although that is not the most accurate method).
Yorkman or imok2 probably can help on this.
RE: Centravac Chiller has wide condenser approach
RE: Centravac Chiller has wide condenser approach
Look at the heat transfer equation Q=UA delta T. Micro fouling (as you define it) would affect U. Macro fouling (as you define it) would affect A. How this affects condenser water flow depends on how your condenser water supply is set up and number of passes in your condenser. If you had a constant pressure water supply then macro fouling would increase the pressure drop across the condenser and reduce water flow overall but water to an open tube might increase somewhat. If you had a typical centrifugal pump supply then as some of the tubes became blocked the head on the pump would increase reducing overall flow but the flow at each open tube would increase (of course depending on the shape of the pump curve). If you had a constant volume supply (like a positive displacement pump then the overall flow would be the same but the flow to each tube would increase dramatically and so would the pressure drop across the condenser. You might have some type of control scheme which tries to maintain a constant leaving water temperature, a constant condensing pressure, or some other type of algorithm. You need to look at how your water flow is controlled also to help diagnose your problem.
RE: Centravac Chiller has wide condenser approach
Scaling tends to decrease the temperature difference.
Did you make any changes in the system before putting into operation?
RE: Centravac Chiller has wide condenser approach
RE: Centravac Chiller has wide condenser approach
RE: Centravac Chiller has wide condenser approach
RE: Centravac Chiller has wide condenser approach
Condenser entering water temp = 34.6 F
Condenser leaving water temp = 78.2 F
Sat Evaporator Temp = 39.5 F
Evap rfgt pressure = -7.6 psig
Saturated Condenser temp = 91.1 F
Condenser Pressure = 5.4 psig
Condenser approach = 12.8
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These measurements are from the microprocessor but our analog gauges that tell us about the actual condition (for example, cond pressure) confirms the above readings.
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imok2. Regarding your comment about fouling, if the tubes were plugged due to fouling or were restricting the flow through the condenser tubes, we would see the increase in condenser leaving water temp. This is not the case. As I have stated earlier, we induce the false load on these chillers and based on my experience in the past years and the way other chillers are operating right now, condenser leaving water temperature should be higher (around 80 F). If we experienced micro fouling (scaling, minerals and algae), then our condition would make sense but never had an issue with this king of fouling and believe me, we track this closely. My thoughts are that the pass partition plate is erroded and some of the inlet condenser water just going straight the outlet side and indicating low condenser leaving water temp. We had seen some erosion during our last clean up.
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Any comments.
RE: Centravac Chiller has wide condenser approach
RE: Centravac Chiller has wide condenser approach
RE: Centravac Chiller has wide condenser approach
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Saturated Condenser Temperature is the measured value and pressure is the calculated. We also have a pressure gauge and it's reporting the same pressure as calculated condenser pressure. The temperature element is located on top of the condenser.
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Purge 0 means that our purge unit is not taking out any non condensables from the chillers. This coould be due the bad purge unit but since the measure Condenser pressure matches calculated condenser pressure, I dont think that is the case.
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Condenser water flow is controlled by a Control Valve that opens or closes based on condender pressure. In winter we throttle a hand valve to limit the intake of cold water to the condenser and thus artificially increase the condenser preesure. That is why you see such high delta T.
RE: Centravac Chiller has wide condenser approach
RE: Centravac Chiller has wide condenser approach
In industrial systems we usually calculate SCT from the discharge pressure measured at the (evaporative) condenser but it is best to measure both in order to see if purging is required (or if you autopurger is working). How does it measure the condensed liquid refrigerant temperature if the probe is at the top of the condenser where (I would assume) the gas enters and has not yet condensed and is probably superheated? (Please humor me since I rarely see a chiller, most of my work is industrial).
RE: Centravac Chiller has wide condenser approach
"The temperature element is located on top of the condenser"
WoW, how does that work!!
RE: Centravac Chiller has wide condenser approach
The condenser approach gives you indication about the condenser performance at design flow condition. As your prime interest is to maintain the pressure in the system, the cooling water flowrate may not be sufficient to cool the liquid to the required temperature.
If your other system is running with same cooling water flowrate but with different refrigerant parameters, then check for scaling on condenser tubes for the faulty chiller.
If you forsee bypassing of the cooling fluid, it is better to throttle the outlet valve (infact, I would always go for controlling the outlet valve).
As already said, temperature sensor at the top of the condenser is not proper. You should check the approach with respect to subcooled refrigerant temperature (i.e at the bottom)
If you can place two pressure gauges one at the top and second at the bottom of the condenser then the difference indicates the presence of noncondensable gases and thus effectiveness of your purge unit.
If both the cooling water inlet and outlet are from one side of the condenser then check the gasket between dome and tubesheet for possible bypassing.
RE: Centravac Chiller has wide condenser approach
If the bypassing is internal to the condenser I don't see how changing the throttling from the inlet to the outlet will affect the bypass. The flow will still split to have equal pressure drops between flow through the tubes and flow through the leak. Centravac suspects a leak in the pass partition in the head (or dome as you call it). You had a good idea in pointing out that it could also be a faulty seal of the gasket between the baffle plate and the tubesheet.
I wouldn't expect much subcooling on the shell side of a condenser unless the liquid refrigerant level in the condenser was over the tubes otherwise the liquid refrigerant would not be in equilibrium with the gas at that pressure. If there is liquid refrigerant backing up over the tubes then in addition to what you state I would also check the level of this backup as I suggested in item #3 in my first response. Liquid refrigerant over the tubes of the condenser basically reduces the size of the condenser. I tried looking at the Trane Centravac manual but it wasn't very detailed in how the liquid gets from the condenser to the evaporator.
I don't see how this does anything but measure the gas pressure drop on the shell side of the condenser. You need to measure the SCT (temperature of liquid refrigerant) and compare that to the pressure (at the same point depending on how accurate you want to be). The liquid must be saturated (not subcooled) otherwise it would tend to indicate non-condensibles even if there aren't any.
RE: Centravac Chiller has wide condenser approach
RE: Centravac Chiller has wide condenser approach
The first comment is rather tongue in cheek as I overlooked the pass partition leak. I suspect this may not be the case as there is a higher dT across the cooling water circuit.
The pressure should be in equilibrium in a flooded type condenser irrespective of liquid level, in the absence of noncondensable gases, as the discharge pressure of the compressor varies as per the condenser pressure.
The method I mentioned is a standard one for manual purging. We can always ignore the refrigerant pressure drop across a condenser when the system is running.
As per your reasoning, systems designed with subcooling of the liquid refrigerant should always err in indicating noncondensables. However, this can easily be overcome if you take pressure and temperature readings in the liquid region.
I wish a very Happy, Prosperous and Peaceful New Year to you all.