centrifugal charge

[reztech]

How do you verify proper refrigerant charge in a centrifugal chiller using only the operating conditions (temp.s, press.'s ...etc.)?

[freonrick]

You need to follow the manufactor procedure for that machine. They do not all charge up the same.

[chillrdude]

There are some differences between manufacturuers, but for me its all about approach temps and discharge superheat.

[ga1279]

It is all based on the original design conditions for flow rates, entering and leaving water temperatures and the number of passes through the evaporator. The more passes the lesser the evaporator approach ( difference between the leaving water and evaporator refrigerant temperature) The idea is to get them as close together as posible (ie) 1pass 12-15*F 2 pass 9-12*F, 3 pass 6-9*F, and 4 pass 3-6*F. On older machines discharge superheat comes into play 25-30*F, Please remember all these conditions are based on the machine running at FULL LOAD anything less and the readings are meaning less, Good Luck ga1279

[ultrahvac]

approach...approach...approach! Get your approach as little as possible under a full load. There are three approach's to be concerned with, they are...Condensor approach, tower approach, and Evaporator approach. You can't go wrong.

[jayguy]

approach...approach...approach! Get your approach as little as possible under a full load. There are three approach's to be concerned with, they are...Condensor approach, tower approach, and Evaporator approach. You can't go wrong.

approach temp varies with load. approach temp goes up as chiller loads up. some older centrifugals had approach temps in the 5-6F range when unloaded. if you added refrigerant to get your approach down (which can be a valid solution, if you are short on refrigerant), you might carry over. this would destroy (over time) a chiller.

lower approach temps = higher effeciency. but due to chiller design, you may not be able to get there without sacrificing reliability.

[waycoolwayne]

approach temp varies with load. approach temp goes up as chiller loads up. some older centrifugals had approach temps in the 5-6F range when unloaded. if you added refrigerant to get your approach down (which can be a valid solution, if you are short on refrigerant), you might carry over. this would destroy (over time) a chiller.

lower approach temps = higher effeciency. but due to chiller design, you may not be able to get there without sacrificing reliability.

what is the unit CVHA ? years old helps R-11?
:)

[ACCMan]

Why do you think it is low on refrigerant?
Are the tubes clean?
Do you have a VFD on the pumps?
Do you have a "full load" based on amps or % shown on the display?
Post the readings from the service display and watch for a response.
Load the machine...if possible
Entering Chilled Water
Leaving Chilled Water
Entering Condenser Water
Leaving Condenser Water
Superheat if displayed
% of Load and amps with name plate amperage rating
Refrigerant Temperature from Cond. and Evap.
Size and Model or Model number
And.....add the info the other guys mentioned...number of passes etc..
Be sure to let the machine settle out before taking the readings...

Don't be in a hurry to add ........

[reztech]

My question is a general question, not specific to anything I've run across recently. I am just getting into the centrifugal service dept. and would like to approach my job with as much information as I can retain. I know that surge is caused by lift that is beyond the capability of the machine and I would like to be able to take the operating condition of a machine and know if the charge has been compromised and is the cause of the surge condition. Not as obvious as fouled cond. tubes.

[chiller mekanik]

Hi,

I first started out as a DX mechanic & I still enjoy the work when I get a chance to do it.

I can appreciate the nature of your question as I use to spend alot of time thinking of "over chraged" as a possible cause of a surge.

Over the years, I cannot recall ever seeing a centrifugal surge as a result of having too much refrigerant in it.
I'm not saying it can't happen, I'm sure if someone worked hard enough they could get it done.

Because centrifugals are not positive displacement & the fact that they are flloded evaporator, in my mind that makes the possibilitie somewhat unlikely.

Before you ever over charge one, if you follow all of the good advice you've read here, you will see many other symptons that would cue you that there is too much refrigerant long before you would actually cause it to surge just from over charging alone.

The first thing you would probably notice is the low discharge superheat, which is a sympton of "carry over".
Another sympton on some machines would be the compressor getting quiter & quiter.
Liquid droplets will usually quiten one down considerably, although its not a sanctioned method of noise control.J/k

Always think in terms of heat transfer.

If the water flow is right, the delta T will also be right.

If the condensers ability to transfer heat is in tact, then the condenser approach will be okay.

Same for the cooler approach.

Good luck!!..

[absrbrtek]

Welcome to the club, if your still doing them next year we will show you "The Secret Handshake". :eek:

[sigma]

approach...approach...approach! Get your approach as little as possible under a full load. There are three approach's to be concerned with, they are...Condensor approach, tower approach, and Evaporator approach. You can't go wrong.




What is tower approach?

[Shophound]

What is tower approach?

Tower approach is the difference in temperature (delta T) between the ambient air wet bulb temperature entering the tower and the temperature of the water leaving the tower. There is design approach and actual approach. Let's say design conditions for where a given tower operates is 95 degree dry bulb, 78 degree wet bulb entering air conditions, and 85 degree leaving tower water temperature. This would give a design approach temperature of seven degrees above the ambient wet bulb temperature on a design day. Actual tower approach will depend on ambient conditions, tower load, tower cleanliness and water quality, etc. It also can depend on how well a tower is operated. Before I could implement a reset schedule for my plant's towers based on design approach temp over wet bulb, there were many days all three towers were online to handle one chiller, and all three were running balls to the wall, VFD's wound out at 60 Hz. This was because the entering condenser water temperature (ECWT) setpoint was too low, forcing the towers to operate at an approach temp lower than seven (manufacturer says below four is way out of spec for the tower type and size selected). Once the reset schedule was implemented and fine tuned, I experienced days with afternoon temps in the low 100's where just two towers ran, and the VFD's on them were not pegged out balls to the wall, either. It gave the chiller's condenser a bit of relief by keeping ECWT below design of 85 (closer to 82...great for the load and ambient conditions) and the towers some relief by keeping the blowers from running at max speed.

The reoccurring factor I see about chiller plants is how everything in the plant is interrelated. Once a plant operator/chiller tech grasps this reality well, it's a really good thing.

[sigma]

Thank you shophound.