Results 1 to 22 of 22
-
13-01-2011, 03:33 PM #1
Head pressure control in evaporating condensers
Hi everybody,
I have not worked with evaporative condensers so I don't have any experience.
Someone told me today that for head pressure control they turn the fans on and off. So far so good...
Then I was told that they use one (1) hi pressure switch and a timer (???). When pressure goes down then HP switch breaks the circuit of first fan and then activates the timer. After say, 60 s if pressure does not go up then second fan is turned off.
Now My question is this : Why use a timer?
Can anybody explain if this method is correct and if so why timer?
Thanks a lot.
LANA
Even Einstein Asked Questions
-
13-01-2011, 04:55 PM #2
Re: Head pressure control in evaporating condensers
You don't have to put it...but it's better because you may start on or off the 2º motor and after 20 seconds, for example, the high pressure it stands again as before...so with this timer maybe less number of start on/off your motor should do.
To make progress is never good enough, I want to do better and better and better
-
14-01-2011, 04:54 AM #3
Re: Head pressure control in evaporating condensers
The timer is used as a delay when you have multiple fans like on a refrigeration rack for a grocery store. The first fan isn't delayed but those there after can be mainly for staging and usually time delay is for less than 10 seconds. This is mainly used with a computerized controller (eg: CPC, Danfoss, Novar, Johnston Controls) I prefer to set up fans on seperate mechanical pressure controls and stage that way. Like R22 systems for instance: 200psig, 225psig, 235psig, by the time u get to 250psig all fans should be on and ur target pressure should be around 190-200 psig. All fans should cycle off before the pressure drops below the hold back set point which is normally 180 psig. Although in the summer your pressures will run higher than the target head pressure.
-
14-01-2011, 07:15 AM #4
Re: Head pressure control in evaporating condensers
Can anybody explain if this method is correct and if so why timer?Last edited by Aik; 14-01-2011 at 07:21 AM.
In some places will have to think ...
-
14-01-2011, 08:45 AM #5
Re: Head pressure control in evaporating condensers
Hi,
Delaying the switching off of subsequent fans by using a delay off timer in conjunction with a head pressure cycling switch works well. The same applies to use of individual head pressure cycling switches for each fan. Look with both arrangements you will achieve the same goal of controlling your head pressure however there are a no. of factors to look at before you consider using one of these methods. Firstly the arrangement of your system is the determining factor which plays an important part in deciding which system to use. Secondly if the situation dictates that you have to have strict or very close control of your head pressure then use of delay timers on fans is a good choice. On the other hand if you are guided by cost and you are not worried about strict control meassures then use of head pressure of HP cycling switches alone is good enough.
I personally use PLCs a lot and I find them very flexible to use in order to achieve this goal. Remember we always treat each case as a different case altogether and the parameters we use are most likely not going to be the same always. This I am refering to the delay off times we allow before we stop the next fan.
My first choice method is use of timers.
-
15-01-2011, 02:55 AM #6
Re: Head pressure control in evaporating condensers
Dear all,
Thanks a lot for the replies.
As I understand, timer may or may not be used. Like in the air cooled condensers in which only Hi pressure switch is used.
Is there any special reason for evaporative condensers?
Again, Thanks.
LanaEven Einstein Asked Questions
-
15-01-2011, 08:13 AM #7
Re: Head pressure control in evaporating condensers
Lana,
Probably a motor of that type can only handle 6-8 starts an hour.
The differential would also have to be 100-200 kpa depending, otherwise it will short cycle.
A lot more plants are installing VFD to keep a more constant head pressure for various reasons
-power savings
-used with TX Valves
-T/syphon oil cooling systems
-
15-01-2011, 08:46 AM #8
Re: Head pressure control in evaporating condensers
Thanks for the reply.
So far I gather that using a timer helps on number of motor cycling and not for any mechanical/refrigeration reason.
Excuse my slow understanding...
Thanks
LANAEven Einstein Asked Questions
-
15-01-2011, 08:42 PM #9
Re: Head pressure control in evaporating condensers
Lana thats how I see it .
-
16-01-2011, 11:42 AM #10
Re: Head pressure control in evaporating condensers
I've had great results on small systems using Danfoss EKC331 controller. This gives me 4 steps and includes time delays, anti short cycling and lead/lag switching. This gives much more stable and more efficient control than pressure switches.
-
16-01-2011, 12:14 PM #11
Re: Head pressure control in evaporating condensers
For 35°C saturated condensing running the evaporative condenser dry gives the following capacities for the corresponding ambient temperatures:
10% at 30°C
25% at 20°C
40% at 10°C
56% at 0°C
67% at -10°C
82% at -20°C
If the ambient is reasonably cold and the system load drops with ambient then a lot of water and water treatment costs can be saved.Only the dogmatist says he will never change his mind. We all know that some of our opinions are wrong but none of us know which they are for if we did then they just wouldn't be our opinions. - JS Mill.
-
16-01-2011, 03:32 PM #12
Re: Head pressure control in evaporating condensers
Hi Lana.
Just for information only.
On Baltimore evaporative Condensers.
The small ones can be controlled by a motorised flap regulating the air flow.
On larger models it is more normal to have high and low speed fan drive motors.
With Discharge Pressure dictating at which speed the fan rotates.
In either variant it is not normal to stop the Fan rotating.
Grizzly
-
17-01-2011, 02:54 AM #13
Re: Head pressure control in evaporating condensers
Thanks everybody for your comments.
One must learn a thing everyday, So must I .
Thanks guys.
CheersEven Einstein Asked Questions
-
17-01-2011, 09:20 AM #14
-
17-01-2011, 09:15 PM #15
Re: Head pressure control in evaporating condensers
-
18-01-2011, 12:07 AM #16
Re: Head pressure control in evaporating condensers
Surprised with this approach. This is waste of condenser energy. Typically, in North America pumps run first than fans(Cycling or VFDs). VFDs are better than fan cycling. Additionally, water pump operation gives system stability. Variation of refrigeration loads doesn't significantly change condensing pressure, because water works as flywheel. Pump cycling create dry and wet surface. This is a way to the scale formation. We only do pump cycling in winter time because it is necessity. I like VFDs,because they allow us to run water pumps all year around.
-
18-01-2011, 12:22 AM #17
-
18-01-2011, 06:11 AM #18
Re: Head pressure control in evaporating condensers
It's a long story Guys.
Suffice to say when we have Had to shut of the pumps.
It's related to plant elevation and Weather conditions.
So yes you are correct and yes we are trying to reduce the condensors efficiency.
This is not that often I may add.
Grizzly
-
18-01-2011, 09:40 AM #19
Re: Head pressure control in evaporating condensers
Segei you are right and nailed the best method with condensers.
The problem with all multi-fan condensers lays in the structural build of it, u can not switch of one fan and let the other run. the air will just short cycle throe the stopped fan, the fan will rotate in the opposite direction and as soon as the fan starts again u have massive starting amps because of the blade spinning in the wrong direction. i have also seen on bigger condensers that the blades can break of be-course of this.
When ordering a evaporative-condenser from the manufacturer u have to specify how u are going to control your pressure, with the fans all cycling or with a VSD or two/three speed motors.
Also remember that if u use multi-speed motors to change the speed of all the fans together and not individually.
If u have water treatment on your condensers u can cycle the water pump when pressures drop to low during winter and use the fans only to control the pressure.
A new method on a multi rack system is to use a multi circuit condenser and control the capacity by adding a circuit or closing a circuit with a valve on the hot gas side of the condenser.THE BEST WAY OF LEARNING IS TO DO IT YOURSELF!!!
-
18-01-2011, 02:32 PM #20
Re: Head pressure control in evaporating condensers
Usually, issue of air short cycling is solved by condenser designers. Assume that condenser has 3 fans(1+2). To prevent air short cycling, this condenser has inside baffle that separate 1 fan and 2 fans compartments. This baffle has a gap at the bottom to drain the water.
To save energy, some PLCs have wet bulb approach feature to control condensing pressure. To maximize energy savings, this approach should be optimum. Wet bulb approach is useful feature for summer operation. For winter operation, condensing pressure should be minimum. This minimum condensing pressure is very important issue.
-
18-01-2011, 02:46 PM #21
Re: Head pressure control in evaporating condensers
[QUOTE=Segei;218488]Usually, issue of air short cycling is solved by condenser designers. Assume that condenser has 3 fans(1+2). To prevent air short cycling, this condenser has inside baffle that separate 1 fan and 2 fans compartments.
Normally u do not see that this is done as the price for such an condenser are much higher than the conventional condensers, and then the contractor struggles with the condensing pressure afterwards and wonder why and how he should control it.
In the past we cycled all the fans together and used the water temperature in the sump to control the cycling. it worked fine as long as the load are high.THE BEST WAY OF LEARNING IS TO DO IT YOURSELF!!!
-
19-01-2011, 04:51 AM #22
Similar Threads
-
Carrier Transicold Container Problem
By Shamel in forum TransportReplies: 39Last Post: 23-01-2012, 11:23 PM -
Capillary Tube Characteristics
By Chef in forum FundamentalsReplies: 9Last Post: 10-03-2009, 01:03 PM -
Benji
By benji in forum Trouble ShootingReplies: 29Last Post: 30-01-2008, 01:25 PM -
Refrigerated display cabinet, low head pressure
By expat in forum CommercialReplies: 10Last Post: 11-12-2007, 09:07 AM