Head pressure control in evaporating condensers

[lana]

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

[Sandro Baptista]

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.

[DCoker]

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.

[Aik]

Can anybody explain if this method is correct and if so why timer?

I think it's good solve. As said DCoker, you may use mechanical pressure controls for each fan... but if you have N fans pressure in system will rise when the last fan turn on (approximately: (N-1)*differential), because mechanical pressure control has differential. If use one pressure control with timer, pressure in system will be approx constant.

[PRESS]

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.

[lana]

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.

Lana

[RANGER1]

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

[lana]

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
LANA

[RANGER1]

Lana thats how I see it .

[charlie n]

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.

[DTLarca]

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.

[Grizzly]

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

[lana]

Thanks everybody for your comments.
One must learn a thing everyday, So must I .

Thanks guys.
Cheers

[Sandro Baptista]

In either variant it is not normal to stop the Fan rotating.
Grizzly

Why? Sometimes the spray water pump is working alone.

[Grizzly]

Why? Sometimes the spray water pump is working alone.

It is not normal does not mean always, You are correct sometimes the pump running alone is suffice.
Sometimes we have been known to shut of the Pumps.
I will rephrase what I said.
The systems I look after don't normally switch off the fans.
Grizzly

[Segei]

It is not normal does not mean always, You are correct sometimes the pump running alone is suffice.
Sometimes we have been known to shut of the Pumps.
I will rephrase what I said.
The systems I look after don't normally switch off the fans.
Grizzly

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.

[Sandro Baptista]

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.

plenty agree

[Grizzly]

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

[Lodiev]

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.

[Segei]

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.

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.

[Lodiev]

[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.

[Segei]

[quote=Lodiev;218490]

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.

These are not special condensers. Typical condensers from Evapco, Imeco, BAC have these baffles.