Evaparative condenser performance

[DesM1]

Re: Evaporative condenser as used on ammonia refrigeration system.
For a given condenser I am looking for a formula which gives the condensing temperature (Tc) for any given web bulb temperature (Twb).
The manufacturer's brochures give the "Heat Rejection Factors" for different combinations of Twb and Tc but they only go down to Tc = 30*C. I want to set up a spreadsheet model of this but for lower temperatures (ie, if the condenser if oversized and the Twb is say, 5*C.
Would appreciate any help.

 

[Yorkman]

I think the reason the manufacturers don't run numbers that low (5[°])C is because most ammonia/recip. systems need to have a min. head pressure in order to creat a pressure differential across the TXV or float to perform correctly. Generally most evaporative condensers should be able to operate at 5-7 degrees of approach to WB temperature.

I'm not a real engineer, but I play one on T.V.
A.J. Gest, York Int./JCI

 

[DesM1]

Thanks Yorkman.
I realise that there is a minimum head pressure but that pressure is quite low, circa 15*C (6 bar.g) and is determined by such factors as defrost requirements, oil separator performance at the lower density, pipe capacity, etc. On pumped systems the expansion valve is not usually an issue.
Very good savings can be made by "floating" the head pressure thereby taking advantage of low ambient conditions. With high energy costs this is now an issue.
I am trying to construct a programme/model in Excel to predict the savings by increased evaporator sizing. It involves making an hour by hour analysis over a year of the new condensing pressure. Using this and the existing condensing pressure it is possible to calculate the saving.
But to do all this I need to know the performance of any given condenser at low temperature conditions.

 

[25362]

Allow me to mention that these savings wouldn't be felt by an oil-flooded screw compressor working with a given internal volume ratio.

 

[DesM1]

Thanks 25362 but I am curious as to why not.
If I enter the two operating conditions into the compressor programme (eg, Sabroe) it gives me a different power requirement and COP.
I would be very interested to know the basis of you comment.

 

[25362]

DesM1,

The ratio of the volume of gas trapped in the thread of the screw at the start of compression to the volume of trapped gas when it begins to open the discharge port is known as internal volume ratio or volume index Vi. If there is no volume ratio control, i.e., the Vi is fixed, and at full load,

Vi = (P[sub]disch., internal[/sub]/P[sub]suction, int.[/sub])[sup]1/k[/sup]​

Assume k = 1.36 (ammonia), and Vi = 5

If the compressor is sucking, say, at 20 psia
its P[sub]disch., internal[/sub] = P[sub]suction, int.[/sub](Vi)[sup]1.36[/sup] = 20[×](5)[sup]1.36[/sup] = 178.5 psia = 12 bara (!) vs your 7 bara

12 bara would be the pressure of the trapped gas just before the rotor lobe uncovers the discharge port. Meaning the compressor would internally over-compress the refrigerant, leading to significant performance penalties.

A variable volume ratio control is obtained with a specially designed slide valve (most units don't have it because of the opex involved); its position would enable to match the pressure of the trapped gas in the rotor grooves to the prevailing system discharge pressure.

 

[Yorkman]

Could you construct your model by plotting out the new condensing pressures and liquid refrigerant temperatures using a P.E. (Mollier) type diagram and ammonia saturaton chart? It would certainly show the effects of reduced liquid temperatures on capacity and required pounds per minute circulated. Or perhap I'm missing the point and the need for the manufacturers data.

I'm not a real engineer, but I play one on T.V.
A.J. Gest, York Int./JCI

 

[DesM1]

To: 25362
Thanks for your very detailed reply.
Yes, I take your point and you are absolutely correct. In this particular case though the compressors are all variable volume ratio control so a saving should accrue.
But I am interested to discuss the case where they are not variable volume ratio control. My understanding is that it would be common practice to select the compressor with a pressure ratio less than the design pressure ratio. In operation, as the ambient temp decreases below the design value the operating Vi will move towards the ideal. If this be so then the COP will improve as the condensing temp/press decreases (until of course, the compressor's Vi is reached).
Would you agree?

 

[DesM1]

Thanks Yorkman.
My basis problem is knowing how evaporative condensers perform at conditions (ie, web bulb temps and condensing temp/press) below that stated in the catelogues. All of the catelogues stop at 10*C wet bulb and 30*C condensing temp.
The question is, for example, what Heat Rejection Factor would apply when the wet bulb is 5*C (assuming full refrigerating load).
Once I know this I can set up the spreadsheet to give the condensing temperature for each hour of the year for the "as is condenser" and for the "new, bigger" condenser. From the difference in degree-hours I can calculate the saving.

 

[25362]

It all depends on the duration of the warm and cold weather periods to better exploit the given Vi. Besides, losses of efficiency (BHP/TR) are't equally distributed for system's ratios above or below the compressor's Vi. Compressor makers should be consulted on this respect.

 

[25362]

Please note there is a difference between slide valves for capacity control and those for variable volume ratio control.

 

[DesM1]

Yes, I know but thanks for mentioning this. The selection programme comes up as "optimal" for port selection and the manual states that this is what happens when the model has variable volume ratio control (ratios grayed out). I will double check it though.

 

[sirsa1]

maybe I can help in another day about your requirement to find the formula on evap condesers work, you need to think about this is a heat exchanger only but is "evaprative" this means the effect of energy rejection is done at evaporative temperature and this temperature of course is affected by TWB and the efficiency heat exchange capacity provides TC and sub-cooling capability, the reported TC is high value because is commercial regular "std" conditions and you have a great point of vew on lower TC to the most low possible to save a lot of maney on energy cost