Evaporative condenser surface area

Dear sirs:
I need to verify the capacity of an old no brand evaporative condenser for ammonia. The data that I have is the total length of 3/4 nominal steel pipe, the amount of air looks fine.
Best regards
Gonzalo

Dear Gonzalo,

If I may ask, what are you trying to accomplish? Are you replacing the condenser? If you can provide some additional information perhaps I can help.

Best Regards,
US Iceman

Dear Iceman:
In this ammonia installation the condensing pressure is over 215 PSIG. The evaporative condensers were loco made (copied from some brand that I don’t know). Besides that there are some other reasons that let to a high condensing pressure I need to check the capacity of the condenser. In the commercial literature I didn’t find information of the surface pipe area versus capacity.
Best regards
Gonzalo

Hi Gonzalo:

The surface area data you are looking for is usually considered confidential information by the manufacturers. If the coil dimensions (height, width, and length) were similar to commercially manufactured evaporative condenser there are several things you can quickly verify. This assumes the copied coil has the same return bend radius for the coil serpentine.

An estimate to use for the water flow is about 5 GPM per square foot of coil coverage. For the airflow, an estimate is about 4 CFM per square foot of coil coverage. For the coil coverage definition I am assuming the overall opening of the air outlet for the condenser. The maximum air velocity should be less than approximately 550 feet per minute. This helps to reduce the amount of water carryover into the drift eliminators. The water coverage would also depend on the spray nozzle selection and quantity. You could check a pump curve for the pump used. Add about 5 psi head loss in addition to the friction head for the water piping. This should get you close to a reasonable estimate.

The actual heat rejection capacity might be estimated from competitive literature from another manufacturer.

Most of this relies on making intelligent guesses about a lot of unknown data. The heat and mass transfer for these condensers is not worth the effort to go into. The estimation guidelines above are the quickest way I can tell you to get a best guess.

On other thing you can easiliy check, but first a question. What is the suction pressure for the system?

One thing I commonly find with high discharge pressure problems is non-condensables in the condenser and , or, receiver. If your suction pressure is above 0 psig, air can still get into the system during service or re-charging new ammonia. The amount of air in the system increases the total pressure of the system high-pressure side.

The partial pressure of any air in the system added to the vapor pressure of the ammonia at condensing temperature is the total pressure that a gauge will indicate. This usually is the cause of this probelm. At least it is the more common one I see frequently.

Don't get me wrong... It may be the condenser is under-sized, contains some scale on the condenser coil, or even high suction pressures. All would affect the discharge pressure in the same way.

I thought I mention the non-condensable issue, just in case the other information does not help you.

Let me know how this develops.

Best Regards,

US Iceman

Dear Iceman
You are right, the suction pressure is -10 in Hg, there is a problem of noncondensables, we will install a purger, also the problem is the height of the condenser above the receiver, the whole installation is not well, we are going to do some changes on it, but I would not like to change piping and then go for a change on condensers.
Thank you for your information it is quite useful, but in this particular case that the equipment was loco built, I will not fell sure about the capacity from the water GPM, or the CFM. If I have the correct amount of pipe surface, and if there is a lack of water or air I can still change the pump or speed up the fan (under certain limits).
Thanks again and
Best regards

Dear Gonzalo,

I suggested the CFM, GPM, and coil size as a general means to compare this condenser with a similar condenser capacity from BAC, Evapco, or another manufacturer. If you could estimate and compare the data between the existing condenser and an other similar condenser, it might provide a basic heat rejection capacity for your use.

Another thing for you to check is the equalizing line size from the receiver to the condenser vapor inlets. This is another area I find that contributes to proper condenser operation.

As you mentioned, the installation can have a direct impact on the condenser capacity also.

If the condenser cannot drain by gravity, you may have some probelms with liquid ammonia hanging up in the condenser coils also.

From what you describe it sounds like you have a bad situation to investigate.

If I can be of any help, please let me know.

Good Luck and Best Regards.
US Iceman