Absortpion Refrigeration - Evaporator Basics

[knudo]

I am doing some development work on a Continous Absorption (aqua-ammonia) refrigeration system. As a mechanical engineer, my knoledge of thermo-dynamics is somewhat sketchy. I am having trouble with the evaporator of the system. About 50cc / min of ammonai liquid is devivered to the evaporator, but it only cools sinnificantly when the ammonia is first introduced into the evaporator. I am suspicious of the evaporator being to small (volume wise) to process this amount of ammonia. Can someone help with some guidance as to how to size the evaporator. I assume that it is desirable to have the maximum wetted area within the evaporator, and that the evaporated ammonia vapour should occupy 1/25th of the available volume. Hydrogen to make up the rest.
Am I on the right track ????

Regards - Kevin

[absrbrtek]

Is this a flooded evaporator? Are you spraying the ammonia over the tubes? Where is the hydrogen comming from?

[knudo]

Thanks Absrbrtek,
The Evaporator is just a length (800mm)of 25mm steel tube that is filled with Hydrogen. The Ammonia is introduced at one end and flows out the other. The Hydrogen is from the initial charge of the system, and is replenised by the Absorber.
I realise that the Evaporator design is extremly basic, but this rig is only a proof of concept. Any assistance would be greatly appreciated.
Regards - Kevin

[absrbrtek]

What is the purpose of the hydrogen? It is a non condensable gas in your application.

What pressure is your evaporator running at? Do you have any temperature and pressure data from while the unit was running? Pressures, refrigerant temps, absorber temp, generator temp, water in, water out etc.

[knudo]

As absorption refrigeration works on the law of partial pressures, the hyrogen is primarily there in a specific ratio to provide approx 14.5psi(g) ammonai pressure at the evaporator. System pressure is 400psi.
The evaporator drops to -5degC initially but then climbs to about 12 deg with an ambient of 20 deg. Generator is 190deg. Absorber runs 40-85 deg C.