Ammonia plant Room and the number of Air changes required.

[Grizzly]

I recently sat the C.I.T.B Safe Handling of Ammonia course.

With our knowledgeable and helpful friends up in Doncaster.
(Other good trainers also run this course and are also on the Forum.)

One fact quoted was that a Plant room fan must be capable of 15 air changes per hour.
As stated in En 378.

One question I did not clarify is when the fan is expected to run. I have always assumed the fan is normally in "stand-bye?"
However I am attending more and more sites where these fans are left running in Manual (Always On Mode)
What's everyone's take on this?
Grizzly

[cduque]

Hello Grizzly,

EN 378 stands that machine room ventilation shall be capable of normal and emergency ventilation. In normal conditions the minimum is 4 air changes per hour, in emergency the minimum is calculated as a function of the NH3 charge:
V = 0,014×m^(2/3) (1)
where
V is the air flow rate in m3/s;
m is the mass of refrigerant charge, in kg.


Anyway it is considered that with 15 air changes per hour the calculation is not needed.

Other Standards in other countrys state other minimum values (for instance IIAR2 states 30 ACH in emergency).

The fan(s) shall be permanently working in the "normal conditions" and work in "emergency conditions" in case of a leak or emergency actuation.

This is my interpretation of the text, others may have other interpretations...

CDuque

[al]

I reckon that they are left on for fear of the auto system not working in an emergency, the few plant rooms i worked in here had fans top and bottom, over kill maybe, all hooked up to a detector, all were in manual!

[cduque]

As ammonia is lighter than air, preferably ventilation should be placed middle to high in NH3 machine rooms, with other fluids, like CO2 or HFC, should be placed middle to bottom. For the control, in the instalations I'm involved in the idea is to have double speed fans, small speed always on and automaticaly switch to high speed with emergency signal.

[Magoo]

the fan motors should be X rated as well, with starters remote from effected area.

[sterl]

Language in more than one of the standards says, in effect, if fans are set to always run you do not need a "low" refrig content detection arrangement. Though if there is a possibility of walking into a room with a refrig level more than the IDLH there should be warning to those accessing.

So how do you effect warning without a means of detection? All it really means is that normal ventilation system does not need to be tied into the refrigerant detection arrangement....