Refrigerant circuit question

[PoorRod]

I was looking at the original 40 tear old HVAC system in building yesterday. The tenant was inquiring about modernizing the system for better performance. What I found was two multizone AHUs which will probably require a bit of maintenance that was neglected over the years, maybe a new control system, and T&B. My main concern was that one AHU services an administrative area, while the other services an area critical to the tenant's operation. Cooling is provided by DX coils in each AHU, connected to 2 reciprocating compressors (the compressors are identical, each rated at 38 tons), cooled by a common water cooling tower. At the site visit I folowed the lines, and I was pretty certain that each DX coil was connected directly to its respective compressor. However, after I left the site and looked at the as-built drawings, I noticed that the mechanical plans showed the two compressors piped in parallel, connected to common suction and liquid lines that served the coils. The ladder diagram seems to indicate that both compressors would run at the same time, but that is not clear. Heat is provided by a hot water boiler supplying water to coils in each AHU with a common pump and circulation loop.

My biggest concern at the site visit was that each AHU cooling coil was independent of the other, such that if the critical AHU lost cooling capacity, there was no way to provide refrigerant from the circuit that served the administrative area. To me, that seems like the biggest liability with this system, and my first though is to replace the DX coils with a chilled water system that could serve both AHUs (and move from old reciprocating compressors to a scroll or rotary chiller). However, looking at the drawings, it seems that the existing system was designed for either compressor to supply refrigerant to either coil.

I am not too familiar with refrigerant systems, so my questions are: Can I provide the redundancy I want with simple piping changes to the refrigerant system? Are two compressors in one circuit common, and if so, what controls would I need to make sure I protect the compressors?

Thanks.

 

[chilleral]

Piping the two units in parallel will not provide the solution you require as if one system loses its charge both will if the pipe work is common.An alternate would be to provide a means to switch one circuit to the other although depending on where the leak occurs this may not provide a solution.

How critical is the area being cooled.
Is the AHU due for an upgrade?If so a chilled system would be a good idea as it removes toxic refrigerant from the cooled areas.

Hope this helps.

 

[PoorRod]

I'm more concerned about a compressor failing than losing the charge. The switching of the circuit sounds like a possibility, but then I'd have to worry about the refrigerant charge being adequate, right? I want to make sure any piping changes don't change my suction and discharge conditions on the compressors to a point where I cause more damage than benefit.

The area being cooled is critical (downtime will cost the tenant big dollars about 50% of the time), and at 40 years of age the system is due for some upgrades. The appeal of providing the redundancy without the expense of changing coils, new piping, and the chiller is compelling at first glance though. On the other hand, the compressors are about 10 feet away from the natural draft boiler, so I have some code issues to address too.

Thanks for the help