Refrigeration Balance Lines

This question is a bit complex, and it depends on the principles of pressure drop due to flow through the condenser(s), compensated by unequal liquid elevations in the traps between the condenser(s) and receiver(s). A short answer is that the receiver may be warmer or cooler than the condensing temperature. Accordingly, in the first case some liquid will evaporate in the receiver(s), and the vapor needs a place to go. In the second case, vapor will condense into the receiver(s), and this vapor needs to come from somewhere. In both cases, the sink/source of the vapor is at the top of the condenser(s), and therefore there must be an equalizing line between top of the condenser(s) and the top of the receiver(s). If not, liquid will be pushed out of the traps (up to the condenser(s) in the first case, down to the receiver(s) in the second). Especially in the first case, one or more condensers may become flooded or "logged" with liquid, reducing the condenser's capacity.

This will no doubt seem complicated and/or vague, until one gets accustomed to the ideas. It is also a common situation for piping mistakes. Some of the prominent evaporative condenser manufacturers have good information on this, but not usually in the basic catalogs. You should ask specifically for recommendations on parallel piping of condensers, and then pay close attention to the fairly small differences in pressure at various points in the refrigerant flow.

In a related matter, look carefully at the proper purge points for removing noncondensable gases (e.g., air) from the system (tapped off the top of the condenser outlet(s)).

the pipe i have often seen from the receiver to the condenser top is intended to provide an escape route from a secured receiver (valves closed above and below) in case of a fire. Having the condenser's capacity available will give you twice the time to get the fire out before the system explodes in a cloud of deadly phosgene gas. There should be a relief valve in this line. The receiver should not have air and noncondensable gases in it. That would interfere with efficiency. The condenser shouldn't, either, but the condenser will cope with them better than the receiver. a line from the receiver to the top of the condenser might be in place to conduct these gases, which tend to accumulate on top of freon to the top of the condenser, where they can be drained or run through a purge unit. I imagine environmental laws now require a purge unit on every large refrigeration system. In the old days, I am secure in saying that I never saw a functioning purge unit. We received no training in them, and typically the tube to the purge unit would have been hacksawed off and the shutoff valve to this line would be used to drain air from the system by dead reckoning any time the unit hiccupped or dropped in efficiency. Often this would be taking place while another person was holding a freon bottle upside down, jetting cold liquid freon directly into the low pressure line while the compressor's valves did a Buddy Rich drum solo inside.