Post
by fredrosse » Sun Sep 05, 2010 6:19 am
Condensing serves two purposes:
1. Recovering condensate to re-use in the boiler, especially needed when operating in salt water areas.
2. Possibly reducing the back pressure on the engine exhaust by maintaining a vacuum in the condenser, thus increasing power output.
The condensing can be accomplished at either vacuum conditions or atmospheric pressure, or even some positive pressure.
A condenser operating at vacuum conditions needs a vacuum pump to remove any non-condensable gasses (air) that leak into the system. In theory, the gain in power from the engine operating with vacuum exhaust conditions far exceeds the power consumed by the vacuum pump. However in practical terms as applied to small launch steam plants, the vacuum pump may consume more power than is gained by the engine. This can occur if the vacuum pump is a very inefficient one, or is grossly oversized for the intended service. As mentioned in a preceding post, this may often be the case for some small launch steam plants.
In addition to the vacuum pump, one needs another pump to remove the condensate and force it up to atmospheric pressure of the hotwell or feedwater tank. For small steam launch systems, (and large historical marine plants) the duty of pumping non-condensables and condensate is combined into a single pump, normally named a “wet vacuum pump”.
Exhausting to atmosphere is the simplest setup, with gravity drain to the hotwell.
Exhausting to some pressure above atmospheric is also employed, and the positive pressure (hopefully only a slight positive pressure) can force the condensate to the hotwell (or feed water tank). Non condensables would also be forced thru, although there should be virtually none, as all parts of the steam system operate at positive pressure, thus no air inleakage.
A caution is required here:
If there is any possibility of getting higher condenser pressures, then a positive vent relief path will be required, unless the condenser and all of its connected piping are capable of withstanding the maximum pressure that the condenser could possibly experience.
On my steam plants with condenser vacuum, I use a large condenser vent pipe with a check valve on the atmospheric discharge connection. If the vacuum pump fails, or the cooling water fails, condenser pressure will rise to atmospheric pressure, and excess exhaust flow will be released at atmospheric pressure. Large industrial plants have a pressure relief device on every condenser, usually set to open at about two atmospheres pressure.
If there is no vent path, it is conceivable that almost full boiler pressure could be applied to the condenser, and most of them are not designed to withstand this condition.
