Trane TRG-TRC016-EN manual Excess Flow

period two

Chilled-Water System Design

notes

Excess Flow

2,000gpm at 54.6°F

[126L/sat 12.6°C]

2,000gpm at 42°F
[126L/sat 5.6°C]
						F
					°
					2
				4
				t		]
				a
			m			C
					°
		p			6
		g .
0				5
				t
0				a
2			/s
		L
	3
[1

1,800gpm at 42°F

[114L/sat 5.6°C]

1,800gpm at 56°F	Figure 61
[114L/sat 13.3°C]

When the flow of chilled water from the production loop exceeds the demand of the distribution loop, the direction of flow in the bypass pipe reverses. Chilled water flows from the supply side of the production loop, through the bypass pipe, and mixes with warm water returning from the distribution loop. This is called excess flow.

In this example, the pumps operating in the production loop are supplying 2,000 gpm [126 L/s] of water, while the distribution pump is pumping

1,800 gpm [114 L/s] to meet the demand of the cooling coils. The result is that 200 gpm [13 L/s] of supply water flows through the bypass pipe to be mixed with the water returning from the production loop. The temperature of the water returning to the chillers decreases to 54.6°F [12.6°C], reducing the load on the operating chillers.

Some excess flow is normal in the operation of a primary-secondary system. The amount of excess flow is almost always less than the flow of one production pump. The energy consumed by pumping this excess water through the production loop is typically very low, because the production pump only needs to produce enough head to push the water through the chiller evaporator and the bypass pipe.

If a pump-and-chiller pair is turned off as soon as this excess flow condition occurs, deficit flow will result and the pump and chiller will be turned on again. To prevent this from happening, a production pump and its respective chiller are not turned off until the excess bypass flow exceeds the capacity of the next production pump that is to be turned off.

Some systems are designed with variable flow also in the production loop. Although this minimizes excess flow in the bypass pipe and further reduces production-energy consumption, it results in a significantly-more-complex control system. This type of system will be discussed in Period Three.

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