Trane SYS-APM001-EN manual Flow, ft.water Flow rate

System Configurations

Experience with actual VPF plants indicates that a minimum evaporator-flow limit of 60 percent for packaged chillers and 40 percent or less for configured chillers work well.

Chiller manufacturers specify minimum and maximum limits for evaporator water flow. Their objective?

•To promote good heat transfer and stable control (minimum flow limit)

•To deter vibration and tube erosion (maximum flow limit)

In the past, the typical range for water velocity in a chiller was 3 to 11 feet per second. Today, manufacturer-conducted testing shows that specific chillers may accommodate evaporator flow rates as low as 1.5 feet per second, depending on tube type. This is good news for VPF systems because it extends the chiller’s ability to operate effectively without the addition of bypass flow.

The minimum flow limit for a chiller can be lowered by selecting an evaporator with more passes (a common option for machines with cooling capacities of 150 tons or more). Granted, more passes may require a higher evaporator pressure drop and more pumping power (Table 13). However, as the system flow rate decreases, the evaporator pressure drop also decreases by approximately the square of the flow rate reduction. Therefore, the pump requires less extra power to work against the pressure drop as the system flow rate drops below the design value.

The other benefit of the added pass is better turndown with a wider evaporator ΔT, which starts at a lower design flow rate for the same cooling capacity. In the case of the two-pass chiller, when using a 15° ΔT, the chiller invoked minimum flow prior to reaching the 50 percent system flow rate. This could cause a more complicated transition from one to two chillers, as discussed in the sections on “Managing transient water flows” on page 59 and “Chiller sequencing in VPF systems” on page 63. The other issue is that more pumping energy will be used in the system that requires bypassed flow more of the time.

Table 13. Effect of number of passes on minimum evaporator flow and pressure drop at reduced flow with packaged chillers1

	Design	Evap. pressure	Evap. pressure	Evaporator pressure	Minimum flow	Evaporator pressure
	flow rate	drop at design	drop at 80%	drop at 50% flow rate,	rate, gpm [L/s]	drop at minimum
	gpm [L/s]	flow, ft.water	flow rate,	ft. water [kPa]		flow rate, ft. water
		[kPa]	ft.water [kPa]			[kPa]
2 pass	180 [11.4]	13.7 [40.9]	9.0 [26.9]	3.5 [10.5]	77 [4.9]	2.6 [7.8]
3 pass	180 [11.4]	42.6 [127.3]	28.7 [85.8]	11.9 [35.6]	52 [3.3]	4.0 [12.0]
2 pass	113 [7.4]	5.6 [16.7]	3.5 [10.5]	flow too low, use min.	77 [4.9]	2.5 [7.5]
(15° ΔT)				2.5 [7.5]
3 pass	116 [7.3]	19.6 [58.6]	12.8 [38.3]	5.0 [14.9]	52 [3.3]	4.0 [12.0]
(15° ΔT)

1 Chillers may have slight differences in capacity, depending on which variable (flow, capacity, or ΔT) is allowed to adjust.

58 Chiller System Design and Control SYS-APM001-EN