Trane TRG-TRC011-EN manual Period three

period three

Capacity Control

notes

other words, the combination of solution temperature and concentration determines the temperature at which the refrigerant will boil (vaporize).

Varying the temperature at which the refrigerant boils in the evaporator changes the capacity of the absorption water chiller. So, in order to control the capacity of the chiller to meet the ever-changing system loads, either the solution temperature or the solution concentration must be varied. Many chiller control strategies vary both simultaneously.

15 psia

[103.4 kPa]

									re
								u
							s
						s			5 psia
					re
				p
			r
		o							[34.5 kPa]
	p
a
v

1 psia				&
[6.9 kPa]			%	'
0.1 psia	$	)	(
[0.69 kPa]

0

0 5

5 5

0

5 6

concentration

50°F	100°F	150°F	200°F	LiBr solution
[10°C]	[37.8°C]	[65.6°C]	[93.3°C]	Figure 41
	solution temperature

A common method used to vary the temperature of the solution is to vary the amount of absorbent solution delivered to the generator. At part load, in response to a changing leaving-chilled-water temperature, less dilute solution is pumped to the generator, reducing the heat energy required to boil off the refrigerant vapor. Reduced heat input results in less refrigerant boiled off (vaporized) in the generator and a less-concentrated solution returning to the absorber (', 56% shown here at part load versus 64.5% at full load shown in Figure 20). This less-concentrated solution has a lower affinity for water vapor and, therefore, the pressure inside the absorber–evaporator sections increases (pressures at $ and )). This increased pressure causes the refrigerant inside the evaporator to boil at a higher temperature, reducing the temperature difference between the chilled water and the refrigerant, thus reducing the chiller’s capacity.

Because less refrigerant is boiled off in the generator, the refrigerant flow rate through the cycle is decreased. Consequently, the heat rejected within the absorber is less. Less heat rejected by the cooling tower typically results in lower-temperature water returning from the tower, which tends to increase the capacity of the chiller and further reduces heat input to the generator.

Varying the solution flow to the generator can be accomplished in several ways. Historically, it has been common to use either a throttling valve or a bypass valve. A throttling valve creates an additional flow restriction in the pipe from the absorber to the generator, allowing the solution pump to ride up its pump curve, reducing the flow rate. A bypass valve diverts a portion of the solution back into the absorber, thus reducing the flow to the generator.

TRG-TRC011-EN 35