Energy Efficiency Cooling Water

Constant flow systems have limited applications for multiple chillers piped in parallel and serving multiple cooling loads. When the system operates near full load, performance is satisfactory as all chillers and pumps are operating. However, constant flow systems have problems during part-load or off-peak conditions. Consider a constant-flow primary-only chilled water system (Figure 70) with three chillers fed by two pumps and a part load condition small enough so that two chillers can handle the load.



	Fig Nº70. Constant-flow primary-only chilled water system.
	Click on image to enlarge

By turning off one chiller and allowing the flow to continue through the down machine, the supply temperature from the plant increases due to the mixing of water from the chiller that has been off, with the water of the chillers that remain “on-line”. This creates a step reset of the supply water temperature and at the same time, pumping energy is wasted unnecessarily in the evaporator of the chiller that was stopped.

The only way to counter the supply temperature degradation is to drop the temperature of the water leaving the active chillers, but this complicates the operation and/or controls, making it an unsatisfactory option. This scenario shows the severe limitations of the constant-flow primary-only scheme to face real life thermal load fluctuations in an efficiently manner.

On the other hand, a primary/secondary constant flow scheme with each chiller having a dedicated primary pump , allows shutting down a chiller and its pump without affecting the secondary flow. It is possible because both circuits are hydraulically independent. This way the system face the variable thermal load without having to keep on-line all chillers and pumping energy saving are realized during periods of low loads.

Now consider a similar system with three identical chillers operating in parallel designed to cool 55 F (12.8 C) chilled water return to 45 F (7.2 C) chilled water supply. A 50 % drop in load could be faced with two chillers, provided that some supply temperature degradation is allowed.



	Fig. Nº71. Constant flow primary/secondary chilled water system
	Click on image to enlarge

It means facing a 50 % cooling load with around 67 % of the total production flow of the plant. It is of course a more efficient strategy than keeping in service all production units and theirs pumps at all loads.

However, chiller sequencing in a constant flow system does not change really the primary loop into a true variable distribution because the flow remains a higher percent than load. Additionally, although some primary flow variation is allowed, it can be done only in a stepped way with as many steps as chillers in the plant.
Shutting down a chiller in a primary/secondary constant flow system with multiple chiller piped in parallel means to break the design condition balance between production flow (primary loop) and distribution flow (secondary loop). As secondary circuit flow remains constant, the excess flow will run through the common pipe in the direction towards the secondary pumps creating a mixing point and further degradation of the supply temperature.

This reduces the cooling capacity of the coils, especially latent cooling capacity which could mean a loss of humidity control in the zones. Additionally, the negative effect of capacity reduction due to higher supply temperature, demands sometimes the selection of the next higher available size for the coils.

The current standard design for central chilled water plants with multiple chillers and multiple cooling loads is the constant-flow primary/variable-flow secondary scheme (Figure 72). The primary pumps are typically constant volume, low head pumps intended to provide a constant flow through the evaporator of the chiller. The most common arrangement in the primary loop is chiller-pump in tandem, so primary pumps are sequenced with chillers. Chiller capacity is staged in response to rising leaving chilled-water temperature (T1 in figure 72). A chiller is staged off when flow in the bypass exceeds the design flow of one chiller.




	Fig. Nº72. Constant-flow primary/variable-flow secondary chilled water system.
	Click on image to enlarge

This can be determine by monitoring the bypass temperature (T2), the return temperature (T3), and mixed return temperature (T4). Since the chilled-water-flow rate in the primary loop is known to be a good approximation, the flow rate through the bypass can be calculated using these temperatures. Alternatively, a flow meter in the bypass line can be used to measure the excess primary flow directly.

The secondary pumps deliver the chilled water from the common pipe to coils then back to the common pipe. These pumps are variable-speed pumps controlled from differential pressure sensors located remotely in the system or from cooling coil valve position, thus secondary flow tracks the continuously variable thermal load. Pumping energy saved this way provides significant energy savings relative to constant-flow schemes (primary-only and primary/secondary). However, primary flow variation remains stepped which avoids achieving the ideal correspondence between load and flow.

Low delta-T syndrome in chilled water plants
When the secondary circuit return water temperature is lower than design temperature, chillers can not be loaded at their maximum capacity. If the chillers in a chilled water plant designed to cool 55 F (12.8 C) chilled water return to 45 F (7.2 C) are receiving their design flow rate at 52 F (11.1 C) rather than the design temperature of 55 F, the chillers will be loaded at the ratio of:

Where:
CHL (%): Percent chiller loading
CWRTR: Real chilled water return emperature
CWSTD: Design chilled water supply temperature
CWRTD: Design chilled water return temperature

In this case:

The delta-T (the difference between return and supply chilled water temperature) in the plant has been lowered from 10 F design condition to 7 F, then chillers will be unloaded a 30 %.

In variable flow systems, it is assumed that delta-T will remain relatively constant at all loads. Then the flow must vary proportionally with the load. Most variable-flow systems are designed based on this assumption and fail to perform well if the delta-T does not stay relatively constant. The fact is in almost every real-world chilled water plant, delta-T falls well below design levels, particularly at low loads. The low delta-T “syndrome”, as it is known, causes the plant operators to run extra pumps and chillers to meet the load, which in addition to reducing the plant’s cooling output capacity, wastes energy. The system may be keeping the building cool but it is inefficient and a lot of chiller capacity is being wasted. Table 1 shows some causes and mitigation measures for low delta-T syndrome.

Table 1 Some causes and measures against low delta-T syndrome:

Cause	Measure
Improper setpoint or controls calibration: A modest drop in supply air temperature of an AHU can cause coil flow rate to be doubled and delta-T to drop in half.	Controls must be calibrated and setpoints must be checked regularly. Use of pressure independent delta-P control valves or automatic flow control valves.
Using three-way valves: Three-way valves by their nature bypass supply chilled water into the return line, causing chilled water return temperatures lower than design. This exacerbates low delta -T problem.	Do not use three-way valves in variable flow system. Two-position bypass valves across supply and return lines, strategically situated and controlled properly is preferred to ensure minimum flow.
Poor two-way valves selection: An improperly sized two-way control valve may consume more water flow when open than the design calls for. With full flow through the coil, at partial loads the delta-T will invariably be lower than design.	Select the control valves considering the pressure drop of the load served and the available differential across the supply and return mains. Manufacturers usually recommend that wide-open control valve pressure drop be equal to or grater than the pressure drop of the coil plus the pipe and fittings connecting them to supply and return mains.
Reduced coil effectiveness: Coil transfer effectiveness is reduced by water side fouling, air side fouling, air side deterioration, non uniform air distribution across the cooling coil, and coil bypass air. Any reduction in coil effectiveness increases the flow rate of water required to deliver the desired leaving water temperature, thus reducing delta-T.	Control water side fouling by proper chemical treatment. Reduce air side fouling by cleaning coil faces and filters periodically.

Variable primary-flow scheme, potential benefits

Among the variable flow schemes, variable-primary flow (VPF) chilled water systems are of much current interest. A VPF system consists of single or multiple chillers with a unique set of pumps that moves the water through the chillers and distribution system to the cooling load. The cooling output at each coil is controlled with two-way valves. A bypass line with a two-way control valve diverts chilled water from the supply into the return line to maintain either a constant or minimum flow through the chiller(s).

The VPF approach has three main potential advantages over the primary/secondary system: energy and operating costs savings, first cost savings, and better ability to deal with low delta-T syndrome. Energy savings are possible because no excess flow recirculates from supply to return through decoupling lines or three-way valves. In theory, every bit of supply water, without any mixing, must pass through a load before returning to the plant. Energy savings are also possible when conditions permit flow to one or more chillers to exceed design flow. If outside wet bulb temperature is below the design value, as it is over 95 percent of the year, the condensing temperature will be also lower, giving each chiller additional capacity. If more water can be put through the chiller, this extra capacity can be tapped.

The first cost of a VPF system is likely to be lower than that of a primary/secondary system simply because two set of pumps are replaced with one.

As regards dealing with low delta-T syndrome, VPF controls permit flow through evaporators to be increased above design value, making it possible to adjust to less than ideal chilled water return temperature.

However, VPF approach is not a panacea. Chiller sequencing requires more care in order to achieve stable operation during simultaneous flow and load changes. Additionally, the low chiller flow bypass control adds further complexity. In short, the VPF system is both less expensive and more efficient than a primary/secondary system, provided that control issues associated with variable primary flow are handled properly.