Constant-flow Chilled Water Systems

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Introduction – Chilled Water Systems[edit]

In a chilled water system, water is cooled down to 4 – 70C by an air conditioner (Chiller) and this chilled water is then distributed throughout the building or industrial complex via the piping system. These pipes are connected to the air conditioner cooling units placed on the respective remises.


Basically, the water is cooled in chiller and then this chilled water is flown through the piping system to the cooling loads or coils. These coils utilize this chilled water to cool the air and supply this cooled air to the building rooms or spaces.


These chilled water systems mainly consist of chillers, pumps, piping, cooling coils, controls and other components on the evaporator side of the chillers. This dynamic system fulfills the cooling requirements of many air conditioning applications in commercial buildings (or industry) and consequently, it is one of the most energy intensive systems in commercial buildings. It is therefore, very much important to understand how these distribution systems react to varying loads and how their components interact with respect to the requirements, in order to design an energy efficient and cost-effective chilled water plant.


These chilled water systems can be broadly classified into two major classes;

1. Constant-flow Chilled Water Systems In old chilled water plants, a constant volume of the chilled water is circulated through the chiller and the building irrespective of the cooling load being large or small. These systems are known as the Constant-flow Chilled Water Systems. If the loads are small, the constant volume of the chilled water is diverted around the cooling coils using three-way valves. This article emphasizes on these Constant-flow Chilled Water Systems.

2. Variable-flow Chilled Water Systems In these variable-flow chilled water systems, variable volume of the chilled water is circulated through the chiller and the building (via piping) depending up on the cooling load size.


Constant-flow Chilled Water Systems[edit]

These include following configurations;

  • Single chiller serving single cooling load
  • Single chiller serving multiple cooling loads
  • Multiple chillers in series serving multiple cooling loads
  • Multiple chillers in parallel serving multiple cooling loads


Single Chiller Serving Single Cooling Load[edit]

Here, the water is circulated between the evaporator and the coil using a constant-volume pump and control is provided by resetting the temperature of the chilled water leaving the chiller. Due to constant water flow, a reliable heat transfer takes place at the evaporator and the cooling coil. A schematic diagram of a Single Chiller serving a Single Coil in a Constant-flow System is shown in Figure – 1 below.


SingleChillerSingleLoadConstFlow.PNG

















Figure – 1 Single Chiller serving a Single Coil in a Constant-flow System

[Chilled Water Plant Design Guide, Energy Design Resources, Taylor Engineering LLC, Available online at http://www.taylor-engineering.com/downloads/cooltools/EDR_DesignGuidelines_CoolToolsChilledWater.pdf]


Chillers must have a sufficient volume of water in its piping system in order to prevent the unstable temperature variations and it may be an issue with single-chiller serving single coil. The reason being, in case, the chiller and coil are coupled closely there might not remain enough water in the piping system constantly. Therefore, often it is required to install a small storage tank if the chiller is closely coupled with the coil. The minimum water volume to be kept in the piping system of the chiller should be verified with the chiller manufacturer, though some general guidelines are as follow:

• Provide 2.4 gallons/ton for a screw compressor.

• Provide at least a 5-minute re-circulation rate for reciprocating or scroll compressors


Single Chiller Serving Multiple Cooling Loads[edit]

For plants with single chiller serving multiple cooling loads (also referred as cooling coils), where the pumping head involved is not very high, say less than 50 ft, a constant-flow chilled water system is cost-effective. A system with single chiller serving multiple cooling loads is depicted in figure – 2 below.


SingleChillerMultipleLoadConstFlow.PNG

















Figure – 2: Single Chiller Serving Multiple Cooling Loads (Constant-flow System)

[Chilled Water Plant Design Guide, Energy Design Resources, Taylor Engineering LLC, Available online at http://www.taylor-engineering.com/downloads/cooltools/EDR_DesignGuidelines_CoolToolsChilledWater.pdf]


Here, three-way valves are used at the cooling coils in order to adjust the load at each coil (or the cooling load). And according to “chilled water design guide” by Energy Design Resources, these three-way valves actually contribute to the variations in the flow. According to this genuine argument, when these three-way valves adjust the flow according to the cooling load requirement there are pressure drops and consequent flow variations. However, as further suggested in the same reference, providing 2 way valves on coils (loads) representing 20% of the design flow gives positive results in terms of keeping the flow constant.


Multiple Chillers in Series Serving Multiple Cooling Loads[edit]

In this configuration, the flow of water passes through each of the chiller in series. The first chiller is termed ‘lead machine’ and the second chiller in series is termed as ‘lag machine’. This configuration is very much effective for the systems involving high temperature difference (in range of 15-200F). When there is less requirement of cooling water, the lag machine is turned off and the lead machine performs the duty and provides the water at the required temperature.


Although this system is efficient, there is one concern of water pressure drop through the series of chillers. The pump head through the evaporator can be minimized by carefully selecting the chiller and/or by reducing the number of passes through the evaporator from two (or three) to one. However, reducing the number of passes through the evaporator would result in some reduction in the efficiency of the downstream chiller.


Another more accurate solution that solves the high pressure drop problem is to pipe the chiller to the distribution loop in a primary/secondary manner as depicted in figure – 3 below;


MultipleSeriesChillersConstFlow.PNG

















Figure – 3: Multiple Chillers in Series Serving Multiple Cooling Loads (Constant-flow System)

[Chilled Water Plant Design Guide, Energy Design Resources, Taylor Engineering LLC, Available online at http://www.taylor-engineering.com/downloads/cooltools/EDR_DesignGuidelines_CoolToolsChilledWater.pdf]


The advantages of this system are;

• This design allows for more than two chillers to be incorporated in series

• Here, the piping arrangement is quite effective for systems designed for high temperature difference (more than 200F)

• An absorption or engine-driven chiller can be coupled with an electric chiller

• This arrangement allows for the flexibility to choose which machine to load (lead machine or lag machine) depending up on the utility rate or other criteria.


Multiple Chillers in Parallel Serving Multiple Cooling Loads[edit]

In this configuration, the chilled water coming from each chiller is provided to each cooling load (cooling coil) incorporated in the network. The configuration can be understood in a better way by depiction of such configuration in figure – 4 below;


MultipleParallelchillersConstFlow.PNG


















Figure – 4: Multiple Chillers in Parallel Serving Multiple Cooling Loads (Constant-flow System)

[Chilled Water Plant Design Guide, Energy Design Resources, Taylor Engineering LLC, Available online at http://www.taylor-engineering.com/downloads/cooltools/EDR_DesignGuidelines_CoolToolsChilledWater.pdf]


As can be observed from the figure – 4, there are two chillers arranged in parallel and the chilled water coming out from each of them is provided to both the cooling loads (Coil A and Coil B) incorporated in the network. Considering this particular case, there are two chillers, namely, chiller – 1 (upper chiller in figure – 4) and chiller – 2 (lower chiller in figure - 4) in parallel, serving two cooling loads, namely, Coil – A and Coil – B.


In this configuration, 50% of chilled water coming from both, Chiller – 1 and Chiller – 2 will be provided to Coil – A and the remaining 50% of the chilled water coming from both the chillers will be provided to Coil – B. (It should be remembered that this ratio will change with respect to the number of chillers and cooling loads incorporated in the network). Now, if both the loads, Coil – A and Coil – B, are to be run on full load requirement, then the system works efficiently. The system works efficiently even if both the loads are to be run at 50% requirement. In such case, both the chillers will be run at 50% flow of their original operational flow in order for system to work efficiently.


But, the performance gets affected when only one load out of these two, is to be run on full load requirement and the other load is to be kept idle fully. In such case, both the chillers will have to be run at full capacity as 50% of the output from both these chillers get distributed amongst the loads, Coil – A and Coil – B, and Coil A would require full portion designated to it, i.e. 50% chilled water from Chiller – 1 and 50% chilled water from Chiller - 2. So, in effect, the loads will receive their portion of chilled water from both these chillers, but since Coil – B is not to be run, it will bypass the chilled water to the return path. Thus, a total of 50% of the chilled water is circulated in the system without need, all the time. This is a major disadvantage of this configuration for constant flow systems. This issue can be addressed in variable flow systems.



See Also[edit]

1. Variable-flow Chilled Water Systems


Sources[edit]

1. Chilled Water Plant Design Guide

2. Chilled Water Systems