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simulations of chilled water cooling coil delta-t characteristics.

by:Mingfa Tech     2020-01-26
Introduction The central chilled water plant is widely used to provide cooling for various facilities such as commercial building areas, university campuses, hospital complexes and industrial workshops.
Among these facilities, chillers are the main contributors to total energy consumption.
Improving the operating efficiency of the chilled water system and minimizing energy consumption is a major challenge for factory operators and researchers.
One of the most effective energy efficiency measures is to maximize the return and supply temperature difference or increment of the system loop chilled waterT.
Benefits of high delta-
T includes reducing the energy use of the pump, increasing the cooling capacity of the system, and improving the performance of the chiller at low loads.
However, almost all real cold water systems are plagued by low delta
T syndrome, especially at low cold loads. Delta-
Demotion is almost inevitable and it could drop to about 1-half to two-
Thirty designed at low load (Taylor, 2002).
Many factors lead to circulating frozen water delta-
T, such as cold water supply temperature, cooling coil inlet, outlet temperature, flow control valve type, three-stage connection type, coil cold load, solar saver, etc.
However, the root cause of low increments in most systems is
T on the coil side.
The first factor is the geometry of the coil. A high delta-
T coil comes with high air and water hydraulic resistance while low delta-
Tcoil limits the capacity of the chiller, wastes pumping energy and may not be able to meet the cooling needs.
Life Cycle Cost research shows that life cycle cost
Cycle costs tend to be optimized in the highest actual increments-T.
Cooling coils with higher design increments-
Due to the pressure drop on the air side and water side, T consumes more pump and fan energy.
However, the growth of energy consumption is much lower than that of energy conservation due to high increments
T on the chilled water system (Taylor et al. 1999).
The first step in improving system increment is also a crucial step.
The performance is to understand the thermal properties of placing an order cold water cooling coil under various operating conditions.
Including research coil increment-
Different geometric parameters and different water
And air conditions.
A variety of studies have discussed how to maintain a higher increment
Chilled water system. Kirsner (1995)trouble-
Shooting low delta-
Tsyndrome for large chillers
Some possible reasons, such as high water supply temperature, dirty coil, or three-way valves.
Finally, it is found that the main reason is that the coil control setting point of the air handling device is tilted downward and the mixing station does not work. Fiorino (1996)
Recommend 25 \"best practices\" to achieve high chilled water increment\"
From component selection criteria to distribution system configuration guidelines.
Provides an example of low increments
T will prevent the building from meeting the cold load under peakcooling load conditions.
It was pointed out that the frozen water delta-
In the variable flow circulation cooling system, T should be equal to the full load design, which is greater than the partial load design. Taylor (2002)
A comprehensive discussion of the causes and mitigation of lowdeltaT.
He talked about why Delta
T degradation usually occurs and how it is designed around this to maintain the efficiency of the chiller, despite the increment of degradation-T.
The focus is on improving low load performance and trying to fully load the chiller. Moe(2005)
It is recommended to use pressure independent control valve to achieve high increment-T across coils. Conventional 2-
The mode control valve is replaced by a pressure-independent control valve at the coil for the control process.
The valve can eliminate the impact of sudden differential pressure changes on coil flow control or authoritative distortion.
These studies on delta-
Degradation based on actual observations or simplified engineering calculations.
However, detailed quantitative analysis of coil performance changes is not possible under various operating conditions.
Some studies claim that coil partial load performance is one of the main reasons for the rapid decrease in increments
At the low cold load end.
During the swing season and winter months, the coil load in the outer area of the building may drop to a very low level, resulting in the flow of water in the pipe and low delta-T.
If the air treatment device (AHU)
Operating in economic mode, the coil temperature entering the air temperature is low, resulting in the corresponding return water temperature is low.
The water film Heat transfer resistance in the tube is only a small part of the entire air. to-
The water heat transfer resistance at the design flow rate, but with the decrease of the water speed, this resistance will rise until it accounts for nearly 90% of the total resistance under the condition of the laminar flow.
However, this conclusion is not consistent with other studies. Landman (1991)
Shows that the temperature part of the returned chilled water rises
Load conditions by using ARI-to simulate the outlet temperature of the cooling coil chilled water under the design load of the variable air volume and constant air volume system 50% respectively
Certified Tranecooling coil performance program. Taylor (2002)
Shows points near the Delta
T starting to drop is a function of the initial coil speed, but for variable air volume, it is usually around 40% of the load (VAV)
The system of the constant volume system and the load of 50%. Duringmost low-
Flow condition, increment-
T will be on top of the design incrementT.
Wang, Zheng and others. (2006)
By using a decoupling model combining effectiveness, the performance of the cooling coil under different conditions is studied
Nanda method and finite element method.
The results show that the main reason for the low return temperature of chilled water is the use of 3-
If the coil is designed, operated, and maintained properly, the cooling coil control valve is allocated not part of the coil.
The convection coefficient on the water side is estimated using dittus-
Boiler equations from the flow to the turbulence.
In fact, this correction should only apply to the Reynolds number (Re)
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