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Low Delta-T Syndrome in
chilled water systems
July 25, 2021 by Guest Author
Delta-T or ΔT in centralised air conditioning systems
refers to the numerical difference between chilled water
supply and chilled water return temperatures. If actual
delta T at the site is less than design delta T of the
chillers, this condition is called low delta-T syndrome.
Low delta T syndrome can be detrimental to overall
energy efficiency of the chiller plant as well as
equipment life.
Cause
:
Low Delta T syndrome is typically caused by lowering of
return chilled water temperature due to inefficient
transfer of heat.
Effect
Low ΔT can lead to improper sequencing of chillers
(operating more chillers than necessary) and pumps,
and consequently increased energy expenditures.
Typical triggers of low delta T syndrome
1.
Oversized equipment
If coils and vales are not sized properly, appropriate
transfer of heat will not take place, resulting in lower
return chilled water temperatures.
2. Scaling and fouling of coils due to improper
maintenance
If maintenance is not carried out properly, heat transfer
surfaces can get damaged which in turn affects the
exchange of heat.
3.
Improper balancing
Incorrect hydronic balancing can result in wasted
pumping efforts during partial load conditions. This can
cause lower return temperatures.
4.
Increased pumping of water
Improper flow values can also lead to low delta t
syndrome.
Explanation for the effect
The quantitative relationship between heat transfer and
temperature change is governed by the formula:
:
Q= mc ΔT
Where:
Q= Chiller load in kW
m= Mass flow rate of chilled water in kg/s
C or Cp= Specific heat capacity of water= 4.186 kJ/kgK
ΔT= Temperature difference between chilled water
supply and return
Consider the example of a theoretical building chilled
water system of the following type:
Total Design Load= 1000 TR
No. of chillers= 4 (250 TR each)
CHWS= 6°C (Chilled water supply temperature)
CHWR= 12°C (Chilled water return temperature)
Design ΔT= 12-6=6
The building has 8 floors, each with a cooling
requirement of 125 TR.
Flow rate calculation:
M=Q/ (Cp* ΔT)
:
;M= Mass flow rate of chilled water
;Q= Heat load or in this case, total chiller plant
capacity=1000TR
;Cp=4.186
; ΔT=6
Thus,
M=1000/ (4.186*6)
=39.82 kg/s
Convert mass flow rate to volume flow rate:
VFR=MFR/Density
=39.82/998.2
=.039 m3/s=.039*60 m3/min=2.39 m3/min = 2.39*264
GPM=631.37 ~ 632 GPM.
Therefore, chilled water flow rate required = 632 GPM.
Flow rate for each floor= 632/8=79 GPM
Consider 2 cases:
1.
Operation at full load
At full load condition, 6°C water that enters the terminal
equipment (AHU, FCU, etc.) or ‘load’ will absorb heat
from the load and increase its temperature to 12°C.
Therefore all 79 GPM of chilled water supply will flow
through the coil without bypassing across the 3-way
valve. In this case, delta T across chiller will be 6.
2.
Operation at partial load
:
At partial load, when the requirement drops to half, the
3-way valves allow bypass of the chilled water and
thereby limit flow through cooling coil. This bypass lets
6°C water mix directly with 12°C return line. Consider
50% of chilled water supply is bypassed at 50% load
(This figure is taken for simplicity of calculation. In actual
practice more than 50% of chilled water is bypassed for
meeting a 50% partial load.). In this case, around 39.5
GPM of water is bypassed.
Thus, temperature of return chilled water line reduces
to
(39.5*6+39.5*12)/79=9
Actual ΔT=9-6= 3
This new ΔT of 3 has an important implication as it has
now changed the loaded capacity of the chillers.
Chiller loading= ΔTA/ ΔTD
; ΔTA= Actual Delta T
; ΔTD= Design Delta T
Chiller Loading= 9/12= 0.75
The typical chiller which has a tonnage rating of 250 TR
now has a usable capacity of only 75% of its original
rating which is (.75*250) 187.5 TR.
If 4 out of 8 floors are vacant with 0 load, the total
cooling requirement reduces to 500 TR. This
requirement of 500 TR can be met with 2 chillers ideally
(250*2), but due to low delta T syndrome, chiller
capacities are reduced and hence 3 chillers need to be
operated to meet the load. This results in increased
energy usage and consequently greater operational
costs.
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