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Achieving High Chilled Water Delta T without Blending Station Zhan Wang, Gang Wang, Ke Xu, Yuebin Yu and Mingsheng Liu Energy System Laboratory, Department of Architectural Engineering University of Nebraska-Lincoln, Omaha, NE 68182, USA ABSTRACT

1. INTRODUCTION

Typically a blending station is designed to

Many buildings installed the chilled water

ensure that its user is able to avoid low

blending station have the problem that by

chilled water return temperature in the district cooling system. When the chilled water return temperature drops to a low limit, building return water is blended into building supply water to reduce primary chilled water flow and finally increase building chilled water return. However, the blending station will cause extra pump power

and

may

cause

humidity

and

temperature issues. Theoretical analysis has been conducted on the blending station performance. The results show that the blending station is not necessary in the building chilled water systems with 2-way modulation valves at end users. Actually the end user valve configuration and control mainly impacts building chilled water temperature. As soon as the water flow control is improved, the chilled water return temperature can be controlled without the blending stations. This paper presents actual system operation data and optimal control measures at three buildings which receive chilled water from a district cooling system.

using blending station to maintain a proper return chilled water (CHW) temperature from the buildings, the actual supply chilled water temperature to the buildings are normally 3-5°F higher than the design supply chilled water temperature required by the end units such as air handling units (AHU), fan coils and induction units. This temperature

is

42°F

(5°C)

which

is

maintained by the district cooling system. With the higher supply chilled water, cooling coil on the AHU side can not work properly as it was designed. It has the following problems on the systems and building environment in regard to the space temperature and relative humidity: (1) The set point of the supply air temperature, 55°F (13°C), can not be maintained with the higher supply chilled water temperature after the blending station. Unwanted moisture can not be removed by the cooling coil. Buildings will suffer humidity problems. (2) The power of circulating pump, either

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constant speed or with a VFD, will

connections. The chilled water temperature

transform to the chilled water as the

differential grew from 4-11°F (2-6°C) to 16-

extra cooling load;

21°F (9-12°C) after all 3-way control valves

(3) For variable air volume (VAV) systems,

in the system were converted to 2-way

the supply air flow rate will increase to

valves. It seems that the use of 3-way

compensate

air

control valves is accused of the lower

temperature. Thus more fan power will

chilled water temperature differential. Wang

transform from the shaft to air flow as

[2006]

the extra cooling load.

performance by the simulation method and

The chilled water blending station is

addressed that the main reason for the low

mainly designed based on an assumption

chilled water return temperature is the use of

that the return chilled water temperature

3-way cooling coil control valves rather than

decreases

load

the partial cooling loads if the cooling coil is

However, several researchers

designed, operated and maintained properly.

confirmed that the chilled water return

The purpose of this paper is to present

temperature can still maintain a high level

that in the real facilities, chilled water return

without the blending station. Landman

temperature can be maintained properly

[1991] compared cooling coil performances

without the chilled water blending station.

between the design condition and off-design

Both first cost of the blending station and

condition. The simulated cooling coil chilled

operation cost can be reduced.

water return temperature increases rather

2. FACILITIES INFORMATION

under

conditions.

the

higher

partial

supply

cooling

than decreases under partial cooling loads. It increases from 58°F (14°C) to 61°F (16°C) with a 42°F (5°C) constant chilled water supply temperature when the supply air flow decreases from the design value by half. Therefore, the partial load is not the main reason for the lower chilled water return temperature.

Kreutzmann

[2002]

demonstrated a district cooling system without a bypass bridge blend at consumer

investigated

the

cooling

coil

Figure 1 depicts a layout of three different buildings in Omaha. The miles in Figure 1 demonstrate the roughly pipeline distance between the central plant and the building. One common thing of the three buildings is that they have installed chilled water blending stations. The other common thing is that their chilled water and steam are provided by the Energy System Company (ESC), a district heating and cooling

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provider.

Notes: Monthly average ∆T =

∑ ∆Ti ⋅ GPM i ∑ GPM i

,

developed by ESC.

Building 1 (0.47 miles)

Figure 2 depicts the schematic diagrams for two conventional blending station

Energy System Company (ESC)

Central Plant

Building 3 (0.46 miles)

connections. The (a) connection with a variable speed circulating pump was applied

Building 2 (0.26 miles)

in building 1 while the (b) connection with Fig. 1: Schematic layout of the three buildings and the central plant

The chilled water rates are provided by ESC based on the monthly average water flow rate per tonnage, shown in Table 1 and Table 2. Delta T (∆T) is the difference between the building return chilled water temperature and the primary supply chilled water temperature, normally 42°F (5°C).

the constant speed circulating pumps was applied in building 2 and 3. The end units, including AHU cooling coils, fan coils, and induction units, use either 2-way or 3-way control valves to maintain the supply air temperature or space temperature at the set point in the buildings. From district cooling plant

To district cooling plant Pressure reducing valve

The higher the delta T, the lower the chilled

VFD

water rates are.

P1

T1

A

Table 1: Winter CHW rates (Oct - Apr) Average flow

Average ∆T

Rates

rate (GPM/ton)

(ºF)

(cents/ton·hour)

< 1.70

≥ 14

22.29

> 1.71

< 14

23.29

Δ

Average

Rates

rate (GPM/ton)

∆T(ºF)

(cents/ton·hour)

< 1.70

≥ 14

22.29

1.71 ~ 2.00

12 ~ 14

23.29

2.01 ~ 2.40

10 ~ 12

24.29

2.41 ~ 3.00

8 ~ 10

25.29

> 3.01