CHILLER PLANT EFFICIENCY

Schematic of Typical Chilled Water System

This is a schematic of a typical chilled water system. It shows the different loops of the system. In the chiller plant, the condenser loop goes to a cooling tower. The chilled water loop goes to the cooling coils. Coming off the coils is the air distribution system which delivers air to the zones.

Chilled Water System Configurations

Unit loop distribution Primary Loop Pumping

Unit loop with primary distribution Primary-Secondary Pumping

This configuration has one set of pumps. It is usually constant speed pumping. Sometimes referred to as Primary Pumping. Note the 3-way valves for temperature control at loads (AHU).

This configuration has one set of pumps for the chiller loop and a separate set to circulate the chilled water throughout the distribution system – to air-handler coils and perimeter systems. The building pumps are often variable speed. It is sometimes referred to as Primary-Secondary pumping. Note the 2-way valves at loads.

For the loops in the chiller plant, there are different ways to set-up pumping. Each set of pumps gives you better control.

More info: Typical chilled water systems are designed for a rise in temperature (ΔT or “dT”) of 10-12 degrees Fahrenheit. To improve pumping efficiency at part-loads, systems have sometimes been retrofitted to primary-secondary pumping.

Chilled Water System Configurations

Source: courtesy Tao and Janis

Unit loop with primary and secondary distribution Tertiary Pumping

This configuration is for multiple buildings and adds another layer of “tertiary” pumps for each building or wing. It is applied where certain loads need further control of flow or pressure.

Chilled Water Distribution – Pumping Strategies

Constant Volume

The amount of water being delivered throughout the system is always the same. Control of cooling to the load required by the zones is by changing the temperature of the CHW and/or by by-pass (3-way) valves at loads.

Constant Primary / Variable Secondary & Tertiary

Same as constant volume, but the control is done at the secondary systems/loops. The temperature is varied/reset to meet the load by changing the volume of the flow.

Variable Primary / Variable Secondary

The volume can be changed at the primary, by taking chillers offline, and by changing the pump speed at the secondary systems.

More detail the various pumping and control strategies associated with the chilled water system configurations.

Recent Systems – Variable Flow Single Loop

All of the systems to the left maintain a constant volume at chillers, because of manufacturer requirements. To achieve better efficiencies, manufacturers have been modifying equipment designs to handle variable flows. Advances in tube design and control of compressor speeds and refrigerant flows make it possible for chillers to handle variable CHW flow. The secondary loop is no longer required in order to provide for variable flow in the building distribution system.

Chiller System at Full Load with Ideal Conditions

This diagram is from a Building Automation System and shows the ideal conditions for the chiller. The load requires 2000 Gallons Per Minute (GPM) and each chiller is supplying 1000 GPM. These chillers are designed for a 12 degrees Fahrenheit ΔT and the supply temperature is 45 degrees Fahrenheit. The load in the zones is using enough cooling so that the return water is 57 degrees Fahrenheit, which means that the temperature dropped 12 degrees Fahrenheit and no chilled water is bypassing the load.

1,000 Ton Plant

2,000 GPM, 12OF ΔT

Exiting temp. 45d

Entering temp. 57d

No bypass

2000 GPM

supplied

2000 GPM

delivered

Running at 500 Tons

Running at 500 Tons

1000 GPM

supplied

1000 GPM

supplied

Building Automation System (BAS)

Building Automation Systems (BAS) will provide a schematic view of the system with current operating conditions. Understanding how to reach these “screens” is very important for achieving best operating conditions and performance.

Chiller System at Part Load In this scenario the one thousand ton plant is only supplying 704 tons, so this is known as a part load condition.

The chiller is still delivering two thousand gallons per minute, but part of the chilled water goes to the bypass and directly back to the chillers. The combination of the bypass and the return water has a drop in temperature of about 8 degrees which is less than the design dT of 12 degrees. This is significant because most chillers lose efficiency at part-load.

Building Automation System (BAS)

CHW - Air Distribution Interface

• More efficient – less air to be moved by fans – but less common.

• Circulates the chilled water through an element located on the ceiling of the conditions space. Similar in function to induction units.

• The lower temperature of the surface area of the “beam” removes heat from the space in which it is located both by radiant effect and by inducing a flow of air across it.

• Ventilation air is provided by a separate system.

• Cools and dehumidifies the air by heat exchange from CHW as air is blown across the coils.

• Multiple rows keep coil surfaces clean for good heat transfer. • Control valve on coil inlet maintains temperature of air

leaving the coil at set-point

Cooling coils Chilled beam

Once the chilled water cools the air, the air needs to be distributed to the zones. The CHW system interfaces to the air-system most commonly in air-handling units (AHU) which contain cooling coils. A more recent technology being introduced are chilled beams.

More info: From air-handling units there may be extensive ducting and additional controls (such as variable air volume boxes to provide conditioned air to spaces. Refer to the separate lesson for air system components.

Chiller Performance Issues

• Vapor-Compression Cycle • Refrigerant charge (low or excess) • Non-condensables, air in system • Oil contamination • Refrigerant level control • Compressor

• Mechanical or control issues: • Fouled tubes: build-up on the heat exchanger from minerals

and other impurities in the water reduces conductivity. • VFD & vortex damper control. • Special issues for Steam chillers:

• Turbine issues, surface condenser vacuum, wet steam. • Part-Load Condition:

• Chilled Water and Condenser Water Temperatures • 1OF change in condenser water (CW) temperature = 1%

- 1.5% chiller performance. • 1OF change in chilled water (CHW) temperature = 0.75%

- 1% chiller performance • Delta T

• Low delta T (<10dF) (excess flows in distribution system)

Actual chiller performance is often far from nameplate ratings. Here are the most common issues that reduce the efficiency of a chiller. Many can be avoided with a regular maintenance program.

Rating Terms for AC Performance

EER

Energy Efficiency Ratio is probably the most familiar term for rating AC performance as it is used on the “EnergyStar” rating label for unitary AC and heat pumps. The EER is the ratio of rated output cooling to its input electrical power, EER = BTU / watts. High-efficiency unitary equipment now achieves EER of 11 to 12 but many models are still in the market with EER of 8 to 10. The rated EER assumes that refrigerant charge is correct, coils clean, etc.

SEER

Seasonal Energy Efficiency Ratio. The EER but instead of using rated input and output, it uses the expected overall performance for a typical year's weather in a given location, taking into account expected part-load performance.

IPLV

Integrated Part Load Value. IPLV is the efficiency of an AC unit (usually a chiller) under various part-load conditions, i.e. various outdoor temperatures when the unit would operate at 25%, 50%, 75% and 100% of capacity. The manufacturer arranges these along a curve that can be “integrated” into a single value for a given climate.

Rating Terms for AC Performance

kW/ton

A ton of refrigeration is the amount of thermal energy released by one ton of ice melting over 24 hours, or 12,000 BTU/hr. The term is left over from the days of using ice for refrigeration. Depending on the efficiency of your system, it will take a certain amount of electrical energy (KW) to produce this ton of cooling effect. This is the most direct, easy-to-use measure of AC system performance.

COP

Coefficient of Performance. Similar to EER, output / input, but the input energy is converted into BTU (1 KW = 3,414 BTU). COP = BTU out / KW x 3414 in

Measuring KW/Ton

KW / Ton measures the electricity input required to produce a ton of cooling output.

1 ton = 12,000 BTU

To determine the output, you need the “delta T” and the flow rate: • For a chiller, you’re measuring flow of water, in GPM

and the formula for cooling effect, in BTU, is Q = 500 x GPM x dT, where 500 is a constant.

• For an Air-Handling Unit (AHU), you’re measuring flow of air, in CFM, and the formula for cooling effect, in BTU, is Q = 1.08 x CFM x dT, where 1.08 is a constant.

To determine the input, consider the boundary of your system: • Are you measuring just the compressor power or

are you also including the system’s pumps and fans, perhaps the cooling tower too?

• Including pumps and fans is more complicated but gives you a more accurate picture of system performance.

The most important measure of AC system performance is Kilowatts per ton. It measures the electricity input required to produce a ton of cooling output. To determine the output, you need the “delta T” and the flow rate. To determine the input, it’s important to consider the boundary of your system. Your on-board controls will usually provide the measurements for both the input and output and may export them to a Building Automation System (BAS).

Comparing Your kW/Ton

Remember that kW/Ton will vary over the course of the season, based on the load. AC systems will usually be less efficient at part-loads, so you would expect higher kW/Ton in mild weather operation. Newer systems, however, may actually have better efficiency (lower KW/Ton) at part-loads.

Type of System Typical kW/Ton

Performance

Older package DX units 1.5 – 2.0 kW/Ton

Chillers 1.2 – 1.8 kW/Ton

New, Premium Efficiency

Chillers

.8 – 1 kW/Ton

Once you know your Kilowatts per ton, how do you evaluate the performance? Onscreen are performance guidelines. If your measurements are much higher than these, you should consider an improvement program.