Designing an Air

Conditioning system

for optimum operation

Prepared by

Franklin Silva

Introduction

The Chilled water system is the single largest

consumer of Electrical Energy in a facility.

The load includes Chillers, Cooling Towers,

Pumps & Air Handling units.

The Electricity consumption varies with the

combination of equipment and its operating

levels

Selection of optimal mix of equipment to match

the load to most efficient operating point is the

objective to conserve energy.

Design

A very accurate heat load is a must.

Choose the best available equipment to match the base

load.

Pressure drops across the evaporator and chilled water

coils should be the lowest possible.

Install accurate temperature controls with capability of

changing the water flow through coils and the air flow.

Recycle condensate by using it for the make up of the

cooling towers

Design Should be based on

operating cost

82.6%

17.4%

Electricity

Plant Room Equipment

Rather than the initial capital Investment. Capital investment is only 17% of the 10 year life cycle cost.

Heat Load Calculation

Calculate Heat Load

Typical load

Profile

0.00

50.00

100.00

150.00

200.00

250.00

300.00

350.00

400.00

450.00

500.00

8am

9am

10am

11am

12pm

1pm

2pm

3pm

4pm

5pm

6pm

7pm

Calculate the Heat Load

Accurately

Shown is a typical Load Profile of

a factory in Sri Lanka

From this we can determine the

Max / Minimum load so that a

chiller with the most efficient

operating conditions can be

selected.

Determine Max / Min

Chilled water systems

runs at.

6 Hours a day at 400 to

460 Tons

3 Hours at 380 to 390

Tons

2 Hours at 260 Tons

1 Hour at 230 Tons0.00

50.00

100.00

150.00

200.00

250.00

300.00

350.00

400.00

450.00

500.00

8am

9am

10am

11am

12pm

1pm

2pm

3pm

4pm

5pm

6pm

7pm

Matching a Chiller to the

System

From the Heat Load calculation a chiller / s is

required to run 6 hours at 460 Tons and the other

6 hours at less than 390 Tons.

Our objective is to have Chiller running at its most

optimum under all varying conditions.

For Reliability it is prudent to have two chillers

running in parallel

They should meet the criteria of running at

optimum at all load conditions.

Selecting a Chiller

System Runs most of the time i.e. 6 hours a

day at 460 Tons

Then at 3 Hours at 380 to 390 Tons

Therefore the system should be in excess of

460 Tons

Since that the system requires 460 Tons the

chiller should operate at the best efficiency at

460 Tons

Consider Flexibility &

Reliability

Shown are the

Chiller data any

Manufacturer should

provide for evaluation

High lighted are the data

that should be

Considered for selection.

Start with capacity

required at Optimum

Conditions.

Capacity required is 460

Tons

Chiller Data

Evap Cond

Refrigerant 134a Fluid Water Water

Rated Capacity 260 Tons By weight % 0 0

Input power in KW 145 Tubes 261 260

Voltage 400/3/50 Passes 2 2

Orifice VARV Valve:2 Fouling 0.0001 0.00025

RLA 244 Ent Temp °F 54 85

Min CIR 305 Fluid Leavig °F 44 94.3

Inrush 920 Fluid flow gpm 624 780

SSS size 14LAK-50 Fluid Press drop 12.3 16.8

Full Load KW/Ton 0.558

Chiller Part Load Data

Load % Cap in Tr Power %

Input

Power in

KW

Chilled

water in

°F

Chilled

water

out °F

Cond

water in

°F

Cond

water

out °F

Eff in

KW/Ton

100 260 100 145 54 44 85 94.3 0.558

90 234 82.8 120 53 44 81 89.28 0.513

80 208 69.7 101 52 44 77 84.31 0.486

70 182 59.3 86 51 44 73 79.37 0.473

60 156 51 74 50 44 69 74.46 0.474

50 130 44.1 64 49 44 65 69.56 0.492

40 104 40 58 48 44 65 68.71 0.558

30 78 35.2 51 47 44 65 67.85 0.654

29.8 77.4 35.2 51 46.98 44 65 67.83 0.659

Usually chillers run best at 70% load

In this case also chiller is at the best

efficiency of 0.473 at 70%

Further analyzing the heat load, we find that

the 460 Tons is only for a very short time.

Chillers will run at 400 Tons most of the time.

Therefore considering the cost of the chillers

it is prudent to match to 400 tons and not 460

which is the peak.

Capacity at optimum

400 Tons is the highest average capacity

required.

From the chiller data it is amply evident that

the chiller operates at its best when it is 70%

to 80%.

Therefore chiller capacity required is 400 ÷

70 x 100 = 571 or 400 ÷ 80 x 100 = 500

The chiller can be either 500 Tons or 571

Tons

Selection of the Chiller

Chiller Selection Part II

If a 571 Chiller is selected it will run at 70%

which has an efficiency of 0.473 KW/Ton where

as a 500 Ton is selected it will run at 80% which

has an efficiency of 0.486

500 Ton chiller is cheaper than the 571 Ton

chiller further efficiency difference is only 0.013

KW

A 10 year life cycle costing of the two chillers

will show that it is beneficial to use a 500 Ton

chiller

Final Selection

Considering all what was discussed and reliability it is

best to have 2 x 250 Ton Chillers.

The closest to our requirement from this manufacturer is

260 Tons which in total 2 x 260 = 520 Tons.

We now have the flexibility of running at lowest load at

an efficiency of 0.474 KW/Ton which basically is the best

from the equipment selected

Select the lowest pressure drop through the Evaporator

and Condenser. This will save energy from the pumps.

Other equipment

Having selected the chillers we need to have

the other components also to run efficiently

for the whole system to be efficient. Such as

1. Cooling Towers

2. Water pumps

3. Air Handling units

4. Controls

Cooling Towers

Decrease in condenser

temperature will reduce

the differential

pressure of the

compressor which in

turn reduces the work

done making the

compressor more

efficient

Further savings

Increasing Chilled water

temperature will also

save energy

Increasing chilled water

temperature is

increasing evaporating

temperature

It is reducing the work

done by the compressor

Same chiller simulated

to operate at higher

temps at 46°F & 48°F

leaving chilled water.

Chiller Part Load Data at 44°F Leaving

Load % Cap in Tr Power %

Input

Power in

KW

Chilled

water in

°F

Chilled

water

out °F

Cond

water in

°F

Cond

water

out °F

Eff in

KW/Ton

100 260 100 145 54 44 85 94.3 0.558

90 234 82.8 120 53 44 81 89.28 0.513

80 208 69.7 101 52 44 77 84.31 0.486

70 182 59.3 86 51 44 73 79.37 0.473

60 156 51 74 50 44 69 74.46 0.474

50 130 44.1 64 49 44 65 69.56 0.492

40 104 40 58 48 44 65 68.71 0.558

30 78 35.2 51 47 44 65 67.85 0.654

29.8 77.4 35.2 51 46.98 44 65 67.83 0.659

Chiller Part Load Data at 46°F Leaving

Load % Cap in Tr Power %

Input

Power in

KW

Chilled

water in

°F

Chilled

water

out °F

Cond

water in

°F

Cond

water

out °F

Eff in

KW/Ton

Chiller

Load %

Efficiency

at 44°F in

KW/Ton

Efficiency

at 46°F in

KW/Ton

Efficiency

at 48°F in

KW/Ton 100 260 100 139 55.99 46 85 94.3 0.5345

100 0.558 0.5345 0.508 90 234 83.5 116 54.99 46 81 89.28 0.496

90 0.513 0.496 0.479 80 208 70.5 98 53.99 46 77 84.31 0.486

80 0.486 0.486 0.462 70 182 60.4 84 52.99 46 73 79.35 0.462

70 0.473 0.462 0.456 60 156 52.5 73 51.99 46 69 74.46 0.468

60 0.474 0.468 0.474 50 130 46.8 65 50.99 46 65 69.57 0.567

50 0.492 0.567 0.508 40 104 42.4 59 49.99 46 65 68.72 0.567

40 0.558 0.567 0.596 30.9 80.4 39.6 55 49.09 46 65 67.95 0.684

Chiller Part Load Data at 48°F Leaving

Load % Cap in Tr Power %

Input

Power in

KW

Chilled

water in

°F

Chilled

water

out °F

Cond

water in

°F

Cond

water

out °F

Eff in

KW/Ton

100 260 100 132 57.99 48 85 94.2 0.508

90 234 84.8 112 56.99 48 81 89.21 0.479

80 208 72.7 96 55.99 48 77 84.26 0.462

70 182 62.9 83 54.99 48 73 79.35 0.456

60 156 56.1 74 53.99 48 69 74.46 0.474

50 130 50 66 52.99 48 65 69.58 0.508

40 104 47 62 52 48 65 68.75 0.596

32.2 83.6 44.7 59 51.21 48 65 68.09 0.706

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

1 2 3 4 5 6 7

Efficiency at 44°F in KW/Ton Efficiency at 46°F in KW/Ton

Efficiency at 48°F in KW/Ton

Basic Facts about Chillers

Chillers consumes the most amount of

energy in any system.

It is in the range of 50% to 60% of the total

energy of the building.

Therefore any saving that can be achieved is

substantial in the overall energy cost.

Lowering the condenser water temp or

increasing the chilled water temp will save

energy. Of course there are limitations.

Selection Criteria of Cooling

Towers

To take advantage of lowering of the condensing

temperature cooling towers should be sized at

least 25% to 30% higher than the chiller heat

rejection.

Cooling tower fans should not be controlled by

VSD’s

Reason is that the cooling tower fan motor is

relatively smaller than the compressor motor.

Reduction in condenser water will save energy

from the compressor motor which is far greater

Lowering Condenser

Temperature

Lowering of condenser water can be achieved by over

sizing of cooling towers.

Over sizing cooling towers will also reduce the drift and

evaporation losses.

There is a limit that cooling towers can be oversized.

Condenser return will not go down below the ambient

dew point.

Therefore dew point must be considered when selecting

Cooling tower is the fastest de rating component in the

system.

Why should we Increase

Chilled water Temperature

All Calculations are based on 44°F/54°F

It does not mean that the chillers should be

run at that temperature all the time.

Chilled water can be increased at low load

situations.

Generally peak time in Sri Lanka is about 6

hours a day.

Rest of the time chillers run at part load.

So the savings can be substantial

How

Slightly oversize AHU’s at design stage using

higher chilled water temperature of say 48°F

Use BMS to override chilled water control of

the chiller at low load conditions

Performance of the Cooling

Tower

Cooling towers should conform to the following

Evaporation loss should be less than 0.83% of

the circulation

Drift loss should be less than 0.005% of the

circulation.

Noise level should be less than 70 db

Basically sizing of cooling towers is a

compromise between the initial cost of the

cooling towers and running cost of chillers and

cooling towers

Pumps

Size the pumps to take care of the design flow requirements against

friction losses.

Use safety factors for losses across valves and fittings as we are

uncertain and estimated values are used.

Avoid over pumping by trimming the impellors after installation to

bring the flow rates to design.

Over pumping increases the resistance artificially resulting in

inefficient operation

Select pumps at highest efficiency possible at design conditions

It is always better to have few high capacity pumps than many

smaller capacity pumps.

Bigger pumps tends to be more efficient than smaller capacity ones

Variable Flow

Variable flow can be easily achieved in 2 pipe

systems

Variable speed drives should be used in 2

pipe systems to reduce the flow rate by

reducing the pump speed to match the

reduced flow require due to closure of

modulating valves

A decoupler shall be used where single set of

pumps are used in a 2 pipe system.

Variable Air Flow

Incorporating VSD to fans will save energy

VSD can be controlled by static pressure only in

the case of VAV’s

In constant flow system static pressure control

does not serve any purpose.

In such cases two thermostat should be used.

One for the supply air to control the chilled water

flow through and other preferably a multi sensor

thermostat to control the room temperature by

adjusting the VSD

Selection of Air Handling Units

If manufacturer’s selection program is available

run several times to make sure that the optimum

unit is selected.

Select the lowest water pressure drop through

the cooling coil.

Select AHU’s at 48°F chilled water inlet.

Try to limit one AHU per zone

Include mixing box so that fresh air and return

air can be modulated to reduce CO2 level

Air Handling Unit

Fan

Vibration Isolators

Cooling coil

Filters

Air Distribution

Power absorbed by the fan depends on the air flow it

needs to deliver against system pressure.

Reduce the back pressure as much as possible.

Connection to the AHU is as important as the duct.

Remember to strictly follow the recommended

connecting methods to the AHU’s.

Controls

Controls must be accurate.

A BMS may be installed to control zones as

well as to predict future requirement after

analyzing Climatological Historical data.

Incorporate demand limiting to help maintain

maximum power demand below a set target

LEED requires multi sensor thermostats.

What Chiller to use

Always analyze part load

Use the best part load chiller to match your heat

load.

A Centrifugal chiller is used generally in

situations where the load is steady as they are

more efficient at almost full load conditions.

A Screw chiller is use under varying conditions

as the have a almost linear part load efficiency

Take Note

Never select equipment just because a

manufacturer recommends it.

Recommend what is best suited for your

project.

Use Air cool chillers unless there is scarcity

of water for make up of cooling towers.

Use Absorption chillers only where waste

heat is available or cheap heating is available

as they are less efficient.

Night Precooling

Night precooling is circulation of cool air

during night time hours.

The structure will serve as a heat sink

Will reduce pull down cooling load

Will reduce contamination

Only energy cost is the power requirement for

the ventilation fans which is minute compared

with the energy consumed by the Air

Conditioning system

Green / Sustainable Design

Minimize natural resource consumption

through efficient utilization.

Minimize emission that negatively impact our

environment.

Minimize solid waste and liquid effluents.

Minimal negative impact on site ecosystem

Maximum quality of indoor environment.

Conclusion

Energy Savings is a On Going Committed

Process

Energy Saving needs dedicated people to

actively monitor the performance and keep

the system fine tuned.

Energy saving should be recognized by the

management and suitably rewarded if not fine

tuning will be very short term.