Energy Performance
Assessment
Of HVAC Systems
Introduction
Air conditioning and refrigeration consume a significant
amount of energy in buildings and in process industries.
The energy consumed in air conditioning and refrigeration
systems is sensitive to
➢ load changes,
➢ seasonal variations,
➢ operation and maintenance,
➢ ambient conditions etc.
Hence, the performance evaluation will have to take into
account, to the extent possible, all these factors.
Purpose of the Performance Test
• The purpose of performance assessment is to verify
the performance of a refrigeration system by using
field measurements.
• The test will measure net cooling capacity (tons of
refrigeration) and energy requirements, at the actual
operating conditions.
• The objective of the test is to estimate the energy
consumption at actual load vis-à-vis design conditions.
Performance Terms and Definitions
Tons of refrigeration (TR): One ton of refrigeration is the
amount of cooling obtained by one ton of ice melting in one
day: 3024 kCal/h, 12,000 Btu/h, or 3.516 thermal kW.
Net Refrigerating Capacity: A quantity defined as the
mass flow rate of the evaporator water multiplied by the
difference in enthalpy of water entering and leaving the
cooler, expressed in kCal/h, tons of Refrigeration.
kW/ton rating: Commonly referred to as efficiency, but
actually power input to compressor motor divided by tons of
cooling produced, or kilowatts per ton (kW/ton). Lower
kW/ton indicates higher efficiency.
Performance Terms and Definitions
Coefficient of Performance (COP): Chiller efficiency
measured in Btu output (cooling) divided by Btu input
(electric power).
Energy Efficiency Ratio (EER): Performance of smaller
chillers and rooftop units is frequently measured in EER
rather than kW/ton. EER is calculated by dividing a chiller's
cooling capacity (in Btu/h) by its power input (in watts) at
full-load conditions. The higher the EER, the more efficient
the unit.
Preparatory for Measurements
• After establishing steady-state conditions, three sets of
data shall be taken at a minimum of five-minute intervals.
• To minimize the effects of transient conditions, test
readings should be taken as nearly simultaneously.
Procedure
1) To determine the net refrigeration capacity
• The test shall include a measurement of the net heat removed
from the water as it passes through the evaporator by
determination of the following:
a) Water flow rate
b) Temperature difference between entering and leaving water
• The heat removed from the chilled water is equal to the product
of the chilled water flow rate, the water temperature difference,
and the specific heat of the water is defined as follows
• The net refrigeration capacity in tons shall be obtained by the
following equation:
Procedure
1) To determine the net refrigeration capacity
Methods of measuring the flow
In the absence of an on-line flow meter, the chilled water flow can
be measured by the following methods
➢ In case where a hot well and a cold well are available, the flow
can be measured from the tank level dip or rise by switching off
the secondary pump.
➢ Non invasive method would require a well calibrated ultrasonic
flow meter using which the flow can be measured without
disturbing the system
➢ If the waterside pressure drops are close to the design values, it
can be assumed that the water flow of pump is same as the
design rated flow.
2) Measurement of compressor power
• The compressor power can be measured by a portable
power analyser, which would give a reading directly in
kW.
• If not, the ampere has to be measured by the available
on-line ammeter or by using a tong tester. The power can
then be calculated by assuming a power factor of 0.9
𝑃𝑜𝑤𝑒𝑟 𝑘𝑊 = 3 × 𝑉 × 𝐼 × 𝑐𝑜𝑠𝜙
3) To determine the heat rejected at the condenser
Heat rejected at condenser = Cooling load + Work
done by compressor
𝑘𝑊
𝐻𝑒𝑎𝑡 𝑟𝑒𝑗𝑒𝑐𝑡𝑒𝑑 𝑇𝑅 = 𝐸𝑣𝑎𝑝𝑜𝑟𝑎𝑡𝑜𝑟 𝑇𝑅 +
3.516
• The shaft power kW absorbed (work done) by the compressor
can be derived by measuring the motor input power multiplied by
motor operating efficiency.
• Heat rejected at the condenser can be measured as under:
Water cooled condenser :
a) Measure the water quantity flowing through the condenser using flow meter.
b) Measure the inlet and outlet temperature of water in the condenser using digital
thermometer.
𝑚𝑐 × 𝑐𝑝 × 𝑡𝑤𝑜 − 𝑡𝑤𝑖 mc - Mass flow rate of cooling water, kg/h
Cp - Specific heat of water, kcal/kg °C
𝐻𝑒𝑎𝑡 𝑟𝑒𝑗𝑒𝑐𝑡𝑒𝑑 𝑇𝑅 = twi - Cooling water temperature at condenser inlet, °C
3024 two - Cooling water temperature at condenser outlet, °C
3) To determine the heat rejected at the condenser
Air-cooled condenser :
a) Measure the air quantity flowing across condenser coil.
b) Measure the inlet and outlet temperatures of air using digital thermometer.
Since this process is normally a sensible heating, the capacity
can be established by calculating only sensible heat gain
Ma - Mass flow rate of air, kg/h
𝑀𝑎 × 𝐶𝑝𝑎 × 𝑡𝑎𝑜 − 𝑡𝑎𝑖 Cpa - Specific heat of air, kcal/kg °C
𝐻𝑒𝑎𝑡 𝑟𝑒𝑗𝑒𝑐𝑡𝑒𝑑 𝑇𝑅 = tai - Temperature of cooling air at condenser inlet °C
3024 tao - Temperature of cooling air at condenser outlet °C
Measurement of air flow
• Air flow may be measured with any of the following instruments:
a) Vane Anemometer
b) Hot wire anemometer
• The measuring instruments should be duly calibrated. The least
count for anemometers should be 0.1 m/s. Air flow rate is calculated
as the multiplication product of the average air velocity in the plane of
measurement and the flow area.
4) Performance calculations
The energy efficiency of a chiller is commonly expressed
in one of the three following ratios:
First calculate the kW/ton rating from the measured parameters.
𝑀𝑒𝑎𝑠𝑢𝑟𝑒𝑑 𝑐𝑜𝑚𝑝𝑟𝑒𝑠𝑠𝑜𝑟 𝑝𝑜𝑤𝑒𝑟, 𝑘𝑊
𝑎 𝑘𝑊 Τ𝑡𝑜𝑛 𝑟𝑎𝑡𝑖𝑛𝑔 =
𝑁𝑒𝑡 𝑟𝑒𝑓𝑟𝑖𝑔𝑒𝑟𝑎𝑡𝑖𝑜𝑛 𝑐𝑎𝑝𝑎𝑐𝑖𝑡𝑦 (𝑇𝑅)
4) Performance calculations
3.516
𝑏 𝐶𝑜𝑒𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑡 𝑜𝑓 𝑝𝑒𝑟𝑓𝑜𝑟𝑚𝑎𝑛𝑐𝑒 (𝐶𝑂𝑃) =
𝑘𝑊 Τ𝑡𝑜𝑛 𝑟𝑎𝑡𝑖𝑛𝑔
12
𝑐 𝐸𝑛𝑒𝑟𝑔𝑦 𝑒𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑐𝑦 𝑟𝑎𝑡𝑖𝑜 𝐸𝐸𝑅 =
𝑘𝑊 Τ𝑡𝑜𝑛 𝑟𝑎𝑡𝑖𝑛𝑔
5) Performance evaluation of air conditioning
systems
• For centralized air conditioning systems, the air flow at the air
handling unit (AHU) can be measured with an anemometer.
• The dry bulb and wet bulb temperatures can be measured at
the AHU inlet and outlet.
• The data can be used along with a psychrometric chart to
determine the enthalpy (heat content of air at the AHU inlet and
outlet)
𝑚 × ℎ𝑖𝑛 − ℎ𝑜𝑢𝑡 m – mass flow rate of air, kg/hr
𝐻𝑒𝑎𝑡 𝑙𝑜𝑎𝑑 = hin – enthalpy of inlet air at AHU, kJ/kg
4.18 × 3024 hout – enthalpy of outlet air at AHU, kJ/kg
• Heat load can also be calculated theoretically by estimating the various
heat loads, both sensible and latent, in the air-conditioned room.
• The difference between these two indicates the losses by way of leakages,
unwanted loads, heat ingress etc.
6) Evaluation of fans / blowers
The following readings can be taken for evaluation.
a) Air flow rate
b) Static pressure developed by the fan pitot tube and
manometer can be used for measuring the differential
head.
c) RPM of the fan using tachometer / stroboscope.
d) Current (amps), Voltage, power factor and power
consumed (kW) by the fan motor using power
analyser.
The above readings establish the fan performance and can
be compared with the design parameter.
7) Evaluation of primary and secondary water
pumps
The following readings can be taken for evaluation.
a) Water flow rate
b) Head pressure developed. (measured with the help
of suction and discharge pressure gauge)
c) RPM
d) Current (amps), Voltage, Power drawn (kW) by the
motor.
The above readings can be compared with the performance
chart or design parameter.
Measurements to be Recorded During the Test
All instruments, including gauges and thermometers, shall be
calibrated over the range of test readings for the
measurement of the following parameters.
Evaporator
a) Temperature of water entering the evaporator
b) Temperature of water leaving the evaporator
c) Chilled water flow rates
d) Evaporator water pressure drop (inlet to outlet)
Compressor
e) Power input to compressor electrical power, kW
Example
In a brewery chilling system, ethylene glycol is used a secondary
refrigerant. The designed capacity is 40 TR. A test was conducted
to find out the operating capacity and energy performance ratios.
The flow was measured by switching off the secondary pump and
measuring the tank level difference in hot well.
Measurements data:
Temperature of ethylene glycol entering evaporator = (-) 1°C
Temperature of ethylene glycol leaving evaporator = (-) 4°C
Ethylene glycol flow rates = 13200 kg/hr
Evaporator ethylene glycol pressure drop (inlet to outlet) = 0.7 kg/cm2
Power input to compressor electrical power, kW = 39.5 kW
Specific heat capacity of ethylene glycol = 2.34 kCal/kg°C
Performance assessment of Vapour
Compression Refrigeration (VCR) system:
The following are the data collected during the energy audit
of chilled water system in a chemical plant. Find out the
kW/TR and COP?
Procedure for Performance Evaluation of Vapour
Absorption Refrigeration (VAR) System
• It should be ensured that the evaporator, condenser and
generator are nearly at same operating conditions
throughout the duration of the test.
• The vapour absorption system auxiliaries include
➢ chiller water pumps,
➢ condenser water pumps,
➢ cooling tower fans
• Performance assessment of these auxiliaries can be
carried out in the same manner as applicable to vapour
compression refrigeration system auxiliaries.
1) Estimation of performance at evaporator side
The performance evaluation involves the measurement of
following parameters.
Refrigeration effect (Qe)
➢ Chilled water flow rate in the evaporator.
➢ Chilled water temperatures at the evaporator inlet and outlet.
Thermal power input (Qin)
➢ Steam mass flow rate in case of steam heated vapour
absorption chilling package.
➢ Fuel flow rate in case of direct fuel fired vapour absorption
package.
Measuring instruments:
• The measuring instruments should be duly calibrated.
• Direct reading thermometers can be used for measuring temperature. The
least count for temperature indicating instruments should be 0.1 °C.
• The method mentioned in VCR system can be used for water flow rate
measurement.
• For steam heated vapour absorption chilling package, the thermal power
consumption may be measured with any of the following instruments:
a) Calibrated in-line steam flow meter.
b) Collection of condensate in calibrated volume (container) for a defined time
period. The time period should be measured with a digital chronometer
(stop-watch) with least count of 1/100 second. The condensate may be
cooled to reduce the flash steam losses.
• For fuel fired vapour absorption systems, the thermal power may be
measured with any of the following instruments:
a) Calibrated In-line fuel flow meter.
b) Fuel level difference (for liquid fuels) for a defined time period in a
calibrated day tank. The time period should be measured with a
digital chronometer (stop-watch) with least count of 1/100 second.
Performance calculations:
➢ Coefficient of performance, COP at evaporator side
𝑁𝑒𝑡 𝑟𝑒𝑓𝑟𝑖𝑔𝑒𝑟𝑎𝑡𝑖𝑜𝑛 𝑒𝑓𝑓𝑒𝑐𝑡,𝑄𝑒 𝑀𝑒 ×𝐶𝑝 × 𝑡𝑖𝑛 −𝑡𝑜𝑢𝑡
𝐶𝑂𝑃 = =
𝑇ℎ𝑒𝑟𝑚𝑎𝑙 𝑝𝑜𝑤𝑒𝑟 𝑖𝑛𝑝𝑢𝑡,𝑄𝑖𝑛 3600×𝑄𝑖𝑛
❖ For Steam heated Vapour Absorption Chilling Packages,
𝑀𝑠𝑡 × ℎ𝑠𝑡 − ℎ𝑐𝑜𝑛𝑑
𝑄𝑖𝑛 =
3600
❖ For Direct Fuel-Fired Vapour Absorption Chilling Packages,
𝑀𝑓 × 𝐺𝐶𝑉
𝑄𝑖𝑛 =
3600
Qe = Refrigeration effect in kW
Qin = Thermal energy input in kW Cp = Specific heat of water, kJ/kg-K
Me = Chilled water flow rate in the evaporator, kg/h Mst = Steam consumption rate, kg/hr
hst = Enthalpy of steam at operating pressure, kJ/kg hcond = Enthalpy of condensate, kJ/kg
tin = Chilled water temperature at evaporator inlet, K Mf = Fuel consumption rate, kg/hr
tout = Chilled water temperature at evaporator outlet, K GCV = Gross calorific value of fuel, kJ/kg
2) Estimation of performance at the condenser
side (water-cooled condenser)
The performance evaluation involves the measurement of
following parameters.
Heat rejected at the condenser (Qc)
➢ Cooling water flow rate in the condenser.
➢ Cooling water temperatures at absorber inlet and the condenser outlet.
Thermal energy input (Qin)
➢ Steam mass flow rate in case of steam heated vapour absorption chilling package.
➢ Fuel flow rate in case of direct fuel fired vapour absorption package
Determination of refrigeration effect and COP:
i. The refrigeration effect and COP can be determined from the overall heat
balance of the condenser and VAR system.
ii. Heat rejected at the condenser (Q) is equal to refrigeration effect heat (Qe) plus
thermal energy input (absorbed) by the system (Qin).
Performance calculations:
COP at condenser side for
steam heated vapour
absorption package
At condenser,
➢ Refrigeration effect heat= Heat rejected in the condenser-
Thermal energy input 𝑄 =𝑄 −𝑄
𝑒 𝑐 𝑖𝑛
𝑁𝑒𝑡 𝑟𝑒𝑓𝑟𝑖𝑔𝑒𝑟𝑎𝑡𝑖𝑜𝑛 𝑒𝑓𝑓𝑒𝑐𝑡, 𝑄𝑒 𝑄𝑐 − 𝑄𝑖𝑛 𝑄𝑐
𝐶𝑂𝑃 = = = −1
𝑇ℎ𝑒𝑟𝑚𝑎𝑙 𝑝𝑜𝑤𝑒𝑟 𝑖𝑛𝑝𝑢𝑡, 𝑄𝑖𝑛 𝑄𝑖𝑛 𝑄𝑖𝑛
𝐻𝑒𝑎𝑡 𝑟𝑒𝑗𝑒𝑐𝑡𝑒𝑑 𝑎𝑡 𝑐𝑜𝑛𝑑𝑒𝑛𝑠𝑒𝑟, 𝑄𝑐
𝐶𝑂𝑃 = −1
𝑇ℎ𝑒𝑟𝑚𝑎𝑙 𝑝𝑜𝑤𝑒𝑟 𝑖𝑛𝑝𝑢𝑡, 𝑄𝑖𝑛
𝑀𝑐 × 𝐶𝑝 × 𝑡𝑤𝑜 − 𝑡𝑤𝑖 𝑄𝑖𝑛 =
𝑀𝑠𝑡 × ℎ𝑠𝑡 − ℎ𝑐𝑜𝑛𝑑
𝐶𝑂𝑃 = −1 3600
3600 × 𝑄𝑖𝑛
COP at condenser side for direct fuel
fired vapour absorption packages
➢ Refrigeration effect heat= Heat rejected in the
condenser - Heat input
𝑄𝑒 = 𝑄𝑐 − 𝑄𝑖𝑛 × 𝜂𝑐𝑜𝑚𝑏
𝑄𝑒 𝑄𝑐 − 𝑄𝑖𝑛 × 𝜂𝑐𝑜𝑚𝑏 𝑄𝑐
𝐶𝑂𝑃 = = = − 𝜂𝑐𝑜𝑚𝑏
𝑄𝑖𝑛 𝑄𝑖𝑛 𝑄𝑖𝑛
𝐻𝑒𝑎𝑡 𝑟𝑒𝑗𝑒𝑐𝑡𝑒𝑑 𝑎𝑡 𝑐𝑜𝑛𝑑𝑒𝑛𝑠𝑒𝑟, 𝑄𝑐
𝐶𝑂𝑃 = − 𝜂𝑐𝑜𝑚𝑏
𝑇ℎ𝑒𝑟𝑚𝑎𝑙 𝑝𝑜𝑤𝑒𝑟 𝑖𝑛𝑝𝑢𝑡, 𝑄𝑖𝑛
𝑀𝑐 × 𝐶𝑝 × 𝑡𝑤𝑜 − 𝑡𝑤𝑖
𝐶𝑂𝑃 = − 𝜂𝑐𝑜𝑚𝑏
3600 × 𝑄𝑖𝑛 𝑀𝑓 × 𝐺𝐶𝑉
𝑄𝑖𝑛 =
3600
Combustion Efficiency Calculations:
• The method described below can be used for estimating
combustion efficiency of a direct fuel fired absorption chilling unit.
• The methodology is the "Indirect Method" to estimate combustion
efficiency (ηcomb), wherein the losses are estimated from flue gas
analysis to estimate efficiency.
Calculations:
Calculations:
Performance assessment of Vapour Absorption
Refrigeration (VAR) system: Example
Performance assessment of Vapour Absorption
Refrigeration (VAR) system: Example
Method 1: Evaporator side
Estimation of performance from refrigeration effect in evaporator
for steam heated vapour chilling packages (Chilling water)
Method 1: Evaporator side
𝑅𝑒𝑓𝑟𝑖𝑔𝑒𝑟𝑎𝑡𝑖𝑜𝑛 𝑒𝑓𝑓𝑒𝑐𝑡, 𝑄𝑒 = 𝑀𝑒 × 𝜌 × 𝐶𝑝 × 𝑡𝑖𝑛 − 𝑡𝑜𝑢𝑡
𝑄𝑒 = 5183200 𝑘𝐽/ℎ
𝑄𝑒
𝑅𝑒𝑓𝑟𝑖𝑔𝑒𝑟𝑎𝑡𝑖𝑜𝑛 𝑒𝑓𝑓𝑒𝑐𝑡 𝑇𝑅 = = 410
3,516 × 3600
𝑇ℎ𝑒𝑟𝑚𝑎𝑙 𝑒𝑛𝑒𝑟𝑔𝑦 𝑖𝑛𝑝𝑢𝑡, 𝑄𝑖𝑛 = 𝑀𝑠𝑡 × ℎ𝑠𝑡 − ℎ𝑐𝑜𝑛𝑑
𝑄𝑖𝑛 = 7974332 𝑘𝐽/ℎ
𝑄𝑒 5183200
𝐶𝑂𝑃 = = = 0.65
𝑄𝑖𝑛 7974332
Method 2: Condenser side
Estimation of performance from heat rejection in Water Cooled
Condenser for steam heated vapour absorption chilling packages
(Cooling water)
