HVAC system consists of a chain of components designed to cool or heat, ventilate a specific area while maintaining a defined environmental cleanliness level. Purpose of HVAC system is to control or maintain temperature – heating, to purify the air – ventilation and to control or maintain humidity – air conditioning. Climate control systems typically account for a substantial part of energy consumption in commercial buildings. HVAC (Heating, Ventilation and Air Conditioning) is the technology of indoor and automotive environmental comfort. HVAC systems use ventilation air ducts installed throughout a building to supply conditioned air to a room through outlet vents, called diffusers; and ducts to remove air through return-air grilles. HVAC System design is based on principles of thermodynamics, fluid mechanics and heat transfer. HVAC system is important in design of medium and large offices and industrial buildings and marine environments. The three central functions of heating, ventilating and air conditioning are interrelated, especially, installation, operation and maintenance costs. The obligations established in global agreements as well as regulations and legislation that limit the energy consumption and greenhouse gases emissions gave a novel importance to the HVAC systems rating.
One of the key points of the inspection procedure is the availability of reliable energy performance data of the main components of the HVAC system. This task is usually easy for heating only systems, such as gas boilers coupled to hydronic heating plants, but much more complex for systems delivering both heating and cooling. In the latter case, in fact, most system equipment (e.g., water chillers, cooling towers, fans, chilled / hot water pumps, fan coils, etc.) are electrically-driven, but the electricity consumption is seldom measured in a disaggregated way. Normally, the only available electrical consumption data are those measured at the main incomer, and therefore, also includes the contribution of lighting and appliances. One of the main problems is that the definition of energy data collection protocols, suitable for an effective inspection and energy auditing process of existing HVAC systems.
Air conditioning and refrigeration consumes 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.
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.
HVAC and Heat transfer Loops in Refrigeration System
The Heating, Ventilation and Air Conditioning (HVAC) and refrigeration system transfers the heat energy from or to the products or building environment. Energy in form of electricity or heat is used to power mechanical equipment designed to transfer heat from a colder, low energy level to a warmer, high energy level. Refrigeration deals with the transfer of heat from a low temperature level at the heat source to a high temperature level at the heat sink by using a low boiling refrigerant. There are several heat transfer loops in refrigeration system as shown in figure 1.
In Figure 1, thermal energy moves from left to right as it is extracted from the space and expelled into the outdoors through five loops of heat transfer.
Indoor Air Loop: In the leftmost loop, indoor air is driven by the supply air fan through a cooling coil, where it transfers its heat to chilled water. The cool air then cools the building space.
Chilled Water Loop: Driven by the chilled water pump, water returns from the cooling coil to the chiller’s evaporator to be re-cooled.
Refrigerant Loop: Using a phase-change refrigerant, the chiller’s compressor pumps heat from the chilled water to the condenser water.
Condenser Water Loop: Water absorbs heat from the chiller’s condenser, and the condenser water pump sends it to the cooling tower.
Cooling Tower Loop: The cooling tower’s fan drives air across an open flow of the hot condenser water, transferring the heat to the outdoors.
Energy related Performance Terms
Refrigeration: Refrigeration is defined as an art of producing and maintaining the temperature in a space below atmospheric temperature.
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.
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 is the unit.
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.
Coefficient of Performance (COP): Chiller efficiency measured in Btu output (cooling) divided by Btu input (electric power).
Depending on applications, there are several options or combinations, which are available for use as given below:
- Air Conditioning (for comfort)
- Split air conditioners
- Fan coil units in a larger system
- Air handling units in a larger system
Refrigeration Systems (for processes)
Small capacity modular units of direct expansion type similar to domestic refrigerators, small capacity refrigeration units.
Centralised chilled water plants with chilled water as a secondary coolant for temperature range over 5°C typically. They can also be used for ice bank formation.
Brine plants, which use brines as lower temperature, secondary coolant, for typically, sub zero temperature applications, which come as modular unit capacities as well as large centralised plant capacities.
The plant capacities upto 50 TR are usually considered as small capacity, 50 – 250 TR as medium capacity and over 250 TR as large capacity units.
A large industry may have a bank of such units, often with common chilled water pumps, condenser water pumps, cooling towers, as an offsite utility.
The same industry may also have two or three levels of refrigeration and air conditioning such as:
Comfort air conditioning (20° – 25° C)
Chilled water system (8° – 10° C)
Brine system (sub-zero applications)
Two principle types of refrigeration plants found in industrial use are: Vapour Compression Refrigeration (VCR) and Vapour Absorption Refrigeration (VAR). VCR uses mechanical energy as the driving force for refrigeration, while VAR uses thermal energy as the driving force for refrigeration.
Selection of Suitable Refrigeration System
A clear understanding of the cooling load to be met is the first and most important part of designing or selecting the components of a refrigeration system. Important factors to be considered in quantifying the load are the actual cooling need, heat (cool) leaks, and internal heat sources (from all heat generating equipment). Consideration should also be given to process changes and changes in ambient conditions that might affect the load in the future. Reducing the load, e.g. through better insulation, maintaining as high a cooling temperature as practical, etc. is the first step toward minimising electrical power required to meet refrigeration needs. With a quantitative understanding of the required temperatures and the maximum, minimum, and average expected cooling demands, selection of appropriate refrigeration system (single-stage or multi-stage, economised compression, compound or cascade operation, direct cooling or secondary coolants) and equipment (type of refrigerant, compressor, evaporator, condenser, etc.) can be undertaken.
Performance Assessment of Refrigeration Plants
- The cooling effect produced is quantified as tons of refrigeration (TR).
1 TR of refrigeration = 3024 kCal/hr heat rejected.
- The refrigeration TR is assessed as TR = Q x Cp x (Ti – To) / 3024 where Q is mass flow rate of coolant in kg/hr Cp is coolant specific heat in kCal /kg deg C Ti is inlet, temperature of coolant to evaporator (chiller) in °C To is outlet temperature of coolant from evaporator (chiller) in °C. The above TR is also called as chiller tonnage.
- The specific power consumption kW/TR is a useful indicator of the performance of refrigeration system. By measuring refrigeration duty performed in TR and the kiloWatt inputs, kW/TR is used as a reference energy performance indicator.
Measurement of Compressor Power
The compressor power can be measured by a portable power analyser which would give reading directly in kW. If not, the ampere has to be measured by the available online ammeter or by using a tong tester. The power can then be calculated by assuming a power factor of 0.9 Power (kW) = √3 x V x I x cosf
The energy efficiency of a chiller is commonly expressed in one of the three following ratios:
- Coefficient of Performance
- Energy efficiency ration
- Power per Ton
Performance Evaluation of Air Conditioning Systems
For centralised 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).
Heat load can be calculated theoretically by estimating the various heat loads, both sensible and latent, in the air-conditioned room (refer standard air conditioning handbooks). The difference between these two indicates the losses by way of leakages, unwanted loads, heat ingress etc.
Case Study: How to select Refrigeration Compressor Motor?
A reciprocating refrigeration compressor of a 100 TR is working at full load with 4.5 ‘C temperature difference across the evaporator. How to Estimate the water flow rate if water is secondary coolant? How to assess the connected motor size (Kw) to the refrigeration compressor?
Let us take the capacity of reciprocating compressor = 100 TR and working fluid is water so specific heat of water = 1 Kcal/Kg ‘c).
Chilled temperature across evaporator = 4.5 C.
So, Chilled water flow rate Q (kg/h) = (100 x 3024) / (4.5 x 1) = 67200 Kg/hr.
Specific power consumption of reciprocating compressor = 0.7-0.9 Kw/ TR.
Hence, motor power = 100 x 0.9 = 90 Kw.
So, connected motor size may be 90 Kw or maximum 110 Kw may be selected.
Energy Conservation and saving opportunities in refrigeration air conditioning plant area:
The following are few major energy savings opportunities in refrigeration plant area.
- Ensure adequate quantity of chilled water and condenser water flows, avoid by pass flows by closing valves of idle equipment.
- Minimise part load operations by matching loads and plant capacity online; adopt variable speed drives for varying process load.
- Make efforts to continuously optimise condenser and evaporator parameters for minimising specific energy consumption and maximising capacity.
- Ensure regular maintenance of all A/C plant components as per manufacturer guidelines.
Other Energy Saving Opportunities
Insulate all cold lines or vessels using economic insulation thickness to minimise heat gains; and choose appropriate (correct) insulation.
Optimise air conditioning volumes by measures such as use of false ceiling and segregation of critical areas for air conditioning by air curtains.
Building Heat Loads Minimisation
Minimise the air conditioning loads by measures such as roof cooling, roof painting, efficient lighting, pre-cooling of fresh air by air-to-air heat exchangers, variable volume air system, otpimal thermo-static setting of temperature of air-conditioned spaces, sun film applications, etc.
Process Heat Loads Minimisation
Minimise process heat loads in terms of TR capacity as well as refrigeration level, i.e., temperature required, by way of:
- Flow optimisation
- Heat transfer area increase to accept higher temperature coolant
- Avoiding wastages like heat gains, loss of chilled water, idle flows.
- Frequent cleaning or de-scaling of all heat exchangers
At the Refrigeration A/C Plant Area
- Ensure regular maintenance of all A/C plant components as per manufacturer guidelines.
- Ensure adequate quantity of chilled water and cooling water flows, avoid bypass flows by closing valves of idle equipment.
- Minimise part load operations by matching loads and plant capacity on line; adopt variable speed drives for varying process load.
- Make efforts to continuously optimize condenser and evaporator parameters for minimizing specific energy consumption and maximizing capacity.
- Adopt VAR system where economics permit as a non-CFC solution.
How Improved Energy Efficiency and Operational Performance?
Connecting HVAC to IoT provides opportunities to improve operational performance and energy efficiency by linking performance to other data sets. These could include weather forecasts, holiday periods and even local usage in smart buildings whose users are tagged to let the intelligent building system know whether they are on site or not and which rooms they are using. It is an example of a form of artificial intelligence (AI) within a smart building as the HVAC systems make real time adjustments to optimise their performance and adjust themselves within a range of operational parameters.
The benefits to the organisation realise themselves in terms of lower energy bills and improved energy efficiency with the potential to reduce costs by around 25-30 per cent per year compared to traditional or conventional HVAC systems.
Methods by which IoT connected HVAC Systems can be made:
- With a WiFi Smart Thermostat
- Cloud Based Data Availability
- Real Time Monitoring and Control
- Local management on site.
In this article, author has tried to explain the methods of energy performance assessment of HVAC and refrigeration systems. At this era, the labor cost, material cost and transportation cost are rising in India and worldwide. So, it is necessary to perform proper Energy conservation and audit.