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HVAC

Introduction

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 described below:

Figure 1: Heat Transfer Loops In Refrigeration System

In the 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:

Air-Conditioning Systems

Depending on applications, there are several types of air-conditioning systems:

  • Air Conditioning (for comfort / machine)
  • Split air conditioners
  • Fan coil units in a larger system
  • Air handling units in a larger system

Refrigeration Systems (for processes)

In many industrial processed refrigeration system are needed. Different technologies are used for different applications:

  • Small capacity modular units of direct expansion type similar to domestic refrigerators, small capacity refrigeration units.
  • Centralized chilled water plants with chilled water as a secondary coolant for temperature range over 50C 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 centralized 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 off site utility. 
The same industry may also have two or three levels of refrigeration & air conditioning such as:

  • Comfort air conditioning (200 – 250 C)
  • Chilled water system (80 – 100 C)
  • Brine system (sub-zero applications)

Electricity is the main energy type used for operation of refrigeration and air-conditioning system.

Design & Operation

Two principle types of refrigeration plants found in industrial use are:

  1. Vapour Compression Refrigeration (VCR) and 
  2. 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.

Vapour Compression Refrigeration (VCR)

Heat flows naturally from a hot to a colder body. In refrigeration system the opposite must occur i.e. heat flows from a cold to a hotter body. This is achieved by using a substance called a refrigerant, which absorbs heat and hence boils or evaporates at a low pressure to form a gas. This gas is then compressed to a higher pressure, such that it transfers the heat it has gained to ambient air or water and turns back (condenses) into a liquid. In this way heat is absorbed, or removed, from a low temperature source and transferred to a higher temperature source. The refrigeration cycle can be broken down into the following stages as shown in the figure.

Figure 2: Schematic of a basic Vapor Compression Refrigeration System

Alternative Refrigerants for Vapour Compression Systems

The use of CFCs is now beginning to be phased out due to their damaging impact on the protective tropospheric ozone layer around the earth. The Montreal Protocol of 1987 and the subsequent Copenhagen agreement of 1992 mandate a reduction in the production of ozone depleting Chlorinated Fluorocarbon (CFC) refrigerants in a phased manner, with an eventual stop to all production by the year 1996.  In response, the refrigeration industry has developed two alternative refrigerants; one based on Hydrochloro Fluorocarbon (HCFC), and another based on Hydro Fluorocarbon (HFC). The HCFCs have a 2 to 10% ozone depleting potential as compared to CFCs and also, they have an atmospheric lifetime between 2 to 25 years as compared to 100 or more years for CFCs (Brandt, 1992).  However, even HCFCs are mandated to be phased out by 2005, and only the chlorine free (zero ozone depletion) HFCs would be acceptable.

Until now, only one HFC based refrigerant, HFC 134a, has been developed.  HCFCs are comparatively simpler to produce and the three refrigerants 22, 123, and 124 have been developed. The use of HFCs and HCFCs results in slightly lower efficiencies as compared to CFCs, but this may change with increasing efforts being made to replace CFCs.

Vapour Absorption Refrigeration (VAR)

The absorption chiller is a machine, which produces chilled water by using heat such as steam, hot water, gas, oil etc. Chilled water is produced by the principle that liquid (refrigerant), which evaporates at low temperature, absorbs heat from surrounding when it evaporates. Pure water is used as refrigerant and lithium bromide solution is used as absorbent.

Heat for the vapour absorption refrigeration system can be provided by waste heat extracted from process, diesel generator sets etc. Absorption systems require electricity to run pumps only. Depending on the temperature required and the power cost, it may even by economical to generate heat or steam to operate the absorption system.

 

Performance Evaluation of HVAC

The cooling effect produced is quantified as tons of refrigeration.

1 ton of refrigeration = 3024 kCal/hr heat rejected.

The refrigeration TR is assessed as

    

The theoretical Coefficient of Performance (Carnot), COPCarnont - a standard measure of refrigeration efficiency of an ideal refrigeration system- depends on two key system temperatures, namely, evaporator temperature Te and condenser temperature Tc with COP being given as:

This expression also indicates that higher COPCarnot is achieved with higher evaporator temperature and lower condenser temperature.
But COPCarnot  is only a ratio of temperatures, and hence does not take into account the type of compressor. Hence the COP normally used in the industry is given by

Payback of energy saving options

Experiences from the past have shown that upgrading the HVAC can reduce energy costs considerably. The following table gives some examples for the payback of such investments.

Table 1: Payback of investment for improving energy efficiency of HVAC

 

 

Improving HVAC Efficiency

  • The evolution of compressor technology has progressed from reciprocating to rotary, to twin rotary, to scroll, to screw, to centrifugal machines, in terms of efficiency. Need-based replacement for renovation and modernization for energy efficiency is a good opportunity.
  • Select most efficient compressors with lowest I kW/TR (integrated kW/TR
  • accounting for part load performance).
  • Rooftop coating adoption, with reflective materials and under-deck insulation, will reduce U factor (heat transfer coefficient, which should be as low as possible) and heat ingress.
  • Maintain optimum wall area (70%) to window area (30%) ratio for minimizing solar heat gain.
  • Adopt energy-efficient low-wattage lighting systems to reduce cooling load.
  • Optimize thermostat settings for energy economy.
  • Adopt false ceilings in air-conditioned spaces to reduce the area to be conditioned.
  • Adopt good filter maintenance practices for better AHU performance.
  • Regular cooling coil cleaning and ultraviolet light application will reduce bacterial effects.
  • Adopt energy-efficient water-cooled condensers in place of air-cooled condensers.
  • Adopt energy-efficient AHU fans.
  • Adopt heat recovery wheels in AHUs.
  • Adopt grooved copper tubes for better heat exchange.
  • Adopt super slit fins for better heat exchange.
  • Adopt heat recovery condensers.
  • Adopt variable frequency drives for pumps and fans for power savings.
  • Adopt cooling towers with 5ºF approach, in place of conventional 7ºF
  • approach units, to optimize condenser performance.
  • Use soft water for condensers to avoid condenser scaling effects.
  • Use FRP blades in cooling tower fans, for power savings.
  • Adopt two-way valves for AHUs, to achieve better operational control.
  • Adopt intelligent building automation and controls.
  • Adopt chilled water storage and ice bank storage, as applicable, for peak demand management and energy savings.
  • Adopt automatic PF controllers for PF improvement.
  • Adopt VAR systems, especially where waste heat steam is available at low cost, as an eco-friendly and cost-effective alternative.
  • Adopt eco-friendly refrigerants.