HVAC, Profounding Human Comfort.
You may be out on vacation having a cozy nap in a tropical country with the perfect temperature set in your room or you may be in a freezing Greenland office heating yourself thanks to the HVAC system installed. Have you ever wondered how factories, power plants, large shopping malls, and large office premises cool their environment and maintain human comfort within the location? HVAC (Heating, Ventilation, and Air Conditioning) systems often provide comfort to us in numerous instances throughout our day-to-day life and keep us, humans, comfortably focusing on work.
Talking deeply about human comfort may lead this article to a long 30 min read which I would rather not do. Let me walk you through what ‘human comfort’ means from the perspective of an engineer. Humans generally feel comfortable between temperatures of 22 °C to 27 °C and relative humidity of 40% to 60%. In fact, different age levels of people comprehend thermal comfort at different temperatures and humidity values. Different gender feels comfort differently. Different clothing styles feel the comfort differently. So can we ever design a building with the perfect thermal comfort conditions for each of its occupants? Maybe not now but soon with the applications of AI(Artificial Intelligence), we would achieve that. This was just about ‘Human Thermal Comfort’. Human Comfort relies not only on ‘Thermal Comfort’ but also on ‘Visual Comfort’, ‘Acoustic Comfort’ and the list keeps ongoing. Considering all the dynamics of Humans, at present we are not providing the best comfort for each occupant in a large building, but we are trying our best to keep them thermally comfortable under the empirical values suggested by the humans themself.
That was some laying down the bricks for the story to begin. Let’s now focus on developing our own HVAC system. Since I myself live in a tropical country let’s not focus on the heating part of the HVAC system but just get the idea behind how the cooling of large spaces is done.
For better understanding let's break down the HVAC system into 4 subsystems as Airside loop, Chilled water loop, Refrigeration loop, and Heat rejection loop. Not all HVAC systems have all the above subsystems but at least contain 2 or 3 of them for sure.
Airside Loop
The first loop is the airside loop, and the first component of this loop is the
conditioned space. The first two comfort requirements mentioned were dry-bulb temperature and humidity. In order to maintain the dry-bulb temperature in the conditioned space, heat (referred to as sensible heat) must be added or removed at the same rate as it leaves or enters the space. In order to maintain the humidity level in the space, moisture (sometimes referred to as latent heat) must be added or removed at the same rate as it leaves or enters the space.
Most HVAC systems used today deliver conditioned (heated, cooled,
humidified, or dehumidified) air to the conditioned space to add or remove
sensible heat and moisture. This conditioned air is called supply air. The air that carries the heat and moisture out of the space is called return air.
Imagine the conditioned supply air as a sponge. In the cooling mode, as it
enters a space, this “sponge” (supply air) absorbs sensible heat and moisture.
The amount of sensible heat and moisture absorbed depends on the
temperature and humidity, as well as the quantity, of the supply air. Assuming a fixed quantity of air, if the supply air is colder, it can remove more sensible heat from the space. If the supply air is drier, it can remove more moisture from the space.
A heat exchanger, commonly known as a cooling coil, is often used to cool and dehumidify the supply air before it is delivered to the space. A typical cooling coil includes rows of tubes passing through sheets of formed fins. A cold fluid, either water or liquid refrigerant, enters one header at the end of the coil and then flows through the tubes, cooling both the tubes and the fins.
Chilled-water loop
In the airside loop, a cooling coil is used to cool and dehumidify the supply
air. As mentioned, the cold fluid flowing through the tubes of the coil maybe
either water or liquid refrigerant. Systems that use water flowing through the cooling coil also contain a chilled-water loop. Chilled water flows through the coil, absorbing heat from the air. The water leaves the coil at a warmer temperature. Thus the water should be cooled before circulating again. This is done using an evaporator which is also and heat exchanger with the principle use of refrigerants to transfer heat. We can sense here that the next loop is the refrigeration loop connected with the evaporator in the chilled water loop.
Refrigeration Loop
The third loop is the refrigeration loop. Recall that in the chilled-water loop, the evaporator allows heat to transfer from the water to cold liquid refrigerant. As heat is transferred from the water to the refrigerant, the liquid refrigerant boils. The resulting refrigerant vapor is further warmed (superheated) inside the evaporator before being drawn to the compressor. The compressor is used to pump the low-pressure refrigerant vapor from the evaporator and compress it to a higher pressure. This increase in pressure also raises the temperature of the refrigerant vapor. After being discharged from the compressor, the hot, high-pressure refrigerant vapor enters a condenser. The condenser is a heat exchanger that transfers heat from the hot refrigerant vapor to air, water, or some other fluid that is at a colder temperature. As heat is removed from the refrigerant, it condenses and returns to the liquid phase. The liquid refrigerant that leaves the condenser is still at a relatively high temperature. The final step of the refrigeration cycle is for this hot liquid refrigerant to pass through an expansion device. This device creates a large pressure drop that reduces the pressure, and correspondingly the temperature, of the refrigerant. The temperature is reduced to a point where it is again cold enough to absorb heat inside the evaporator. Therefore the refrigeration cycle is complete.
For a technical perspective of the refrigeration cycle, let me bring out a Temperature-Entropy (TS) graph for a basic refrigeration cycle. Examine clearly the state transition regions denoted in different color patterns. State 1 to state 2 the temperature rises while the entropy remains the same, and the pressure increases too. Therefore that's the action of the compressor. State 2 to state 4 via state 3 is achieved by the condenser where the pressure remains constant but the state transition occurs from the vapor state to liquid state, therefore, releasing the heat(latent heat). State 4 to state 5 pressure is lowered with the expansion valve and thus the temperature drops as well and the state of the refrigerant occupies a mixed state of liquid+vapor. Finally from state 5 to state 1 latent heat is absorbed from the evaporator and therefore the refrigerant occupies a vapor state.
Heat Rejection Loop
The fourth loop is the heat-rejection loop. In the refrigeration loop, the condenser transfers heat from the hot refrigerant to air, water, or some other fluid. In a water-cooled condenser, water flows through the tubes while the hot refrigerant vapor enters the shell space surrounding the tubes. Heat is transferred from the refrigerant to the water, warming the water. The water flowing through the condenser must be colder than the hot refrigerant vapor. A heat exchanger is required to cool the water that returns from the condenser back to the desired temperature before it is pumped back to the condenser. When a water-cooled condenser is used, this heat exchanger is typically either a cooling tower or a fluid cooler (also known as a dry cooler).
In a cooling tower, the warm water returning from the condenser is sprayed over the fill inside the tower while a propeller fan draws outdoor air upward through the fill. One common type of fill consists of several thin, closely spaced layers of plastic or wood. The water spreads over the surface of the fill to increase the contact with the passing air. The movement of air through the fill allows heat to transfer from the water to the air. This causes some of the water to evaporate, a process that cools the remaining water. The remaining cooled water then falls to the tower sump and is returned to the condenser.
So we’ve discussed in brief terms the main subsystems of an HVAC system that is used in large factories, shopping mall complexes, your office premises, large warehouses. Hope you guys got a clear understanding of the operation of an HVAC system and a glimpse of human thermal comfort. May this blog be an inspiration for someone out there to create the ultimate human comfort optimized system that someday hopefully everyone will be benefitted from.
Ciao Fellas!
