what is FCU, AHU AND FAHU?

FCU:

FCU is the abbreviation used for FAN COIL UNIT that are available for either DX or chilled water system that houses refrigerant or chilled water coil respectively. Beside the type of coil used, the other components are common such as the blower fan & filters. FCUs are usually available from 0.75 to 5 TR from various brands across the globe.

Fan Coil Unit, as the name suggests the unit houses the Blower(Fan), the evaporator Coil (for DX System) / Heat Exchanger Coil( for sytems other than DX), Filter and sometimes Heater Coil(Electric). Usually takes the hot air from room and cool it then suply to room.

FCU is fan coil unit which is intalled for samll capacities and have less options than AHU like no humidity control and no special options for heat recovery of air filters.

Fan Coil Unit, as the name suggests the unit houses the Blower(Fan), the evaporator Coil (for DX System) / Heat Exchanger Coil( for sytems other than DX), Filter and sometimes Heater Coil(Electric). Usually employed for upto 4 to 5tons , some manufacturers do make higher capacities but will be belt driven which could be noisy.

AHU:

AHU is generally a bigger system than FCU. AHU is more complex than the FCU and that AHU are often used in bigger establishments or spaces. The AHU system usually channels air through ducts whereas the FCU may have or don't have any ductworks. AHU system treats outside air while FCUs basically recycle or re-circulates the air. AHU have sections for reheating and humidifying whereas the FCU may have heaters but no Humdification. FCU are often observed to be noisier than the AHU.

AHU is the abbreviation used for AIR HANDLING UNIT; is an advance type of FCU beyond 5 TR capacity. They are either available in standard sizes or custom size & body construction. In addition to the standard components (blower fan & filter), it has advance filters, UV light, mixing chambers, etc. depending upon the requirement & construction.

AHU is the abbreviation used for AIR HANDLING UNIT; is an advance type of FCU and 

normally made as to customer demand-such as heater,uv lamps,carbon filter, hepa filter, pre filter and bag filters are normally using in all AHU'S with return air duct + some fresh air also and suply the cool purified air to the premises by using blower and motor running  with or without VFD.

AHU means Air Handling Units and are available in number of varities and tonnages from small upto large capacities. Thay are available with number of modifications which are normlly not available in FCUs as mentioned above.

Air Handling Unit, think of AHU as a bigger FCU. AHU typically houses Blower, Heating or Cooling Coil(or both) and Filters. AHU's can be given provision for adding Fresh Air(Outside Air) , Humidifier and UV lights (Seen a demonstation conducted by carrier and their studies shows no or very very less amount of mould forming at the coils and ofcourse bacteria and viruses) for killing organic substances. AHU's are be available for larger tonnages too.

FCU is an indoor unit with small tonage capacities used with central air conditioning systems such as chillers system.

AHU is an indoor and also can be used as outdoor also used with central air conditioning systems such as chillers system but have a wide range of capacities having  a great static pressure of fans to deliver the air through air ducts to big cooling zone areas.

FAHU:

FAHU is the abbreviation used for FRESH AIR HANDLING UNIT. These are usually centralized units employed to induce fresh air quantities to the confines spaces. They come into picture wherever there are limitations to fresh air intake either directly or through AHUs. FAHUs are either of normal construction having 100% fresh air through a blower fan or Treated FAHU that employs an additional cooling coil to induce treated air into the confined space without deteriorating the indoor conditions. It all depends upon the selection of the designer to provide an optimum HVAC solution.

Fresh Air Handling Unit, same as a AHU but dosent have air recirculation option(100% Fresh Air is used). The Return air is extracted to the atmosphere either used Heat Wheel or coils like heat exchanger coil,pre cooling coil and cooling coil with pre filter, bag filter,UV lamp,HEPA filter,Carbon filter { where it is applicable }and for some units using Electronic filter also.Then suply the purified cool air to the rooms with belt driven motor or direct drive controlled by VFD.

When the AHU is used for fresh air only then it is known as FAHU (Fresh Air Handling Unit). means there is no return duct only fresh air supply to the area.

Fresh Air Handling Unit, same as a AHU but dosent have air recirculation option(100% Fresh Air is used). The Return air is extracted to the atmosphere, usually used in places like hospitals where contaminated return air is not advised to be reused. The extract air is most likely to be of at a lower temperature that the fresh air taken by the FAHU, so inorder to increase the efficiency of the system a heat exchanger(usually Heat wheel or Cross flow Plate type HX) is used where the temperature of fresh air is transfered to the extract air.

FAHU is same like AHU with fresh air100%.

Dampers

Control Dampers:

For controlling air distribution, such as

Fire damper:

A thermally actuated damper arranged to automatically restrict the passage of fire and/or heat at a point where an opening violates the integrity of a fire partition or floor.

Smoke damper: 

A damper arranged to control passage of smoke through an opening or a duct.

Volume control damper (VCD): 

A device used to regulate the flow of air in an HVAC system.

Common types: Š 

• Opposed blade dampers (e.g. in AHU).
• Parallel blade dampers.
• Butterfly dampers (e.g. in VAV box).
• Linear air valves (e.g. in fume hood).
• Specialty dampers.

Damper Sizing

• Typically chosen based on duct size and convenience of location.
• Proper selection and sizing provides the following benefits: Š 

1. Lower installation cost (damper sizes are smaller).
2. Smaller actuators or a fewer number of them are required.
3. Reduced energy costs (smaller damper, less overall leakage).
4. Improved control characteristics (rangeability) because the ratio of total damper flow to minimum controllable flow is increased.
5. Improved operating characteristics (linearity).

Selecting and Sizing Dampers:

The three basic damper applications are: „ 

• Two-position duty.
• Capacity control duty.
• Mixing duty.

2 way and 3 way valves

2-Way and 3-Way Valves

2-way valves are pretty simple and straight-forward. A 2-way valve is any type of valve with two ports: an inlet and an outlet port, typically labeled “A” and “AB” respectively. 2-way valves are used in many applications, from basic on/off to more complex variable flow applications with pumps and VFDs. The type of valve you need for an application depends on the amount of flow, the degree of control, shut-off, and pressure drops over the valve.

 Fig: 2 way valve

Fig: 3 way valve connections


3-way valves have, yes, three ports, labeled “A”, “B”, and “AB”. Port “AB” is common to the “A” or “B” port. 3-way valves are commonly found in constant flow/volume pumping systems and can be either mixing or diverting valves. 3-way valves can be piped in the supply or return. If in the supply, then a diverting valve is used. If piped in the return, a mixing valve is used. Ball valves can be piped to be mixing or diverting, but globe valves require different bodies for mixing or diverting. 

Mixing applications have the 3-way valve configured with two inputs from the supply piping and one output to the return piping, thus mixing together two inputs before sending it out. Mixing valves are most commonly used with modulating control but can be on/off.

Diverting applications have the 3-way valve configured with one input from the supply side and two outputs to the return piping. In general, diverting valves are more expensive than mixing valves.

2 Way Valve:

2 Way (Or 2 Ports) Valve is passing the water in one direction only. so if the valve is fully close it will trap the water before it. this will lead to a pressure increase in this branch

3 Way Valve:

3 Way (Or 3 ports) Valve is passing the water in two directions.
so if the valve is fully open the full amount of water will be moving in one direction, if it closed the water will pass to the other direction, if the valve is partially open then percent of the water will flow through direction 1 and the remaining will pass through the other (for Diverting Valve which installed in the supply line).

In another cases if we install the valve in the return line so if the valve is open water will flow through the unit (Cooling coil as example) then pass through main direction. if the valve is close the water will by pass the unit and then flow through the other direction through the valve (Mixing Valve which installed in the Return line).

This will not cause the pressure rising. Why we use the 2 way valve? when you use the 2 way valves in HVAC system in all the equipment's in your building this means that you will not need all the chilled water to go through your system all the time if you don't need much cooling. so you will be able to reduce the speed of the secondary pumps of your system. this will lead to a huge energy saving in the running cost of your building (depend on the number of Pumps and their sizes). Also you will be able to reduce the size of your pumps. But in this case you have to use Variable Speed Pumps. Also you should have 2 sets of Pumps, One set constant with Speed for Chillers and another set to serve the building. ans also By Pass Valve to guarantee the min flow of the chillers. Otherwise you can use one set with a variable flow chillers. This has to be a decision in the mechanical design stage depend on the cost calculation of the project.

Advantages of 2-Way Valves:

•  „Less expensive to buy and install.
• Result in variable flow which reduces pumping energy.
• Reduced piping heat losses and pump energy.
• Potentially lower costs for pumping and distribution systems.
• System balancing is reduced or eliminated.

Disadvantages of 2-Way Valves:

• Most chillers and some boilers cannot handle widely varying flow rates.
• Differential pressures will increase across control valves, reducing system controllability.

Control Valve Ratings „

• Flow coefficient.
• Close-off rating: Š The maximum pressure drop that a valve can withstand without leakage while in the full closed position.
• Pressure drop: Š The difference in upstream and downstream pressures of the fluid flowing through the valve.
• Maximum pressure and temperature: Š The maximum pressure and temperature limitations of fluid flow that a valve can withstand.

Location of Control Valves:

• „ At the outlet on the top of cooling/heating coils.

1. Avoid coil starvation from water flow (lower pressure)Š
2. Flow of water from the bottom to the top (avoid air bubble).

• Flow measuring & balancing device should be placed after the control valve.
• Provide a means of shut-off to allow a proper means for servicing.

Conclusion:

1. use the 2 way Valve in the system that can withstand the variable water flow
2. use the 3 way valve in the systems that needs a constant water flow.

Selecting & Sizing Valves:

Control valve selection depends on: „ 

• The fluid being controlled.
• Valve style: 2-way or 3-way.
• Control mode: modulating or 2-position.
• Maximum fluid temperature.
• Maximum inlet pressure.
• Desired flow characteristic.
• Maximum fluid flow rate.
• Desired pressure drop when valve is full open.
• Turn-down ratio.
• Close-off pressure.

Flow Characteristic Selection: 

The desired flow characteristic is a function of: „ 

• The heat transfer device being controlled and its flow versus capacity characteristic.
• The control of fluid supply temperature.
• The control of the differential pressure across the valve.

Four Pipe HVAC System

Four Pipe HVAC System:

The system's piping consists of four insulated pipes, two supply and two return lines. One set is dedicated to chilled water, kept between 60 degrees F and 40 degrees F. Another set of pipes is dedicated to hot water, generally kept between 150 degrees F and 200 degrees F. The pipes run to air handlers, which use the chilled or hot water to change the air temperature.

Air Handlers:

• Air handlers in a four-pipe HVAC system can be custom designed to meet a wide variety of heat or cooling demands. They also usually are versatile in that they can be kept in mechanical rooms, on the roof of the building, or with smaller units, in the space above ceilings.

Boilers and Chillers:

• The water in the system runs through two separate systems in order to change the temperature. Cold water is brought down in temperature through chillers, which can be located on the ground or the roof of the building. Chillers come in a wide variety of types to fit different budget and efficiency needs. Water is heated up using a boiler. The boiler is kept either inside or outside. As with the chillers, a number of boiler types are available, for different efficiency and budget needs.
• 
  
  

Advantages Compared to Two-Pipe HVAC:

• The four-pipe HVAC system has a number of advantages over a two-pipe system. Four-pipe systems have separate heating and cooling fan coil units and separate pipes for heating and cooling. This means that hot or chilled water is always available, so the system can immediately change over from heating to cooling mode. Two-pipe systems have to be manually switched over, which is not only inconvenient but time-consuming. Four-pipe systems also can cool some rooms while heating others, offering great flexibility in a building with a variety of heating and cooling needs.

Disadvantages Compared to Two-Pipe HVAC:

• Four-pipe HVAC systems have a number of disadvantages compared to a two-pipe system. They are more expensive to install and maintain and have twice as many valves, coils, controls and pipes to maintain. They also are twice as prone to congestion due to the increased piping.

Propeller Fans

Propeller Fan:

A propeller is a type of fan that transmits power by converting rotational motion into thrust. A pressure difference is produced between the forward and rear surfaces of the airfoil-shaped blade, and a fluid (such as air or water) is accelerated behind the blade. 

The propeller fan sometimes called as PANEL FAN, is the most commonly used of all fans. It can be found in Industrial, Commercial, Institutional and Residential applications. It can exhaust hot and contaminated air or corrosive gases from factories, welding shops, foundries, furnace rooms, laboratories, laundries, stores or residential attics or windows.

Sometimes several propeller fans are installed in the walls of a building operating in parallel and exhausting the air.

Propeller dynamics, like those of aircraft wings, can be modelled by either or both Bernoulli's principle and Newton's third law. A marine propeller of this type is sometimes colloquially known as a screw propeller or screw, however there is a different class of propellers known as cycloidal propellers - they are characterized by the higher propulsive efficiency averaging 0.72 compared to the screw propellers average of 0.6 and the ability to throw thrust in any direction at any time. Their disadvantages are higher mechanical complexity and higher cost

Types of Fans used in Construction.

Generally there are 6 types of fans used in Mechanical Construction.

1. Axial Flow Fans:

There are four types of axial-flow fans. Listed in the increasing order of static pressure.

They are:

a. Propeller fans(PFs).

b. Tubeaxial fans(TAFs).

c. Vane axial fans(VAFs).

d. Two Stage Axial Flow fans.

2. Centrifugal Fans:

There are six types of centrifugal fan wheels in common use. Listed in the order of decreasing efficiency, They are:

a. Centrifugal fans with AirFoil(AF)blades

b. Centrifugal fan with Backward Curved(BC) blades.

c. Centrifugal fan with Backward Inclined(BI) blades.

d. Centrifugal fan with Radial Tip(RT) blades.

e. Centrifugal fan with Forward Curved(FC) blades.

f. Centrifugal fan with Radial blades(RBs).

3. Axial Centrifugal Fans.4. Roof Ventilators.5. Cross Flow Blowers.6. Vortex Regenerative Blowers.