AIR HANDLING UNITS

HVAC has been very important in the field of construction. It is very necessary to know about it before we get involved in it. There are various things we must notice and take care of. There are various terms we must be familiar with. Today in this tutorial, I will write about the AHU. 

AHU consists of Various parts like Filter, Humidistat,  Heating, and cooling coils, Filters, Motors or Fans, Electronic control, and so on.  Each part has its own set of standards to meet to get approval from the clients. For Example: 

Fan → AMCA

Motor: NEEMA

Coil: AHRI

AHU: EUROVENT

Fine Filter: 

Pre-filter:

HEPA Filter:

MERV filter:

VFD:

Starter Panel:

Thermal Break in the panel: The inside of the panel which is cold is isolated using the insulation or thermal break. 

Puff: The panel isolated is filled with puff insulation. 

The thickness of the panel: The thickness of the panel is mostly 50mm. 

Humidistat: The velocity of the air should be medium as high velocity doesn't provide enough time to dehumidify the air. So, the vapor or moisture gets enough time with the coil to condense. 

Humidifier

Cooling/Heating Coils: The coils can be 3 rows, 4 rows, 6 rows, 8 rows, and so on. 

The Fan in the AHU can be Plugged type or Electronic control fan which has VFD combined. 

Motor: It has its own set of standards that it must keep up with. Like IE1 and so on. The brand of the motor which is used in the motor 

Coil: The coil carries the refrigerant which is inserted inside the hole of the fins. There is 6 rows type (Mostly for more than 6000 CFM), 4 rows type, and 3-row type cooling coils. The header carries to various rows in cooling coils.  Also, the thickness of the Coil matters. The pitch of the coils in row deep is arranged to compensate the bend of the tube. 

Also, the pipe materials like SS, GI matter the efficiency and also the overall costs. 

AHU: AHU are of various types. Some are floor mounted, some are ceiling suspended. Some are manual and some are electronic control. 

ESP/ISP of AHU:

External static pressure is calculated by calculating the loss of pressure outside through the duct. For example, the filter and Pre-filter are assumed to have pressure loss of  15 & 20 mm (i.e. 15*9.8 +20*9.8pa) of Water.  The pressure loss through the duct should be calculated. Also, the pressure loss through the diffuser should be calculated using the catalog like Airmaster. 

Internal ISP: Filter: 20 mm of H2O, Pre-filter: 15 mm of H2O

1. Pressure Drop for Filter

    Fine Filters: 0.0981 inch/wg

    Pre Filters: 0.19-.75 inch/wg

    HEPA Filter: 0.98-2.95 inch/wg

2. Pressure Drop for Coil

    2 Row Coil: .11 inch of Wg- 8 Fins

    4 Row Coil: 0.22 inch of wg-8 Fins

External ESP: VCD: 0.4 Inch of H2O (0.4X250 Pa)

                        Diffuser: 0.15 Inch of (0.15X250 Pa)

For Example: For 4000 CFM AHU, the approx. value of ESP is 40mm of H2O. And for 16000 CFM, approximately about 70mm of H2O. 

VFD:
This controls the CFM through the AHU. In the absence of VFD, there will be a higher flow of air in case of low static pressure but if VFD is there, it will control the speed of the fan and hence the CFM. 

 

COIL SELECTION BASICS (ISHRAE JOURNAL)

Types of Coils 

There are direct expansion (DX) coils and chilled water coils. Some coil manufacturers fabricate coils from 5/8 inch OD copper tubes, others from 1/2 inch copper tube and still others use 3/8 inch tubes. Selection of the tube size is a matter of manufacturer's choice and market demand. Price, as always, plays a major part in the tube size selection.

The selection method is for 5/8 inch tubes.

In a coil, copper tubes are arranged parallel to one another, either in staggered pattern or non- staggered pattern, along the length L' of the coil. A staggered pattern is more commonly used. For 5/8 inch tubes, the triangular pitch is 1.75 inch or 1.5 inch. For 1/2 inch tubes it is 1.25 inch and for 3/8 inch tubes it is 1 inch. 

Plate or ripple fins are used to enhance the heat transfer area. Thus the primary surface area (outside area of bare copper tubes) is enhanced greatly by adding a secondary area of fins. The total area including fins is called "outside surface area," for use in the calculations, in this article. 

The cross-section (L x H) across which air flows is called the face area or the finned area. Thus L is finned length and H is fin height Fins are arranged perpendicular to the tubes. Fin spacing varies between 8 and 14 fins per inch of tube.

A Average air velocity across the face area is called coil face velocity or simply face velocity. Thus

The number of rows of copper tubes in the direction of air flow is termed as depth of coil (rows deep). Coils with 3,4,6 or 8 rows are commonly used. 

Refrigerant or chilled water enters the first row and leaves the coil from the last row. A coil in which chilled water or refrigerant is supplied to all the tubes in the first row (also referred to as tubes high or tubes in face) is called a maximum or full circuit coil. Thus a typical coil of 17.5 inch height which has 10 tubes in face (based on 1.75 inch pitch) will have a maximum of 10 circuits. 

If the supply is given to alternate tubes in face, we get a half-circuit coil with 5 circuits as against 10 circuits. The U-bends at the end of the tubes can be arranged, at the time of manufacturing, to obtain the number of circuits desired.

Face velocity is restricted to 500 fpm to avoid carryover of condensate from the coil. The value of 500 fpm is very commonly used for coil sizing and it works very well for cfm/ton in the range of 500 to 600. If the cfm/ton ratio falls below 500, (this generally happens when room sensible heat factor goes below 0.8 due to high room latent load) a 4-row coil at 500 fpm becomes inadequate. A 5-row coil is not very common. Hence by lowering face velocity, a 4- row deep coil can be selected at 400 fpm, when cfm/ton is about 400. As the cfm/ton ratio reduces further, 6-row or 8-row coils have to be selected. This situation is encountered when the occupancy and/or fresh air components are high. 

Thus based on the face area and number of rows, a quick coil estimate can be done. 

While actually sizing the coil, the initial assumption of face velocity may have to be changed to arrive at an acceptable selection. Thus the procedure is required to be repeated with a new value of face velocity. This is known as iteration.

Cooling Coil Calculations

Air Handling Unit (AHU) Selection Guide

Introduction:

Selection of AHU includes the type of AHU, filters, coils, panels, fans, and a few other components. Any change in specification of these items directly affects the performance, quality, capacity, and cost of the air handling units. This document explains various terminology and standards used in the industries at the macro level. The readers are advised to refer to the latest standards & manufacturer data to get specified accurate information.

Click here for Air Handling Unit (AHU) performance calculation.

Types of AHU

• Horizontal floor mount - All major components are aligned to the base tray and it makes maintenance-friendly.
• Vertical floor mount – Designed to occupy less space
• Ceiling suspended – Hanged from the ceiling inside the “false ceiling” and zero floor space.

A typical AHU consists of the below components in place

Filters

European Standards EN 779& EN 1822

These standards are describing the classification & characteristic requirement of filters used in building ventilation & industrial process.

Based on the particulate size it classifies the filters as below

• Classes G1, G2, G3 & G4 – Defining the requirement of coarse dust particulate size > 10µ
• Classes M5, M6, F7, F8 & F9 - Defining the requirement of fine dust particulate size 1µ - 10µ
• Classes E10, E11, E12, H13, H14, U15 & U16 – Defining the requirement of suspended particulate sizes <1µ

Based on the type of filtration the filters are grouped together as below

• Coarse filter requirements are defined under classes G1-G4 of E 779
• Medium filter requirements are defined under classes M5-M6 of E 779
• Fine filter requirements are defined under classes F7-F9 of EN 779
• High-efficiency filter requirements are defined under classes E10-U16 of EN 1822

ISO 29463

It is derived from EN1822 and both are based on MPPS – Most Penetrating Particulate Size. MPPS is the particulate size at which an air filter gives minimum resistance. But they have some differences in leakage test methods. Both standards are describing the requirement for high efficiency EPA, HEPA & ULPA filters.

ISO 16890

It replaces EN 779 from the year 2018. The EN 779 classifies the filters based on the filtration efficiency of particulate matter size of 0.4µ. Whereas ISO 16890 classifies the filters based on a spectrum of particulate sizes from 0.3µ to 10µ. It has three major classifications used by WHO (World health organization) as below

PM1 – Particulate sizes less than 1µ

PM2.5 – Particulate sizes less than 2.5µ

PM10 – Particulate sizes less than 10µ

The particulate matter >10µ are not breathed, the particulate matter <10µ can penetrate bronchi, the particulate matter <2.5 µ can penetrate pulmonary alveoli and the particulate matter <1 µ can penetrate alveoli capillary barriers.

European standard EN 15805

It describes the header framing dimensions of filter used in air intake systems and air handling units.

MERV–Minimum Efficiency reporting value

It is described in ASHRAE 52.2 to report the effectiveness of filters. It classifies the filters from MERV1 to MERV16. Higher the rating filters will allow fewer dust particles. This standard is intended to assist the end-user and specifier in their selection of appropriate filters for various applications.

Coil

All standard values in this document are referring to water as tube side fluid. Please refer AHRI 410 & ASHRAE standard for other fluids.

Chilled water details:

As per AHRI 410: 2001 the acceptable range of values are

• Chilled water inlet temperature 35° F ~ 65ׄ° F
• Water velocity - 1 fps ~ 8 fps
• Minimum fin surface temperature >32° F
• Minimum tube surface temperature >32° F

Typical values are

• Chilled water in & out temperature 45° F & 55° F
• Estimated Δt - 10° F
• Chilled water pressure drops - max. 20 ft H2O
• Flow rate - 2.4 gpm/ton
• Water velocity – 2fps~3fps

An increase in flow rate causes the velocity & pressure drop to increase. Then the water stays very little time inside the coil, which causes the air leaving temperature to grow and gives a reduced air Δt. To achieve the required air temperature and coil performance we may have to increase the number of fins/ft, which results in a price increase for the coil.

The maximum pressure drop across the coil falls between 20 ~ 24 ft H2O. But it is advisable to keep as low as possible and typical values fall less than 10 ft H2O. The pressure drop in the coil is controlled by the number of tubes in the coil.

The size, length & width of a coil is decided based on the fin length, design air face velocity, and required tonnage of the coil.

Fins

Aluminum is the standard material for fins used in most applications. Copper fins are recommended for corrosive environments. The number of fins various between 8 fins/inch to 14 fins/inch based on the heat transfer requirement.

When there is an increase in water flow rate the heat transfer rate reduces across the coil due to higher velocity, which requires fewer fins/inch. The required number of fins/inch will be close to 8. There will be a decrease in price for the coil, but we will end up with higher velocity & pressure drop.

Typical aluminium fin thickness falls between 0.006” ~ 0.0095”.

Tubes

With ½” tube OD, the performance will be increased slightly, and not a major difference between 5/8” tube OD. Like tube OD with an increase in tube thickness, there will not be a big difference in performance, only the coil life can be improved. The typical thickness for coils is 0.025” ~ 0.035”.

Number of rows in coil

A typical number of rows of a coil for AHU is 4. An increase in the number of rows results in more Δt, low discharge air temperature, more moisture removal& higher dehumidification & more air pressure drops for the fan.

Typically, an increased number of rows will be used where the intake to an AHU is 100% fresh air, like operation theatres.

Drain pan

A drain pan for individual coil section is expected, because the condensation in the upper coil may block the airflow in the lower one. And the main drain pan is required at the bottom of the full coil, which drains out the water.

Reynold’s number of an AHU coil is dependent on the tube inner diameter, fluid velocity & tube type. Reynold’s number can be controlled based on the circuit & adding a turbulator. Adding a turbulator may result in a higher pressure drop.

Refer latest AHRI 410 for the method of calculating Reynolds number and acceptable values.

Fouling

Fouling is the formation of sediments and any other matters that form inside the tube over a period. Fouling increases pressure drop reduces heat transfer, and obstruct fluid flow.

There is no straight method to calculate the fouling and it is the value directly added to thermal resistance for design & manufacturing of coil. This defines the duration for cleaning frequency.

The unit of fouling is ft²-°F-ht/Btu. Value zero will be considered for fouling when possible and the typical value will be 0.0005 for internal & 0.001 for external.

Air side details

As per AHRI 410: 2001 the acceptable range of values are

• Air face velocity – 200 fpm ~ 800 fpm
• Entering air dry-bulb temperature – 65° F ~ 100° F
• Entering air wet-bulb temperature – 60° F ~ 85° F

Typical values are

• Air face velocity – 500 fpm
• Entering air dry-bulb temperature –65° F ~ 80° F
• Entering air wet-bulb temperature – 60° F ~ 70° F
• Estimated Δt - 20° F ~ 25° F

When the air face velocity is too low heat transfer will not take place due to lack of turbulance. At the same time when the air face velocity is too high, the heat transfer will be very less with a higher air pressure drop. Also, moisture carry-over will happen at high velocity which results in water droplets all over the AHU. The air pressure drop will be very minimal at 400 fpm of face velocity, but there will be a slight increase in the cost of the AHU.

With the increased number of coil rows and fins, we get a higher air pressure drop.

As a rule of thumb 400 cfm is considered for 1TR capacity in a typical AHU. Refer to performance calculation for more details.

ASHRAE standard 33 – defines the equations for calculating air side pressure drop, sensible cooling, dehumidification, and test requirements.

AHU Panels

Outer skin & inner skin are usually made of galvanized steel. Aluminum and stainless steel materials are considered as alternate for specific requirements.

The insulation is selected based on the required thermal resistance & sound attenuation.

Single skin AHU

• Mainly for ventilation
• Outer casing shall be of – 0.5mm thick; with powder coated.
• Insulation of 25mm thick fibre glass or 15mm foam
• Suitable for lesser flow up to 2200 cfm

Double skin AHU

• Mainly for air conditioning
• Inner Skin – 0.4~0.7mm thick; galvanized steel
• Outer Skin – 0.4~0.9mm thick; with powder coated
• Insulation of 50mm thick fibre glass or 25mm foam; insulation is sandwiched between inner skin and outer skin.

Fans

Forward curved fans	Backward curved fans
Forward curved fans will have many small blades curved in the direction of rotation.	Backward curved fans will have less quantity of longer blades curved opposite to the direction of rotation.
Flow is tangent to the rotation	Flow is in radial direction
Requires a scroll house to convert the kinetic energy into static energy	No housing is required
Smaller in size for a given air flow	Larger in size for a given air flow
Single inlet is suitable for high pressure low volume Double inlet is suitable for high volume, low pressure system	Suitable for higher efficiency & high pressure
Can run with AC motor	Requires an EC motor

Typically, forward curved fans are used in AHUs due to the operational requirement, cost & size.

Operating above or below the optimum design criteria can cause noise and reduce the efficiency of the system for both forward & backward curved fans.

Internal static pressure (ISP) = Total pressure drop caused by all components within the system, like., filters, coils, mixing box and other components.

External static pressure (ESP) = It is the pressure developed in supply/return system / ducting, like., duct pressure loss, volume control dampers, diffusers, and other components.

Total static pressure (TSP) = ESP + ISP

Each component (filters, dampers, diffusers, grills, ducts, etc) in the given system produces some static pressure to the fan. The static pressure to the fan is directly proportional to the flow and at the same time, the fan cannot be operated above or below the optimum levels.

Additional information

Additional details to be considered in selecting an AHU are given below and we are not covering in this article.

• Drip eliminator,
• Anti-corrosive coating to the coil
• Variable frequency drives for fans
• Humidifiers
• De-humidification
• Belt drive / Direct dive for the fans
• Selection of pulley and belts
• Volume control dampers