ahu cooling coils
DESCRIPTION
Cooling CoilsTRANSCRIPT
AHU Cooling Coils
Dehumidification: The process in which the moisture or water vapour or the humidity is
removed from the air keeping its dry bulb (DB) temperature constant is known as the
dehumidification process. This process is represented by a straight vertical line on the
psychrometric chart starting from the initial value of relative humidity, extending downwards
and ending at the final value of the relative humidity represented in Grains /Lb of moisture
content in air.
Air may be dehumidified by the following methods:
Cooling - condensation of air vapour
Adsorption of water vapour
Absorption of water vapour
1. cooling the air - vapour condensation
In a cooling system the humidity is reduced by cooling the air below dew point. A part of the
moisture in the air is condensed and drained out. Cooling coils in an AHU perform this function.
2. Adsorption
In an adsorption system the humidity is reduced with an adsorbent material as silica gel or
activated alumina.
Adsorption is a physical process in where moisture is condensed and held on the surface of the
material without any change of in the physical or chemical structure of the material. The
adsorbent material can be reactivated by heat.
Temperature for reactivation: 160 - 1700C
Heat required for reactivation: 4800 - 4800 kJ/kg water removed
Adsorption has a high carbon footprint due to the heat supplied for reactivation of the
Adsorption agents.
Adsorption Agents:
Silica gel - SiO2
Silica gel - SiO2 - is a hard, adsorbent, crystalline substance and very porous. Voids are about 50 -
70% by volume and adsorb water up to 40% of its own mass. The bulk density of silica gel is 480
- 720 kg/m3 and a specific heat capacity of 1.13 kJ/kgK.
Activated alumina
Activated alumina is about 90% aluminium oxide Al2O3 and very porous. Voids are about 50 -
70% by volume and adsorb water up to 60% of its own mass. The bulk density is 800 - 870 kg/m3
and a specific heat capacity of 1.0 kJ/kgK.
3. Absorption
In an absorption system the humidity is reduced with an absorbent material such as a calcium
chloride solution.
Absorption involves a change in the physical or chemical structure of the material and in general
is not easy to reactivate the material.
Cooling Coils: Careful selection and application of cooling coils in HVAC air handling unit
applications is very important. The main issues to consider while selecting filters and its ongoing
maintenance are:
1. The coils shall be constructed from copper tubes with a wall thickness of 0.5mm and
Aluminium fins with a maximum fin spacing of 3mm; this has been taken based on a life
cycle assessment on increased heat transfer and air flow resistance.
2. Fins shall be rippled in order to reduce air resistance and pressure drop.
3. Coil casings shall be galvanised frame complying with C4 classification with an optional
stainless steel casing where extended hygiene requirements are required.
4. The resistance to air flow through the coil shall not exceed 10 Pa / row of coil, with the face
velocity not exceeding 2 metres per second.
5. The tube velocities shall be limited to less than 1 m/sec to improve holding time and
increasing heat transfer.
6. The heat exchanger shall be capable of being entered from both sides or, up to 1.6 mts
interior height, of being cleaned without removing other fitted parts.
7. All coils shall have double sealed test points on Flow and Return headers.
8. The maximum fin depth for core to remain accessible for cleaning at 300 mm if pipes offset,
450 mm if pipes in line. For greater fin depths, heat exchanger shall be split. The coil depth
shall be limited to 8 row coils to ensure maintainability.
9. The coils shall be coated with epoxy / vinyl coatings for corrosion resistance. For urban
saline environments additional post coating proprietary treatments like Blygold / Heresite
treatments shall be carried out.
10. The resistance to water flow through the coils shall not exceed 50 KPa.
11. All coils shall be hydraulically pressure tested to 22 bar at the factory and a working
pressure of 16 bar.
Specification:
The coil design shall ensure equal fluid flow through all coil circuits. The coils shall be
constructed from copper tubes and Aluminium fins with a maximum fin spacing of 3mm, all
electro-tinned after manufacture to BS 1872. Tube wall thicknesses and coil construction shall
be suitable for the system operating pressures and temperatures with a minimum wall thickness
of 0.5mm. Fins shall have a minimum thickness of 0.25mm and make firm and continuous
contact with the primary tubes, the fins shall be rippled in order to reduce air resistance and
pressure drop. Where more than one primary tube is required, tube rows shall be staggered in
the direction of air flow.
The heat exchangers sealed within the AHU casing by means of sealing strips to prevent bypass
leakage. The coils shall not protrude beyond the normal unit casing and shall be provided with
an adjacent access section for cleaning. Coil casings shall be galvanised frame complying with C4
classification with an optional stainless steel casing where extended hygiene requirements are
required.
The resistance to air flow through the coil shall not exceed 10 Pa / row of coil, with the face
velocity not exceeding 2 metres per second. Tubes shall terminate in one pair of tubular
headers for each coil or coil section.
No moisture shall be permitted to carry over into downstream sections. Cooling coils without
droplet eliminators shall be preferred.
The tube velocities shall be limited to less than 1 m/sec. All coil and coil section connections
shall be on the same side with coil sections mounted in tiers one above the other. All coils shall
have double sealed test points on Flow and Return headers.
The heat exchanger shall be capable of being entered from both sides or, up to 1.6 mts interior
height, of being cleaned without removing other fitted parts. The maximum fin depth for core to
remain accessible for cleaning at 300 mm if pipes offset, 450 mm if pipes in line. For greater fin
depths, heat exchanger shall be split. The coil depth shall be limited to 8 row coils to ensure
maintainability and ease of access.
The coils shall be coated with epoxy / vinyl coatings for corrosion resistance, for urban saline
environments additional post coating proprietary treatments like Blygold / Heresite treatments
shall be carried out.
All coils shall be hydraulically pressure tested to 22 bar at the factory and a working pressure of
16 bar with three copies of the test certificate supplied by the manufacturer.
All coils shall be provided with a drip pan to allow for unhindered drainage of condensation, the
condensate drain pan shall be in stainless steel with a minimum of 2 mm and externally insulted
with PE foam insulation.
The coils shall have screwed header connections up to 50mm size and flanged connections for
65mm size and above. Screwed connections shall be made using ground-in spherical seated
unions. Pipework serving the coils shall be fitted with unions (up to 50mm) or flanges (65mm
and above) in positions to facilitate minimal disruption of pipework upon coil removal. Tubes
shall terminate in one pair of tubular headers for each coil or coil section. All coil and coil
section connections shall be on the same side with coil sections mounted in tiers one above the
other. Pipe connections shall be arranged to facilitate air venting and drainage with a means
provided for venting and drains all coils. Self sealing test plugs with extended bodies to clear the
insulation vapour seal shall be provided on the inlet and outlet connections at each coil.
Pipe connections shall be arranged to facilitate air venting and drainage with a means provided
for venting and draining all coils. Self sealing test plugs shall be provided on the inlet and outlet
connections at each coil. The resistance to water flow through the coils shall not exceed 50 KPa.
Connecting pipes shall be insulated where they pass through casing, with external connecting
pipes to the coil adequately insulated.