Important Points To Know When Replacing Your Coils On Your HVAC Equipment

Before you think about replacing your corroded coils in your HVAC equipment, it may be good to know about the basic construction of a chilled water coil. There is no difference between a chilled water coil and a hot water coil in construction, as they are both a water coil.

Tube Size

Most of the chilled water coils are constructed with either 1/2″ or 5/8″ OD tubes. The selection of tube size can vary from different original equipment manufacturer, dependent on the availability and which is more economical for the factory. Either size can be considered to substitute for each other, which makes replacement of coil much easier.

Number of Rows

Chilled water coils can usually ranged from 3 rows, up to 12 rows in depth. Hot water coils have lesser rows from 1 to 2 rows.

Number of Tubes per row

1/2″ OD tubes are on a 1.25″ center to center height between each tubes, while 5/8″ OD tubes are on 1.5″ center to center height. For example, if a chilled water coil has a 30″ fin height, there will be 24 nos of 1/2″ OD tubes per row or 20 nos of 5/8″ OD tubes per row.

Fins per Inch (fpi)

‘fpi’ refers to the number of fins in one inch distance. In order words, 10 fpi refers to a fin pitch (distance between each fins) of 2.5mm and 12fpi refers to a fin pitch of 2.1mm. While it may be more economical to produce any coil cheaper by making them 12 fpi with less rows rather than 8 or 10 fpi, cost is not alway the deciding factor when it comes to sizing of coils. Just remember that deep coils increase the air pressure drop and are very difficult to clean.

Fins material

Fins are usually produced in aluminum because of their great heat transfer characteristic at an economical cost. However, some end users prefer to have copper fins for more corrosion, despite copper is two or three times more expensive than aluminium. For the same economical reason, coil coatings on aluminium fins are very popular among some users, such as blue fins, heresite, blygold, etc.

Coil Frame / Casing material

Most of the commercial coils are constructed with galvanised steel casing, due to lower cost. However, I will strongly recommend my clients to have their chilled water coils built with 304 stainless steel casings, because these casings are stronger and stackable. The most important feature of stainless steel is that they are more resistance to rust, where it can be a problem where water droplets are almost present most of the time on the cooling coil. For a hot water coil where there is no condensation, galvanised steel casing is usually acceptable.

Circuiting of Coil

Circuiting of coil can be really tricky, as it is only visible from the side of the coil and usually obstructed by the equipment panel. Circuiting is the number of tubes that are feed from the header. As a rule of thumb, we must keep the water velocity over 0.3 m/s and below 1.85 m/s (0.9 – 1.25 m/s is most optimum). In addition, the number of tubes that are feed into the header must divide evenly into the number of tubes in the coil.

Overall size of a coil

Replacing your chilled water coil can be easy and you rarely need to worry about the performance of the new coil. When you replace a 20 year old coil, it is dirty with probably damaged fins. The coil is probably operating at 1/2 of its capacity at best. When you put a new coil on the job, the performance will definitely be acceptable. Your main concern is now making sure that the new coil physically fits into the internal space allowed.

When it comes to coil replacement, a slightly smaller coil is always better than too large. It is always safer since you can work with a smaller coil and you definitely do not want to have an out of dimension coil that can’t fit into the internal space of your HVAC equipment. Therefore, it is important to engage a Coil Expert to ensure the right sizing of your replacement coils.

Please reach out to JA Redden Pte Ltd to be your Coil Replacement Specialist!