Coil Bypass: Fighting Summertime Humidity (January 2001)
For most industrial process environments, particularly pharmaceutical and micro-electronic industries, however, a fixed amount of supply air would be required. Thus the 50° to 55° supply air tends to overcool. The prevailing way of countering overcooling is to provide reheat coils to heat the air back up, resulting in simultaneous heating and cooling, and thus increased energy consumption.
This article explores other avenues for summer humidity controls. The emphasis is on the simplest method: coil bypass. Using this technique, a certain amount of air is directed to bypass the cooling coil and then to mix with the coil discharge air, realizing "free reheat." This approach requires slightly lower coil leaving air temperature. This article will take a close look from the perspective of psychrometrics to see how much energy can be saved using this simple method, and its pros and cons.
A Typical ExampleThe best way to understand this situation is to look at a typical system as illustrated in Figure 1. The air-handling unit (ahu) shown here serves a pharmaceutical process area of 1,000 sq ft with a ceiling height of 20 ft. The current good manufacturing practice (cGMP) requires the space to have a minimum of 20 air changes (ach) for ventilation and air conditioning. This leads to a fixed total air supply of approximately 7,000 cfm. Unlike most commercial buildings where vav systems are widely employed, this 7,000 cfm will be maintained all the time, regardless of space cooling or heating load.
The ahu consists of a mixing plenum, pre-filter bank, a preheating coil, a chilled water coil, a supply fan, a reheat coil, and a final filter bank.
The space served by the ahu has 85% sensible heat ratio, and is to be maintained at 72° year around with relative humidity of no more than 60%. The space total cooling load is estimated around 32,000 Btuh.
The ahu takes approximately 6,500 cfm of return air and mixes it with 500 cfm of fresh makeup air at the condition of 92° db (drybulb) and 80° wb (wetbulb) (ho = 43.6 Btu/lb) to produce a mixed air at roughly 73.5° db and 64° wb (hm = 29.3 Btu/lb).
Energy Usage At Design ConditionsAccording to Figure 2, a psychrometric chart, the room condition at 72° and 60% rh has approximately an enthalpy (hr) equaling 28.2 Btu/lb. Assuming the supply air has an enthalpy (hs), then
Q = 4.5•cfm•(hr - hs)
Where Q is the total cooling load at 32,000 Btuh, and cfm is the supply air volume at 7,000 cfm. The supply enthalpy (hs) is then
hs = hr - Q / 4.5 cfm = 28.2 - 32,000 / (4.5•7,000) 27.2 Btuh
The supply air condition can thus be determined on the psychrometric chart at the intersection of the 0.85 sensible heat ratio line and the constant enthalpy line at 27.2 Btuh. It is approximately 68 db and 61 wb.
In order to produce this supply air, the mixed air at 73.5° db and 64° wb (hm = 29.3 Btu/lb) will be cooled down through the chilled water coil to roughly 59° db and 57.8° wb (hc = 25 Btu/lb), and then reheated through the reheat coil back up to 68° db (The actual reheat can be slightly less considering the motor heat added to the airstream; for ease of comparison, this motor heat is not taken into account here).
Therefore, the chilled water coil load is
Qcw = 4.5•cfm•(hm - hc) = 4.5•7,000•(29.3 - 25) = 135,450 Btuh
And the reheat coil load is
Qrh = 1.1•cfm•(T1 - Ts) = 1.1•7,000•(68 - 59) = 69,300 Btuh
Modified SystemFigure 3 shows the ahu modified from that was shown in Figure 1. The only difference between the two is that a bypass section at the chilled water coil is added. Now, instead of 100% of the mixed air going through the coil, only a certain portion gets cooled.
Now assume this portion of air gets cooled down deeper, say, to 54° db and 52.8° wb (h2 = 22 Btu/lb), and then mixed with the bypass air at 73.5° db and 64° wb (hm = 29.3 Btu/lb) to produce the supply air at an enthalpy hs = 27.2 Btu/lb. The portion of the bypass (x) can be estimated as follows:
hs = x•hm + (1-x)•h2 or
x = (hs - h2) / (hm - h2) = (27.2 - 22) / (29.3 - 22) = 0.71
In other words, roughly 70% of the mixed air can bypass the chilled water coil to produce the ideal supply air at 68° without being reheated.
Now the chilled water coil load for the new system is reduced to
Qcw' = 4.5•cfm•(1-x)•(hm - h2) = 4.5•7,000•0.3•(29.3 - 22) = 69,000 Btuh
And the reheat coil load is simply cut down to zero.
Comparing with the conventional system in Figure 1, the modified system in Figure 2 cut the energy from 204,750 Btuh to 69,000 Btuh, by more than 65%.
Capital Investment For The New SystemMost energy improvement measures come with a price tag or in an economical term, capital investment. A simple payback is the capital investment divided by the estimated annual energy dollar savings. A more elaborated lifecycle cost analysis often makes such a measure more attractive than a simple payback indicates. Now take a look at what capital investment it would take to implement such a design improvement.
To cool the mixed air down to 54° db and 52.8° wb, rather than 59° db and 57.8° wb, would take a deeper coil. Most likely the number of rows would increase from four to six. However, reducing the face area of the coil by more than 50% would undoubtedly more than offset the cost increase due to the row increase.
The only other investment is the bypass air damper, which is a very small part of the overall ahu cost and most likely would be offset by the reduced cost due to the decreased coil size. Therefore, there is almost no capital cost involved if this is a new design project.
Why Is This Method Not Being Widely Employed?A question that naturally arises is why this design concept is not being widely employed. There may be two major reasons:
First, this design concept is not particularly attractive for commercial applications where vav systems dominate and reheat coils are not even allowed by many state energy codes. Only in most pharmaceutical and micro-electronic setups would this new design approach realize tremendous energy savings. Unfortunately, energy conservation frequently is the least important consideration in those industries. With millions of dollars worth of products depending on their validated hvac systems, people are very reluctant to try anything unconventional. And regardless how simple and straightforward this modified system is, it is something unconventional.
Secondly, this design concept may have occasionally been employed somewhere, but it definitely never receives any public attention. Its energy savings potential has rarely been analyzed, at least not to my knowledge.
These two reasons compound to make such a design concept not widely used. It is this author's hope that this article will bring this design concept to life. If industrial process owners can see the tremendous benefits associated with this design concept and how little risk it has, they could start implementing such systems and realizing significant energy savings at no cost. It would be good not only for the environment, but also for their business' bottom line.
ConclusionA chilled water or DX cooling coil bypass design can accomplish summer humidity controls with significant energy savings compared with the conventional overcool-then-reheat design which is widely used in industrial environments.
This design approach is particularly useful for constant air volume systems where desired room rh in summer is under 60%. Theoretically speaking, more than 50% of primary energy can be saved while no capital investment is required, as shown in this article through a psychrometric analysis. Engineers should consider adopting this design approach to help industrial clients realize significant energy savings. ES