Background History
This report is presented a room environment condition inside a room to be at 70o F dry bulb at a Relative Humidity between 50 % to 60%. Their experience was they can meet the temperature at 70 o F dry bulb at 60% relative humidity and there were reports of condensation on the ceiling and walls which later produces molds. The Chart 1 below represents the above condition in a Psychrometric Chart.
Condensation is most likely cause by the small temperature difference between the room condition dew point at 55.4 o F dry bulb and the coil supply air dew point at 53.3 o F dry bulb. At this small difference it is likely excessive condensation occurs especially during rainy season.
In Chart 2.0, which is the same as Chart 1, except the small dew point temperature difference between the Room Temperature and Supply Air temperature is increased by changing the chilled water temperature setting to 40o F / 50 o F.
Purpose
This report was made to determine the most effective air conditioning system solution to have a room environmental condition for a room at 70o F between 50% to 60% humidity without having condensation occurring in the operating room.
Methodology
The process that will be used is thru a Psychrometric Chart Analysis method by comparing different room conditions and plotting an air conditioning system in a psychrometric chart. The comparison will be based on an air conditioning system shown below Chart 2, where there is a enough dew point temperature difference between the room condition (about 5o F) and the supply air dew point to prevent excessive condensation.
Data Evaluation
Case 1
The room condition is set at 70o F dry bulb at 50% relative humidity and the chilled water supply temperature is at 45o F and 55o F. This case will result to excessive condensation because the dew point temperature of the room condition is lower than the supply air dew point temperature. Refer to Psychrometric Chart 4.0
Case 2
In Psychrometric Chart 5.0 the same room condition as in Case 1 at 70o F dry bulb at 50% relative humidity but the chilled water supply is lowered to 35o F /45o F in order to provide a supply air dew point 43o F to prevent condensation.
Case 3
This Case, refer to Psychrometric Chart 6.0, complies with the requirement of SLMC – BGC where the room condition for operating rooms is 70o F dry bulb between 50% and 60% relative humidity. The Chart shows a condition at 70o F dry bulb and 55% relative humidity. In this case the chilled water supply should be 38o F / 48o F to provide a supply air dew point temperature of 46o F dry bulb.
Case 4
In Psychrometric Chart 7.0 shows a Desiccant System. The purpose of using a desiccant system is to achieve low dew point temperature. This is done by air passing thru a grid of small air passages in a slow rotating heat wheel that absorbs water vapor using impregnated silica gel as a medium to drop the humidity of the air.
At a 4o F difference in dew point temperature between the supply air dewpoint at 46o F dry bulb and the 50o F dry bulb room air dew point would prevent the chances of condensation.
Case 5
This is like Psychrometric Chart 7.0 except that the Room Condition is set at 70o F dry bulb at 55% humidity. The difference between the room condition dew point temperature and the supply air temperature is 7o F which would be enough to prevent condensation in the room.
Analysis
In Table 1.0 shown above summarize the different cases or options that are considered in this report. Our point of interest is the dew point difference. Using Psychrometric Chart 3.0 as our way to explain about the importance of dew point temperature. This Chart shows the standard air conditioning cycle where the sensible load is represented by the blue line and the latent load represented by the orange line.
Table 1.0 Air Handling Unit System Description
| Description | Dew Point Difference | Air Handling Unit System | System Data | Chart |
| Case 1 | 2.8o F | Chilled Water System | 45o F / 55o F | Chart 4 |
| Case 2 | 7.5o F | Chilled Water System | 35o F / 45o F | Chart 5 |
| Case 3 | 7.0o F | Chilled Water System | 38o F / 48o F | Chart 6 |
| Case 4 | 4.4o F | Desiccant System | Desiccant | Chart 7 |
| Case 5 | 7.0o F | Desiccant System | Desiccant | Chart 8 |
The orange line represents the capacity of the air handling unit to supply cold air to handle latent loads in the room that can potentially cause condensation when cooling is not enough or the length of the orange line is short. Please note the length of the orange line is the difference between the dew points of the supply air and the room conditions.
The Chart 10.0, shown below, is the condition of the SLMC-BGC described above (1. Background History) where condensation occurs in certain rooms in the building. The latent load orange line is short indicating that the air handling units producing supply air into the room do not have enough coldness to handle latent loads.
To solve the problem in Chart 1.0 the chilled water temperature should be reduce to 40o F to bring down the supply air temperature to 50o F that would lengthen the latent load line in blue. This change in chilled water temperature will increase the dew point difference between the supply air and the room condition air.
The Chart 12.0 shown below is a Desiccant System. This system intents to reduce the supply air temperature to 55o F dry bulb by using desiccant to remove water vapor and to keep the air dry at 28% relative humidity and cool the air thru sensible cooling at constant humidity ratio. The latent load is no longer a factor because water vapor is removed during the desiccant process.
Recommendation
Based on the above information and analysis we are recommending the use Chart 6.0 in Case 3 and Chart 8.0 in Case 5. The decision will be based on the least cost.
Prepared By:
Fernando S. Guevara
The Engineer's Point Of View 