When installing MD-R Series gas dryers, the following rules apply:
- Sample pressure equal to or greater than purge pressure.
- Sample gas pressure not to exceed 80 psig.
- Temperatures must not exceed 120°C.
- Purge air of -40°C dew point at a flow rate of approximately two to times sample flow is required to achieve rated performance.
- Sample and purge air must flow counter-current to each other.
- If sample dew point is above ambient temperature, inlet of dryer must be heated. Call factory for details.
1. STANDARD METHOD
The most efficient way to set up MD-R Series dryers is to have sample gas enter through the Nafion tube (wet sample gas inlet) and purge gas flowing countercurrent to the sample (refer to Figure 1). Purge gas should be instrument air or other dry gas. If no dry purge gas is available, the reflux or split sample purging method shown to the right may be used.
2. REFLUX METHOD
The reflux setup, shown in Figure 2, returns dry sample gas back to the dryer for use as the purge after it has gone through the analyzer. Since this method uses all of the dry sample gas as the purge gas, only the sample gas flow required for analysis passes through the dryer. This results in high drying efficiency.
The vacuum on the purge gas should be at least 15” of Hg, with a higher vacuum preferable. This vacuum level is necessary to provide the desired 2:1 purge-to-sample gas flow ratio.
3. SPLIT SAMPLE METHOD
The split sample method, shown in Figure 3, diverts some of the sample from the main stream to be used as the purge gas. More sample gas passes through the dryer than is required for the analysis, lowering the drying efficiency when compared to the reflux method. The following equation can be used to determine the purge flow rate required for the split sample method. Any units may be used as long as they are consistent.
Vp = Vs / (Ps/2Pv) – 1
Vp = Purge flowrate (indicated on flowmeter) Ps = Sample pressure (in absolute units)
Vs = Sample flowrate (indicated on flowmeter) Pv = Purge pressure (in absolute units)
NOTE: Pressure units must be in absolute terms. For proper operation, be sure that the pressure ratio, Ps/Pv is greater than 2. Vacuum pump on purge exhaust is optional.