Case Study
For this Newsletter of combined Process and Acoustical Engineering pointers for our clients, we are offering a case-study for the selection of engine silencers and coolers for a typical mid-size gas driven reciprocating compressor. The parameters are as follows:
- Operator needs to install a 1200 hp Gas engine drive compressor.
- Project budget cost is some $2.4M.
- The nearest resident is at 600 metres from the plant site in a rural area.
- Fuel gas is valued at $2 per MCF and electricity at 4 cents/kw hr.
- Cooler fan power is 40 hp.
- Without special acoustical treatment to the exhaust and fan, the expected noise level at the resident is calculated at 45 dBA—which is 5 dBA above the nighttime allowable, although 5 dBA below the daytime allowable.
A) The exhaust silencer options are:
- Install a stock reactive (hospital grade) silencer at $5,000 for an extra back- pressure of 10” w.g. or,
- Install a custom low back-pressure silencer at $7,500 for no extra back-pressure.
The difference is evaluated on the basis, that in this case the 10” extra back pressure costs the unit 1.5% more fuel at 8 cu.ft.per hp hr, or 0.015 x 8 x 1200 x 8760 x 2/1000=$2,522 per year. This analysis shows the better silencer to pay off in about one year at full load.
B) The fan options are:
- Install a low noise fan at a differential cost of about $2,000.
- Install fan intake and discharge silencers at a cost of about $25,000.
- Install a separate electric drive fan and Variable Frequency Drive (VFD) to slow the fan down at night at an extra cost of $60,000.
In this case, due to the large (5 dBA) target reduction of noise, the low noise fan option looked marginal, although it is most often the preferred choice in cases where the target noise reduction is 4 dBA or less, due to the high cost of options 2 and 3.
Option 2 is a straight cost adder with little ancillary benefit other than the fact that the discharge fan silencer may help prevent warm air re-circulation and assist with extra exhaust silencing if the exhaust is mounted on the cooler body. Fan silencers will also cause a slight airflow pressure drop, which will need to be accounted for in the cooler performance.
Option 3 looks disproportionately expensive but needs a second look if engine power is at a premium. By disconnecting the fan from the engine jackshaft and installing electric drive and a VFD for $60,000, several advantages emerge:
- An extra 40 hp of compression is purchased at only $1500 per horsepower compared to the base installation at some $2000 per hp.
- Instead of running the fan at full speed year round connected to the engine jackshaft, the VFD runs the fan at 100% speed only for some 2—3 months a year and at peak daytime ambient only. Nighttime fan speeds drop to < 75% in the summer and < 30% in the winter.
- As fan power varies as the speed to the exponent 3 and noise varies as speed to the exponent 5, the average year round fan power drops to about 15hp and the fan nighttime noise (which is the design environmental concern) drops some 5 to 8 dBA compared to full speed summer daytime operation.
- With the above gas and electric parameters, the energy payoff slightly favors the electric drive option (although this may vary on a case by case analysis).
- Fan belt life is dramatically improved due to the soft-start characteristics of the VFD.
- Operator does not have to change fan blade pitch twice a year to get best summer cooling and avoid winter hydrate problems in the gas cooler.
- Fan speed control gives the Operator better engine temperature control than louvers and combined with thermostat control gives a better clean-running radiator section.
In summary, the exhaust and cooler noise control options are not always obvious if we do not consider the combined process and acoustical factors to give the best combined capital and operating costs.
